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Publication numberWO2007101174 A2
Publication typeApplication
Application numberPCT/US2007/062842
Publication date7 Sep 2007
Filing date27 Feb 2007
Priority date27 Feb 2006
Also published asWO2007101174A3
Publication numberPCT/2007/62842, PCT/US/2007/062842, PCT/US/2007/62842, PCT/US/7/062842, PCT/US/7/62842, PCT/US2007/062842, PCT/US2007/62842, PCT/US2007062842, PCT/US200762842, PCT/US7/062842, PCT/US7/62842, PCT/US7062842, PCT/US762842, WO 2007/101174 A2, WO 2007101174 A2, WO 2007101174A2, WO-A2-2007101174, WO2007/101174A2, WO2007101174 A2, WO2007101174A2
InventorsSonia Melle Hernandez, Ana N. Gomez, S. Thomas Picraux, John Devens Gust, Mark Hayes, Solitaire Lindsay, Antonio A. Garcia, Joseph Wang, Terannie Vazquez-Alvarez
ApplicantArizona Board Of Regents For And On Behalf Of Arizona State University
Export CitationBiBTeX, EndNote, RefMan
External Links: Patentscope, Espacenet
Digital magnetofluidic devices and methods
WO 2007101174 A2
Abstract
Disclosed are devices and methods for moving and controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields. For example, droplets can be moved, immobilized, dispensed, coalesced, and/or divided. Also disclosed is a digital magnetofluidic device comprising a hydrophobic surface; a magnetically active fluid droplet in contact with the surface; and a magnetic field coupled with at least a portion of the droplet. Also disclosed is a digital isoelectric focusing method using the devices and methods. Also disclosed are digital microelectrochemical detection methods and digital microelectrochemical reaction methods. This abstract is intended as a scanning tool for purposes of searching in the particular art and is not intended to be limiting of the present invention.
Claims  (OCR text may contain errors)
CLAIMSWhat is claimed is:
1. A digital magneto fluidic device comprising:
a. a hydrophobic surface; b. a magnetically active fluid droplet in contact with the surface; and
c. a magnetic field coupled with at least a portion of the droplet.
2. The device of claim 1, wherein the hydrophobic surface is a superhydrophobic surface.
3. The device of claim 1, wherein the magnetic field has a strength of at least about 0.05 nN, at least about 0.1 nN, at least about 0.2 nN, at least about 0.3 nN, at least about 0.4 nN, at least about 0.5 nN, at least about 0.6 nN, at least about 0.7 nN, at least about 0.8 nN, at least about 0.9 nN, at least about 1 nN, about 0.1 nN, about 0.2 nN, about 0.3 nN, about 0.4 nN, about 0.5 nN, about 1 nN, about 2 nN, about 5 nN, or about 10 nN.
4. The device of claim 1 , wherein the magnetic field is produced by a permanent magnet or an electromagnet.
5. The device of claim 1 , wherein the magnetic field is rotating.
6. The device of claim 1, wherein the magnetically active fluid droplet comprises an aqueous fluid.
7. The device of claim 6, wherein the aqueous fluid comprises at least one of water, sea water, saliva, blood, semen, plasma, urine, lymph, serum, tears, vaginal fluid, sweat, plant or vegetable extract fluid, or cell or tissue culture media, or a mixture thereof.
8. The device of claim 6, wherein the magnetically active fluid droplet further comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof.
9. The device of claim 6, wherein the magnetically active fluid droplet further comprises ampholytes.
10. The device of claim 6, wherein the magnetically active fluid droplet further comprises at least one of a chemically active agent, a chemical labeling agent, or a radioactive agent or a mixture thereof.
11. The device of claim 1 , further comprising an electric field coupled with at least a portion of the droplet.
12. The device of claim 1, wherein the magnetically active fluid droplet comprises at least one of a paramagnetic material, a diamagnetic material, or a ferromagnetic material or a mixture thereof.
13. The device of claim 12, wherein the at least one of a paramagnetic material, a diamagnetic material, or a ferromagnetic material or a mixture thereof is present in the droplet at a concentration of from about 0.05% (w/v) to about 5% (w/v), from about 0.1% (w/v) to about 10% (w/v), from about 0.5% (w/v) to about 5% (w/v), from about 1% (w/v) to about 10% (w/v), or from about 0.1% (w/v) to about 1% (w/v).
14. The device of claim 1, wherein the magnetically active fluid droplet comprises paramagnetic particles.
15. The device of claim 14, wherein the paramagnetic particles comprise functionalization.
16. The device of claim 14, wherein the paramagnetic particles comprise polysiloxane-coated iron particles.
17. The device of claim 15, wherein the functionalization comprises at least one of a molecular recognition moiety, an optical tag, an acidic moiety, a basic moiety, a cationic moiety, and anionic moiety, a hydrophilic moiety, a hydrophobic moiety, or a stimulus- responsive molecule or a mixture thereof.
18. The device of claim 1, wherein the magnetically active fluid droplet comprises an aqueous solution or suspension of at least one of iron, nickel, or cobalt or a mixture thereof.
19. The device of claim 1, wherein the magnetically active fluid droplet comprises an aqueous suspension of paramagnetic carbonyl iron particles.
20. The device of claim 1, wherein the magnetically active fluid droplet has a contact angle with the superhydrophobic surface.
21. The device of claim 20, wherein the contact angle is magnified relative to a smooth surface.
22. The device of claim 20, wherein the magnetically active fluid droplet is in motion across the surface of the superhydrophobic surface, thereby creating an advancing edge contact angle and a receding edge contact angle.
23. The device of claim 20, wherein the magnetically active fluid droplet has a contact angle hysteresis that is decreased relative to a smooth surface.
24. The device of claim 1, wherein the contact angle between the magnetically active fluid droplet and the superhydrophobic surface is at least about 120░, at least about 130░, at least about 140░, at least about 150░, at least about 155░, at least about 160░, or at least about 165░.
25. The device of claim 1, wherein the contact angle between the magnetically active fluid droplet and the superhydrophobic surface is from about 120░ to about 180░, from about 130░ to about 180░, from about 140░ to about 180░, from about 150░ to about 180░, from about 155░ to about 180░, from about 160░ to about 180░, from about 165░ to about 180░, from about 140░ to about 160░, from about 150░ to about 170░, or about 160░.
26. The device of claim 1, wherein the hydrophobic surface comprises at least two regions of differing hydrophobicity.
27. The device of claim 26, wherein the hydrophobic surface comprises a wettability gradient.
28. The device of claim 26, wherein the hydrophobic surface comprises at least two different superhydrophobic materials having differing superhydrophobicities.
29. The device of claim 26, wherein the hydrophobic surface comprises at least two superhydrophobic materials having differing roughnesses.
30. The device of claim 1, wherein the hydrophobic surface comprises poly(te/t-butyl acrylate)-δ/oc^-poly(dimethylsiloxane)-δ/oc^-poly(fert-butyl acrylate).
31. The device of claim 1 , wherein the hydrophobic surface comprises superhydrophobic isotactic polypropylene.
32. The device of claim 1, wherein the hydrophobic surface comprises superhydrophobic boehmite or superhydrophobic silica.
33. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic fluorine-containing nanocomposite coating prepared from a sol gel prepared from tetraethoxysilane, lH,lH,2H,2H-perfluorooctyltriethoxysilane, and silica.
34. The device of claim 1, wherein the hydrophobic surface comprises polytetrafluoroethylene coated mesh film.
35. The device of claim 1, wherein the hydrophobic surface comprises fluorinated dislocation-etched aluminum.
36. The device of claim 1, wherein the hydrophobic surface comprises a multiplicity of carbon nanotubes.
37. The device of claim 36, wherein the hydrophobic surface comprises a multiplicity of carbon nanotubes coated with polytetrafluoroethylene.
38. The device of claim 1, wherein the hydrophobic surface comprises a multiplicity of carbon nanotubes coated with a zinc oxide thin film.
39. The device of claim 1, wherein the hydrophobic surface comprises a multiplicity of superhydrophobic amphiphilic poly( vinyl alcohol) nanofϊbers.
40. The device of claim 1, wherein the hydrophobic surface comprises anode oxidized aluminum.
41. The device of claim 40, wherein the hydrophobic surface further comprises a superhydrophobic coating comprising residues of 1H,1H,2H,2H- perfluorooctyltrichlorosilane or 1 H, 1 H,2H,2H-perfluorodecyltrichlorosilane.
42. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic micropatterned polymer film having micro- or nano-scale surface concavities.
43. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic porous poly(vinylidene fluoride) membrane.
44. The device of claim 1, wherein the hydrophobic surface comprises superhydrophobic microstructured zinc oxide.
45. The device of claim 44, wherein the hydrophobic surface comprises conductive superhydrophobic microstructured zinc oxide.
46. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic block copolymer of polypropylene and poly(methyl methacrylate).
47. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic block copolymer of fluorine-end-capped polyurethane and poly(methyl methacrylate).
48. The device of claim 1, wherein the hydrophobic surface comprises superhydrophobic low- density polyethylene.
49. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic film deposited by microwave plasma-enhanced chemical vapor deposition of trimethyltrimethoxysilane and carbon dioxide.
50. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic polystyrene microsphere/nanofϊber composite film.
51. The device of claim 1 , wherein the hydrophobic surface comprises a superhydrophobic coating comprising residues of 2-(3-(triethoxysilyl)propylaminocarbonylamino)-6-methyl- 4[lH]pyrimidinone.
52. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic calcium carbonate and poly(7V-isopropyl acrylamide) hierarchical structure.
53. The device of claim 1, wherein the hydrophobic surface comprises superhydrophobic electrospun polystyrene trichomelike structures.
54. The device of claim 1, wherein the hydrophobic surface comprises a superhydrophobic copolymer comprising poly((3-trimethoxysilyl)propyl methacrylate-r-polyethylene glycol methyl ether methacrylate).
55. The device of claim 1, wherein the hydrophobic surface comprises microscale features produced by sol-gel etching.
56. The device of claim 1, wherein the hydrophobic surface has roughness and a hydrophobic layer.
57. The device of claim 56, wherein the roughness is a well ordered microstructure.
58. The device of claim 56, wherein the roughness is a well ordered nanostructure.
59. The device of claim 56, wherein the roughness is a random fractal geometry.
60. The device of claim 1, wherein the superhydrophobic surface comprises a nanoscale structure.
61. The device of claim 60, wherein the nanoscale structure is grown by a vapor- liquid-solid technique, by a chemical or physical vapor deposition onto patterned substrates, by dry plasma deposition of pattered substrates, by wet etching of a patterned substrate, or by deposition of separately fabricated nanostructured materials.
62. The device of claim 60, wherein the nanoscale structure is grown by a vapor-liquid-solid technique.
63. The device of claim 61, wherein the separately fabricated nanostructured materials are nanodots or nano wires.
64. The device of claim 60, wherein the nanoscale structure comprises a nanowire.
65. The device of claim 64, wherein the nanowire comprises at least one magnetically active material.
66. The device of claim 64, wherein the nanowire comprises at least one magnetically inactive material.
67. The device of claim 64, wherein the nanowire comprises silicon, zinc oxide, alumina, silicon dioxide, titanium, tungsten, tantalum, iron, nickel, or alloy nanowire or a mixture thereof.
68. The device of claim 64, wherein the nanowire comprises a silicon nanowire.
69. The device of claim 64, wherein the nanowire is in one or more of a random array of nanowires, an ordered array of nanowires, or a hierarchically patterned array of nano wires.
70. The device of claim 64, wherein the nanowire has a diameter of from about 1 nm to about 100 micrometers, from about 10 nm to about 100 micrometers, from about 10 nm to about 200 nm, from about 20 nm to about 500 nm, from about 20 nm to about 100 nm, or from about 20 nm to about 50 nm.
71. The device of claim 26, wherein the hydrophobic layer comprises a hydrocarbon.
72. The device of claim 71, wherein the hydrophobic layer comprises a perfluorinated hydrocarbon.
73. The device of claim 26, wherein the hydrophobic layer further comprises at least one stimulus-responsive molecule.
74. The device of claim 73, wherein the stimulus comprises at least one of light, heat, pH, a biologically active molecule, or solution chemistry or a combination thereof.
75. The device of claim 73, wherein the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form and second form have different effects on the wetting of the surface.
76. The device of claim 73, wherein the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form is more hydrophilic than the second form.
77. The device of claim 73, wherein the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form is more polar than the second form.
78. The device of claim 73, wherein the stimulus-responsive molecule has predominantly a polar form when exposed to light having a first wavelength.
79. The device of claim 78, wherein the stimulus-responsive molecule has predominantly a nonpolar form when exposed to light having a second wavelength.
80. The device of claim 73, wherein the stimulus-responsive molecule is a photochrome.
81. The device of claim 80, wherein the photochrome isomerizes under two different wavelengths of light.
82. The device of claim 80, wherein the photochrome comprises an organic molecule.
83. The device of claim 80, wherein the photochrome is covalently attached to the surface.
84. The device of claim 80, wherein the photochrome is a spiropyran.
85. The device of claim 84, wherein the spiropyran is an indolinospiropyran.
86. The device of claim 73, wherein the photochrome comprises a spirooxazine, benzo- naphthopyran, naphthopyran, azobenzene, fulgide, diarylethene, dihydroindolizine, photochromic quinone, perimidinespirocyclohexadienone, or dihydropyrene or a combination thereof.
87. A method of inducing linear movement of a fluid droplet on a surface comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface; b. coupling a magnetic field with at least a portion of the droplet; and c. varying the magnetic field intensity across the surface.
88. The method of claim 87, wherein the magnetic field has an intensity sufficient to overcome friction between the magnetically active fluid droplet and the hydrophobic surface but insufficient to overcome the surface tension of the magnetically active fluid droplet.
89. The method of claim 87, wherein the magnetic field has an intensity of about O.lnN.
90. The method of claim 87, wherein the magnetic field has an intensity of about InN.
91. The method of claim 87, wherein the hydrophobic surface further comprises at least one stimulus-responsive molecule.
92. The method of claim 87, wherein the magnetic field is varied so as to produce a droplet speed of about 0.5 cm/s, about 1 cm/s, about 2 cm/s, about 3 cm/s, about 4 cm/s, about 5 cm/s, about 6 cm/s, or about 7 cm/s.
93. The method of claim 87, further comprising the step of rotating the magnetic field.
94. The method of claim 99, wherein the magnetically active fluid droplet comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof.
95. The method of claim 99, wherein the magnetically active fluid droplet further comprises at least one of a chemically active agent, a chemical labeling agent, or a radioactive agent or a mixture thereof.
96. The method of claim 87, wherein the magnetically active fluid droplet comprises paramagnetic particles.
97. The method of claim 96, wherein the paramagnetic particles comprise functionalization.
98. The method of claim 97, wherein the functionalization comprises at least one of a molecular recognition moiety, an optical tag, an acidic moiety, a basic moiety, a cationic moiety, and anionic moiety, a hydrophilic moiety, a hydrophobic moiety, or a stimulus- responsive molecule or a mixture thereof.
99. The method of claim 87, further comprising the steps of: a. positioning an additional fluid droplet in contact with the hydrophobic surface; b. varying the magnetic field intensity so as to move the magnetically active fluid droplet substantially toward the additional fluid droplet; and c. contacting the magnetically active fluid droplet with the additional fluid droplet with a force sufficient to overcome surface tension of the magnetically active fluid droplet or the additional fluid droplet, thereby coalescing the droplets.
100. The method of claim 99, wherein the second fluid droplet comprises a magnetically active fluid.
101. The method of claim 99, wherein the second fluid droplet comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof.
102. The method of claim 99, wherein the second fluid droplet comprises particles.
103. The method of claim 99, wherein the second fluid droplet comprises paramagnetic particles.
104. The method of claim 103, wherein the paramagnetic particles comprise functionalization.
105. The method of claim 104, wherein the functionalization comprises at least one of a molecular recognition moiety, an optical tag, an acidic moiety, a basic moiety, a cationic moiety, and anionic moiety, a hydrophilic moiety, a hydrophobic moiety, or a stimulus- responsive molecule or a mixture thereof.
106. A method of immobilizing a fluid droplet on a surface comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface; and b. coupling a stationary magnetic field with at least a portion of the droplet.
107. The method of claim 106, wherein the hydrophobic surface is a superhydrophobic surface.
108. A method of immobilizing a fluid droplet on a surface comprising the steps of: a. positioning a fluid droplet in contact with a surface having a more hydrophobic region and a less hydrophobic region; and b. contacting the droplet with the less hydrophobic region.
109. The method of claim 108, wherein the more hydrophobic surface is a superhydrophobic surface.
110. The method of claim 108, wherein the fluid droplet comprises a magnetically active fluid.
111. A method of dispensing a fluid droplet from a reservoir comprising the steps of: a. positioning a fluid within a reservoir having an opening; b. increasing the pressure within the reservoir, thereby dispensing at least a droplet of the fluid.
112. The method of claim 111, wherein the fluid is a magnetically active fluid.
113. A method of dispensing a fluid droplet from a reservoir comprising the steps of: a. positioning a magnetically active fluid within a reservoir having an opening; b. coupling a magnetic field with at least a portion of the fluid; and c. moving the magnetic field substantially away from the reservoir, thereby dispensing at least a droplet of the fluid.
114. The method of claim 111 or 113, wherein the reservoir comprises a substantially enclosed chamber.
115. A method of dividing a fluid droplet comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface; b. coupling a first magnetic field with at least a first portion of the droplet; c. coupling a second magnetic field with at least a second portion of the droplet; and d. varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet.
116. The method of claim 115, wherein the hydrophobic surface is a superhydrophobic surface.
117. A digital isoelectric focusing method comprising the steps of: a. providing a magnetically active fluid droplet comprising: i. ampholytes, ii. a first protein having a first isoelectric point, and iii. a second protein having a second isoelectric point different from the first isoelectric point; b. positioning the droplet in contact with a hydrophobic surface; c. coupling an electric field with the droplet, thereby generating a pH gradient within the droplet; d. allowing the first protein to migrate along the pH gradient to the first isoelectric point; e. allowing the second protein to migrate along the pH gradient to the second isoelectric point; f. coupling a first magnetic field with at least a first portion of the droplet, wherein the first portion comprises the first isoelectric point; g. coupling a second magnetic field with at least a second portion of the droplet wherein the second portion comprises the second isoelectric point; and h. varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet.
118. The method of claim 117, wherein the hydrophobic surface is a superhydrophobic surface.
119. The method of claim 117, wherein the providing step is performed before the electric field is coupled with the droplet.
120. A method of digital microelectrochemical detection comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface; b. contacting an electrode with the droplet; and c. measuring an electrochemical property.
121. The method of claim 120, further comprising the steps of: a. coupling a magnetic field with at least a portion of the droplet; and b. varying the magnetic field intensity across the surface.
122. The method of claim 120, wherein the contacting step further comprises contacting a reference electrode.
123. The method of claim 120, wherein the electrochemical property is the current or the potential of the droplet.
124. The method of claim 120, wherein the method is chronoamperometric detection.
125. The method of claim 120, wherein the droplet further comprises an electrochemically active species.
126. The method of claim 120, wherein the electrochemically active species is glucose or a derivative thereof.
127. The method of claim 120, wherein the electrochemically active species is dopamine or a derivative thereof.
128. A method of digital microelectrochemical reaction comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface, the droplet comprising at least one electrochemically active species; b. contacting an electrode with the droplet; and c. applying electrochemical energy, thereby oxidizing or reducing the at least one electrochemically active species.
129. The method of claim 128, further comprising the steps of: a. coupling a magnetic field with at least a portion of the droplet; and b. varying the magnetic field intensity across the surface.
130. The method of claim 128, wherein the electrochemical energy comprises a voltage potential or a current.
131. A digital isoelectric focusing method comprising the steps of: a. positioning a magnetically active fluid droplet in contact with a hydrophobic surface, the droplet comprising: i. ampholytes, ii. a first protein having a first isoelectric point, and iii. a second protein having a second isoelectric point different from the first isoelectric point; b. contacting an electrode with the droplet, thereby generating a pH gradient within the droplet; c. allowing the first protein to migrate along the pH gradient to the first isoelectric point; d. allowing the second protein to migrate along the pH gradient to the second isoelectric point; e. coupling a first magnetic field with at least a first portion of the droplet, wherein the first portion comprises the first isoelectric point; f. coupling a second magnetic field with at least a second portion of the droplet wherein the second portion comprises the second isoelectric point; and g. varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet.
Description  (OCR text may contain errors)

DIGITAL MAGNETOFLUIDIC DEVICES AND METHODS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States Application No. 60/777,679 filed February 27, 2006, which is hereby incorporated herein by reference in its entirety.

ACKNOWLEDGEMENT

[0002] This invention was made with government support under Grants Nos. CTS-0102680, CHE-0352599, and DMR-0413523 awarded by the National Science Foundation. The United States government has certain rights in the invention.

BACKGROUND

[0003] Controlling droplet movement under the influence of a stimulus is a capability of continued and growing interest. Although drop dynamic behavior on a superhydrophobic surface is interesting from a scientific and technologic point of view, little is known about aqueous drops moving on a flat non-patterned superhydrophobic surface by mechanisms different from gravity. There are several examples of technologies that can benefit from key advances in this field, such as superhydrophobic surfaces capable of self-cleaning by the action of a rolling drop or micro fluidics devices that take advantage of new effects and better performance derived from manipulating fluids at small scales. Quere, D., Fakir droplets. Nature Materials, 2002. 1: p. 14-15; Gould, P., Smart, clean surfaces. Materials Today, 2003. 6(11): p. 44-48; Nguyen, N. -T. and S. T. Wereley, Fundamentals and applications of microfluidics . 2002, Norwood, MA: Artech House. A number of fascinating phenomena have been reported in the literature treating the dynamic behavior of non- wetting drops. Most of them focused on drop dynamics on patterned non-wetting surfaces. Examples of these are studies on the dynamics of drops rolling over an inclined superhydrophobic surface through the action of gravity or by spreading on a flat, patterned superhydrophobic surface. Quere, D. and D. Richard, Viscous drops rolling on a tilted non-wettable solid. Europhysics letters, 1999. 48(3): p. 286-291; Mahadevan, L. and Y. Pomeau, Rolling droplets. Physics of fluids, 1999. 11(9): p. 2449-2453; McHaIe, G., et ah, Topography driven spreading. Physical review letters, 2004. 93(3). Relaxation and contact line dynamics have been studied in drops generated by drop-wise condensation on superhydrophobic geometrically patterned surfaces that grow until they become large enough to touch and coalesce. Beysens, D., Phase transition, contact line dynamics and drop coalescence, in International workshop on dynamics of complex fluids . 2004, Yukawa Institute at Kyoto University: Kyoto, Japan. Other studies used a water drop placed between a hydrophilic and a superhydrophobic patterned surface in order to measure fluid pressure (water) effects on contact angle. Journet, C, et al, Carbon angle measurements on superhdrophobic carbon nanotube forests: effect of fluid pressure. Europhysics letters, 2005. 71(1): p. 104-109. Also, the contact angle of a drop on a superhydrophobic surface can be modified using light. Rosario, R., et al., Lotus Effect Amplifies Light-Induced Contact Angle Switching. J. Phys. Chem. B, 2004. 108: p. 12640- 12642.

[0004] Digital microfluidics, alternatively referred to as discrete microfluidics, is an alternative paradigm for manipulation of discrete droplets, where processing is performed on unit-sized packets of fluid which are transported, stored, mixed, reacted, or analyzed in a discrete manner. This concept can be demonstrated using electrowetting arrays for droplet transportation without the use of pumps or valves. Fair, R. Digital Microfluidics. 2004 [cited 2005 11/01]; Available from: http://www.ee. duke.edu/research/microfluidics/; Srinivasan, V., V.K. Pamula, and R.B. Fair, Droplet-based microfluidic lab-on-a-chip for glucose detection. Analytica Chimica Acta, 2004. 507(1): p. 145-150; Ren, H., et al., Dynamics ofelectro- wetting droplet transport. Sensors and Actuators B: Chemical, 2002. 87(1): p. 201-206.

[0005] The physics of scale require that microfluidic devices exploit new approaches to fluid movement because of an inherently large ratio of liquid surface area to volume. One promising method is the control of movement of fluid droplets by magnetic fields. Magnetic fields can be easily imposed by permanent or electromagnets, can be accurately controlled, and are typically mild enough to pose no danger to biological materials. However, moving water-based droplets with magnetic fields has not been effectively demonstrated previously because, inter alia, the droplet movement is retarded by the typically low contact angle between the droplet and the surface.

[0006] Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide for digital microfluidic methods with the driving force being the use of magnetic fields. SUMMARY

[0007] Disclosed is a digital magnetofluidic device comprising a hydrophobic surface; a magnetically active fluid droplet in contact with the surface; and a magnetic field coupled with at least a portion of the droplet.

[0008] Also disclosed is a method of inducing linear movement of a fluid droplet on a surface comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; coupling a magnetic field with at least a portion of the droplet; and varying the magnetic field intensity across the surface.

[0009] Also disclosed is a method for combining two drops comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; coupling a magnetic field with at least a portion of the droplet; and varying the magnetic field intensity across the surface, and further comprising the steps of positioning an additional fluid droplet in contact with the hydrophobic surface; varying the magnetic field intensity so as to move the magnetically active fluid droplet substantially toward the additional fluid droplet; and contacting the magnetically active fluid droplet with the additional fluid droplet with a force sufficient to overcome surface tension of the magnetically active fluid droplet or the additional fluid droplet, thereby coalescing the droplets. One or more of the droplets can optionally further comprise one or more reactive components, for example, at least one of a biologically active agent, a pharmaceutically active agent, a chemically active agent, a chemical labeling agent, or a radioactive agent or a mixture thereof. Coalescing the droplets consequently mixes the components of the drops.

[0010] Also disclosed is a method of immobilizing a fluid droplet on a surface comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; and coupling a stationary magnetic field with at least a portion of the droplet.

[0011] Also disclosed is a method of immobilizing a fluid droplet on a surface comprising the steps of positioning a fluid droplet in contact with a surface having a more hydrophobic region and a less hydrophobic region; and contacting the droplet with the less hydrophobic region. [0012] Also disclosed is a method of dispensing a fluid droplet from a reservoir comprising the steps of positioning a fluid within a reservoir having an opening; and increasing the pressure within the reservoir, thereby dispensing at least a droplet of the fluid.

[0013] Also disclosed is a method of dispensing a fluid droplet from a reservoir comprising the steps of positioning a magnetically active fluid within a reservoir having an opening; coupling a magnetic field with at least a portion of the fluid; and moving the magnetic field substantially away from the reservoir, thereby dispensing at least a droplet of the fluid.

[0014] Also disclosed is a method of dividing a fluid droplet comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; coupling a first magnetic field with at least a first portion of the droplet; coupling a second magnetic field with at least a second portion of the droplet; and varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet.

[0015] Also disclosed is a digital isoelectric focusing method comprising the steps of providing a magnetically active fluid droplet comprising ampholytes, a first protein having a first isoelectric point, and a second protein having a second isoelectric point different from the first isoelectric point; positioning the droplet in contact with a hydrophobic surface; coupling an electric field with the droplet, thereby generating a pH gradient within the droplet; allowing the first protein to migrate along the pH gradient to the first isoelectric point; allowing the second protein to migrate along the pH gradient to the second isoelectric point; coupling a first magnetic field with at least a first portion of the droplet, wherein the first portion comprises the first isoelectric point; coupling a second magnetic field with at least a second portion of the droplet wherein the second portion comprises the second isoelectric point; and varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet. In a further aspect, the electric field can be provided by contacting an electrode. [0016] Also disclosed is a method of digital microelectrochemical detection comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; contacting an electrode with the droplet; and measuring an electrochemical property.

[0017] Also disclosed is a method of digital microelectrochemical reaction comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface, the droplet comprising at least one electrochemically active species; contacting an electrode with the droplet; and applying electrochemical energy, thereby oxidizing or reducing the at least one electrochemically active species.

[0018] Additional advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice. Other advantages will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE FIGURES

[0019] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate several embodiments and together with the description serve to explain the principles of the invention.

[0020] Figure 1 shows three photographs showing the difference between a hydrophobic and superhydrophobic surface. The left hand side of this silicon substrate contains nanowires while the right hand side does not. The entire surface is covalently coated with a fluorinated hydrocarbon. The water drops from the needle adhere to the right hand side of the sample while they slide off the left hand side of the sample.

[0021] Figure 2 shows an example of how the selection of a particular rough surface can increase the light-induced contact angle change.

[0022] Figure 3 shows a drop of liquid sitting on a fractally rough composite surface made up of solid and air.

[0023] Figure 4 shows experimental contact angles on the rough surface. [0024] Figure 5 shows a schematic diagram and still image of a water drop containing aligned paramagnetic particle chains and a rare earth magnet. The schematic illustrates a magnetic field line and the effect of geometry on the angle of paramagnetic particle chain alignment. The magnet is moved to the right and the drop slides along the superhydrophobic surface due to the paramagnetic particle chain's action pushing against the surface tension of the drop.

[0025] Figure 6 shows a sequence of three consecutive still images showing a millimeter-size drop with paramagnetic particles sliding on a superhydrophobic surface sample due to the magnetic field of a permanent magnet moving below the surface. The drop is displaced from position in (a) to (c) by the action of the magnet. Pictures were taken at 10 frames per second.

[0026] Figure 7 shows a distribution of paramagnetic particle aggregated inside a moving drop. Five consecutive frames (a) to (e) of a moving drop taken at 10 frames per second show the relatively homogenous and stable distribution of chains sliding with the moving drop.

[0027] Figure 8 shows a sequence of sketches showing a barrier with a meniscus of liquid before applying a magnetic field, after applying a field and pulling the drop through the barrier, and finally after the surface tension causes the drop to snap off leaving a new meniscus caught between the barriers.

[0028] Figure 9 shows a sequence of sketches showing the use of pressure to apply a fluid droplet to a surface defect.

[0029] Figure 10 shows a schematic illustrating proteins contained within a droplet before migration as a function of isoelectric point under an applied electric field.

[0030] Figure 11 shows a schematic illustrating proteins contained within a droplet after migration as a function of isoelectric point under an applied electric field.

[0031] Figure 12 shows the chemical structure of spiropyrans and their photoresponsive equilibrium.

[0032] Figure 13 shows the chemical structure of dihydroindolizines and their photoresponsive equilibrium. [0033] Figure 14 shows the chemical structure of dithienylethenes and their photoresponsive equilibrium.

[0034] Figure 15 shows the chemical structure of dihydropyrenes and their photoresponsive equilibrium.

[0035] Figure 16 shows SEM images of nanowires growing on a silicon oxide surface seeded with gold nanodots. After 8 minutes of growth a dense array of randomly oriented, long and thin silicon nanowires with gold caps is evident.

[0036] Figure 17 shows a blood droplet sliding off a superhydrophobic surface.

[0037] Figure 18 shows a urine droplet sliding off a superhydrophobic surface.

[0038] Figure 19 shows a saliva droplet sticking to a superhydrophobic surface.

[0039] Figure 20 shows coalescence of two drops on a superhydrophobic surface sample, (a) A 4 microliter drop containing paramagnetic particles on the right of the figure was displaced by the action of a permanent magnet toward a 6 microliter pure water drop pinned on a surface defect, (b) The two drops coalesce when they become close enough to touch, (c) The combined drop is removed from the surface defect due to the paramagnetic particles and the external magnetic field. Depinning is due to the use of surface tension as a lever and the paramagnetic particles as the fulcrum.

[0040] Figure 21 shows still frames from a movie showing the splitting of a water drop using magnetic fields, a) Two permanent magnets were placed below the drop, b) The stress placed on the drop by moving two magnets away from each other is evident in the distortion of the drop and the partial split seen at the upper part of the drop, c) After the split, the drop volume is about half of what is seen in (a) and (b). The other half of the drop is out of the field of view of the microscope and thus not seen in these still sequences, d) The remaining drop regains spherical shape.

[0041] Figure 22 shows a polished silicon wafer bearing random silicon nanowires with diameters of 20-50 nm prepared by a vapor-liquid-solid technique. [0042] Figure 23 shows direct comparisons of water contact angles on adjacent polished and nanowire areas.

[0043] Figure 24 shows (a) A water drop of 6 μl resting on a nanowire surface under a magnetic field. Paramagnetic particle chains are vertically aligned along the magnet axis, (b) The same water drop sliding to the right and under the influence of a magnet, which is positioned beneath the surface. Paramagnetic particle chains push against the lateral surface of the drop. A force diagram accompanies the drop images.

[0044] Figure 25 shows the minimum magnetic field needed to displace the drop, B^ as a function of particle concentration for drops of different sizes. Each point is an average of at least ten measurements and indicates that drop size does not affect B^

[0045] Figure 26 shows a sequence of frames from a video of a 30 microliter water drop moving from left to right on a silicon nanowire superhydrophobic surface with an iron particle concentration of 5% by the action of a permanent magnet below the surface.

[0046] Figure 27 shows a silicon nanowire superhydrophobic surface, a) Water drop showing a static contact angle higher that 170░. The drop is held by the pipette tip to prevent it from rolling off the surface, b) Top view SEM of the nanowire surface, c) Side view SEM. On the SEM images it is possible to distinguish from among the gold-capped nanowires with different directions covering the surface. The nanowire growth conditions were 460 0C, 1 Torr, 18 minutes, and the nanowire average length was 2.5 μm. Scale bar: 500nm.

[0047] Figure 28 shows low density polyethylene surfaces (LDPE). a) Water drop on LDPE superhydrophobic surface, showing a static contact angle of approximately 150░. b) Top view SEM of 50X magnified image shows that the polyethylene crystals do not cover the surface homogeneously, however, the sample is still superhydrophobic. c) Top view SEM of 1500X magnified image shows the floral-like low density polyethylene structures that cover the polyethylene slab, making the surface superhydrophobic by a combination of multidimensional surface roughness and the intrinsic hydrophobicity of polyethylene.

[0048] Figure 29 shows a sequence of frames from a video showing movement of water drops in three dimensions, on a low density polyethylene surface, a) Drop moving from right to left on a horizontal surface, b) Drop moving upwards along a vertical surface, c) Drop moving from left to right on an upside-down surface. All drops contain 5 wt% siloxane- coated, iron microparticles. On the right of all frames a pendant drop from a pipette tip is shown as a gravity reference.

[0049] Figure 30 shows square-wave vo Mammograms for dopamine illustrating the absence of carry-over between "Sample" and "Blank" doplets. Four repetitive measurements performed on (a) "Blank" (0.1 M phosphate buffer) and on (b) "Sample" (10 mg/1 dopamine) drops (30 μl). A "Wash" drop served to clean the electrode between the 'Sample' and "Background" measurements. Conditions: SWV parameters, frequency 25 Hz; step potential 4 mV; amplitude 25 mV. 0.1 M phosphate buffer (pH 7.0).

[0050] Figure.31 shows a schematic depicting the setup for electrochemical measurements using magnetically-controlled droplet movement. Drops (30 μl) were moved into and out of the electrode assembly using magnetic fields generated by cylindrical bar magnets. Three separate magnets (M) were used to simultaneously move three different drops: "Blank" (B), "Sample" (S), and "Wash" (W). The electrode assembly consisted of a Pt wire working electrode (WE), a Pt wire counter electrode (CE), and an Ag/ AgCl wire reference electrode (RE). Inset shows a photograph of a typical solution droplet, containing the paramagnetic particles (bottom) in contact with the three-electrode assembly (top).

[0051] Figure 32 shows quantitative voltammetric measurements using the droplet-based electrochemical microfluidic system. Background-subtracted SWV for increasing dopamine concentrations: 5, 10, 15, 20, or 25 mg/1 (curves a - e). Inset shows the corresponding calibration plot. Conditions, as in Figure 30.

[0052] Figure 33 shows photographs illustrating the sequence of events during the digital- micro fluidic electrochemical enzymatic assays of glucose. A) Movement of the 15 μl glucose "Sample" drop towards the electrode assembly; B) movement of the 15 μl GOx "Reagent" drop towards the electrodes and the "Sample" drop; (C,D) merger of the two drops around the electrode assembly and amperometric measurement of the reaction product; (E, F) removal of the combined drop from the electrode assembly.

[0053] Figure 34 shows current-time chronoamperometric recordings obtained at the Graphite/ Pt-on-carbon/ mineral oil (5.5/2.7/1.5 composition ratio) electrode. Increasing glucose concentrations of 0, 2, 6, 10 rnM (a-d respectively) were analyzed in 30 μL combined drops of glucose oxidase (15μl) and glucose (15 μl). Resultant drop glucose concentrations measured were 0, 1, 3, and 5 mM. Potential step to +0.65 V (vs Ag/ AgCl). Inset shows the corresponding calibration plot.

[0054] Figure 35 shows (a) SEM image showing the polysiloxane-coated carbonyl iron microparticles. (b) Magnetization curve for both uncoated and polysiloxane-coated carbonyl iron microparticles.

[0055] Figure 36 shows still frames from a video showing the movement of a 20μl water drop containing 2% carbonyl iron particles. The drop moves from left to right by the action of a permanent magnet that is manually displaced below the surface and reaches a maximum speed of about 2 cm/sec.

[0056] Figure 37 shows a schematic force diagram illustrating that the vertical component of the magnetic force Fm deforms the drop surface at the contact line towards the superhydrophobic surface. This results in an increase in the advancing contact angle based on the relationship θ a = π - a a .

[0057] Figure 38 shows a sequence of still frames from a video showing coalescence of two albumin solution drops.

[0058] Figure 39 shows a sequence of still frames from a video where an albumin drop is split by the action of two bar magnets being separated underneath the superhydrophobic surface.

[0059] Figure 40 shows a sequence of still frames from a video showing a dopamine aqueous solution drop being moved towards an electrode by the action of a magnet, and pulled away from the electrode after the measurement is completed.

DETAILED DESCRIPTION

[0060] Before the present compounds, compositions, articles, devices, and/or methods are disclosed and described, it is to be understood that they are not limited to specific synthetic methods unless otherwise specified, or to particular reagents unless otherwise specified, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

A. DEFINITIONS

[0061] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, example methods and materials are now described.

[0062] Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this pertains. The references disclosed are also individually and specifically incorporated by reference herein for the material contained in them that is discussed in the sentence in which the reference is relied upon. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein may be different from the actual publication dates, which may need to be independently confirmed.

[0063] As used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a component," "a polymer," or "a particle" includes mixtures of two or more such components, polymers, or particles, and the like.

[0064] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as "about" that particular value in addition to the value itself. For example, if the value "10" is disclosed, then "about 10" is also disclosed. It is also understood that when a value is disclosed that "less than or equal to" the value, "greater than or equal to the value" and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value "10" is disclosed the "less than or equal to 10" as well as "greater than or equal to 10" is also disclosed. It is also understood that throughout the application, data is provided in a number of different formats and that this data represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point "10" and a particular data point 15 are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

[0065] A residue of a chemical species, as used in the specification and concluding claims, refers to the moiety that is the resulting product of the chemical species in a particular reaction scheme or subsequent formulation or chemical product, regardless of whether the moiety is actually obtained from the chemical species. Thus, an ethylene glycol residue in a polyester refers to one or more -OCH2CH2O- units in the polyester, regardless of whether ethylene glycol was used to prepare the polyester. Similarly, a sebacic acid residue in a polyester refers to one or more -CO(CH2)SCO- moieties in the polyester, regardless of whether the residue is obtained by reacting sebacic acid or an ester thereof to obtain the polyester.

[0066] As used herein, the terms "optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0067] As used herein, the term "coupled," when referring to an object (e.g., a droplet) and a magnetic field or an electric field, refers to a state of magnetic or electric communication. That is, an object is coupled with a field when the filed can exert a force upon the object and the object experiences that field. One example of an object coupled to a magnetic field is a solution droplet containing magnetic species moving under the influence of a magnetic field across a superhydrophobic surface. [0068] As used herein, the term "magnetofluidic" refers to devices and methods for moving and controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields. "Digital magnetofluidics" refers to the manipulation of discrete droplets.

[0069] As used herein, the term "hydrophobic" refers to a surface where water drops have a contact angle above about 90 degrees but below about 120 degrees.

[0070] As used herein, the term "superhydrophobic" refers to a surface where water drops have a contact angle above about 120 degrees and up to 180 degrees.

[0071] As used herein, the term "wettability" refers to the relative degree to which a fluid spread evenly across a surface or will spread into or coat a solid surface in the presence of other immiscible fluids. The surface wetting of a material is normally expressed as an angle. Poor wettability is indicated by a high value, for example, greater than about 40░.

[0072] As used herein, the term "contact angle" refers to the equilibrium angle of contact of a fluid on a rigid surface, measured within the fluid at the contact line where three phases (liquid, solid, gas) meet. For example, water sheeting on glass has zero contact angle, but water beading on an oily surface or plastic can have a contact angle of 90░ or greater.

[0073] As used herein, the terms "rough" or "roughness" refer to surface irregularities. Roughness height is the height of the irregularities with respect to a reference line. The roughness width is the distance parallel to the nominal surface between successive peaks or ridges which constitute the predominate pattern of the roughness. Typically, the unit pattern of surface irregularities is smaller than the size of a material, for example a droplet, placed on the surface. A "fractally rough" surface has a geometry as described by Mandelbrot's definition of fractal geometry; that is containing fractions of dimensions.

[0074] Disclosed are the components to be used to prepare the compositions as well as the compositions themselves to be used within the methods disclosed herein. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutation of these compounds may not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a particular compound is disclosed and discussed and a number of modifications that can be made to a number of molecules including the compounds are discussed, specifically contemplated is each and every combination and permutation of the compound and the modifications that are possible unless specifically indicated to the contrary. Thus, if a class of molecules A, B, and C are disclosed as well as a class of molecules D, E, and F and an example of a combination molecule, A-D is disclosed, then even if each is not individually recited each is individually and collectively contemplated meaning combinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considered disclosed. Likewise, any subset or combination of these is also disclosed. Thus, for example, the sub-group of A-E, B-F, and C- E would be considered disclosed. This concept applies to all aspects of this application including, but not limited to, steps in methods of making and using the compositions. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the methods.

[0075] It is understood that the compositions disclosed herein have certain functions. Disclosed herein are certain structural requirements for performing the disclosed functions, and it is understood that there are a variety of structures that can perform the same function that are related to the disclosed structures, and that these structures will typically achieve the same result.

[0076] Disclosed are methods of moving and controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields. Small water drops (volume 5-20 μL) that contain fractions of paramagnetic particles as low as, for example, 0.1% wt/wt can be moved on a superhydrophobic surface at relatively high speed (7 cm/s) by displacing a permanent magnet. An aqueous drop pinned to a surface defect can be combined with another drop that contains paramagnetic particles thus making it possible to move the newly formed drop. A drop can also be split using two magnetic fields. This new approach to microfluidics has the advantages of faster and more flexible control over drop movement and manipulation.

B. MICROFLUIDICS

[0077] Microfluidic devices are, essentially, tiny, sophisticated devices that can analyze samples or otherwise manipulate fluids and materials at small scales typically below one millimeter in characteristic length. Continuous flow systems have generally been the default approach towards microfluidics such as the so called lab-on-chip bioassay systems. Fluid droplet based microfluidic applications, however, have become increasingly popular because of their ability to enable spatially and temporally resolved chemistries. Typical microfluidic devices can have one or more channels with at least one dimension less than 1 mm and can be used with common fluids including, for example, whole blood samples, bacterial cell suspensions, protein or antibody solutions, and various buffers.

[0078] Molecular diffusion coefficients, fluid viscosity, pH, chemical binding coefficients, and enzyme reaction kinetics can be measured by using microfluidic devices. Microfluidic devices can also be used in many applications relating to clinical diagnostics, for example, capillary electrophoresis, isoelectric focusing, immunoassays, flow cytometry, sample injection of proteins for analysis via mass spectrometry, polymerase chain reaction (PCR) amplification, DNA analysis, cell manipulation, cell separation, cell patterning, chemical and materials synthesis, and chemical gradient formation.

[0079] For example, in a microfluidic device, the cells, DNA, or proteins that are used to test the candidate drug efficacy can be reduced so that a small amount of a candidate drug can be mixed with its target and the result recorded. This can reduce the time needed to screen all of the drug candidates and can allow as many tests as possible to be run simultaneously. For example, a microfluidic device can require only a single drop of blood for a battery of twenty to thirty tests and can provide nearly immediate results. Microfluidic devices can also help pharmaceutical companies, for example, screen for new drugs by allowing tests to be run on an extremely small scale and in a simultaneous fashion.

[0080] The small size and parallel nature of microfluidic devices can create significant advantages. First, because the volume of fluids within these channels is very small, usually only several nano liters, the amounts of reagents and analytes used are quite small, compared with traditional analysis methods. Second, fabrication techniques used to construct microfluidic devices can be relatively inexpensive and are compatible with elaborate, multiplexed devices and with mass production. Third, microfluidic devices can be fabricated as highly integrated devices for performing a plurality of functions on the same substrate chip.

[0081] Fluids are typically driven through microfluidic devices by either pressure driven flow or by electro-osmotic pumping. In pressure driven flow, the fluid can be pushed through the device by using a positive displacement pump, for example, a syringe pump. Pressure driven flow can be both relatively inexpensive and quite reproducible. Pressure driven flow can be useful for continuous flow systems but is less useful for fluid droplet based lab-on-chip applications. In electro-osmotic pumping, an electric field can be applied across the microchannels of the micro fluidic device. Ions near the surface of the walls of the microchannels move towards the electrode of opposite polarity, resulting in motion of the fluid near the walls and transfers via viscous forces into convective motion of the bulk fluid. Electro-osmotic pumping can be useful for both continuous flow systems based lab-on-chip applications.

[0082] Other pumping devices that can be used with micro fluidic devices include, without limitation, mechanical micropumps, such as centrifugal pumps (CD technology), peristaltic pumps, reciprocating pumps, rotary pumps, sonic pumps, ultrasonic pumps, surface acoustic wave (SAW) pumps and nonmechanical micropumps, such as capillary pumps, thermocapillary micropumps, electrocapillary (electrowetting) micropumps, electro-hydro dynamic (EHD) pumps, EHD static pumps (EHD injection pumps), EHD dynamic pumps (traveling or EHD induction pumps), electrokinetic pumps, electro-osmotic pumps, electrophoretic pumps, magneto-hydro dynamic (MHD) pumps, and dielectrophoretic pumps.

[0083] Microfluidic devices have a variety of applications including, without limitation, chemical microplants, lab-on-a-chip (LOC) devices, micro total analysis systems (μTAS), microfactories, microseparation systems, and point-of-care (POC) devices.

[0084] Chemical microplants are miniaturized chemical plants. A chemical microplant is generally best suited for a distributed processing of materials at the point-of-use. Such distributed processing could avoid central storage and transportation of toxic substances. Another application could be for substances that are needed only in small quantities.

[0085] Lab-on-a-chip (LOC) devices are small chips that can contain microfluidic channels narrower than a human hair. These devices take advantage of the properties of liquids and gases to separate and better allow microsensors to analyze their constituent elements.

[0086] Micro Total Analysis Systems (μTAS) are miniaturized systems fabricated by the use of micromechanical technology capable of providing total chemical analysis on a microliter scale. The microdevice, fully integrated for example onto a silicon substrate (chip), can perform sample handling, reagent mixing, sample component separation, and analysis. A major area of interest has been the transfer of separation techniques such as capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) to the chip format, coupled with detection systems such as spectrophotometric or conductometric detectors. Micro TAS can be also used in biochemistry for DNA chip analysis and drug discovery studies.

[0087] Microfactories provide micro-scaled production. This involves parallel production. Explosive reactions or reaction demanding intensive heat exchange can be divided into safer microreactions, but still providing the same volume of production.

[0088] Microseparation systems are miniaturized separation systems.

[0089] Point-of-care (POC) devices involve diagnostic testing carried out when a patient visits the clinic, with the results available at that visit. Such devices usually consist of a disposable test cartridge and a reading device, usually hand-held or desktop-sized.

[0090] Micro fluidic devices can be fabricated from a variety of materials. Silicon (Si) has been used extensively in micro fluidic devices. Silicon can be an especially good material for microfluidic channels coupled with microelectronics or other microelectromechanical systems (MEMS). It also has good stiffness, allowing the formation of fairly rigid microstructures, which can be useful for dimensional stability. In these applications, the silicon surface is actually a silicon oxide that naturally forms upon exposure of silicon to air or that is formed by another oxidation method. When a material is referred to as "silicon," the material can include silicon bearing such an oxide surface.

[0091] Generally, a photoresist is spun onto a silicon substrate. The photoresist is then exposed to ultraviolet (UV) light through a high-resolution mask with the desired device patterns. After removing the excess unpolymerized photoresist, the silicon wafer is placed in a wet chemical etching bath that anisotropically etches the silicon in locations not protected by photoresist, resulting in a silicon wafer in which microchannels are etched. A glass coverslip can be used to fully enclose the channels and holes are drilled in the glass to allow fluidic access. For straighter edges and a deeper etch depth, deep reactive ion etching (DRIE) is an alternative to wet chemical etching.

[0092] Another material suitable for microfluidic device is polydimethylsiloxane (PDMS). Generally, liquid PDMS is poured over a mold and cured to cross-link the polymer, resulting in an optically clear, relatively flexible material that can be stacked onto other cured polymer slabs to form complex three dimensional geometries.

1. SURFACE WETTING

[0093] Over the past 70 years, pioneers such as Wenzel and Cassie and Baxter have made notable contributions to the understanding of surface wetting. Cassie, A. and S. Baxter, Wettability of porous surfaces. Trans. Faraday Soc, 1944. 40: p. 546-551; Wenzel, R.N.,

Resistance of solid surfaces to wetting by water. Industrial and Engineering Chemistry, 1936. 28: p. 988-994. Recently there has been a renewed interest in this subject and researchers have concentrated their attention on nanostructured materials, actuation of liquid contact angle changes using external fields, and surface analysis measurements. A major goal in this area has been to control phenomena related to wetting such as capillary rise and fall and the movement of liquids along surfaces using an external stimulus such as light or electric fields. The interest in studies focused on water resides on the obvious ubiquity of the fluid and its importance in biomedicine and environmental studies. It was noted that even though plants can repel water using the so-called Lotus effect, the intrinsic contact angle of their leaves can be below 90 degrees. This effect was indicated to be non-ergodic since the same leaf can be fully wetted or non- wetted depending upon its history. An explanation for this phenomenon is that leaf surfaces feature roughness at multiple length scales. Otten, A. and S. Herminghaus, How Plants Keep Dry: A Physicist's Point of View. Langmuir, 2004. 20(6): p. 2405 -2408. This property can be mimicked to create artificial superhydrophobic surfaces. Lai, S. C. S. (2003) Mimicking nature: Physical basis and artificial synthesis of the Lotus-effect.

University Leiden, Leiden (NL) (http://www.home.wanadoo.nl/scslai/lotus.pdf), 1-31. When placed over a superhydrophobic surface, water drops tend to minimize their contact with the surface by becoming spherical, and tend to slide or roll off the surface extremely easily, as if they were repelled by the surface. The Lotus effect can be described by the photographs in Figure 1. 2. ROUGHNESS

[0094] One approach to preparing microscopically rough surfaces has been the use of photolithographic methods. For example, standard photolithography with a resist can be used to prepare surfaces with defined surface feature (pillar arrays) dimensions in an n-type silicon substrate. The height of the surface features, h, is specified by the etch depth.

[0095] In another approach, x-ray lithography techniques, such as (S)LIGA, can be used to define high aspect ratio structures in nickel. The process consists of exposing a sheet of PMMA bonded to a wafer using X-ray lithography. The PMMA is then developed and the exposed material is removed. Nickel is then electroplated up in the open areas of the PMMA. The nickel over-plate is removed by polishing, leaving high aspect ratio nickel parts. The PMMA is removed, and the nickel parts may remain anchored to the substrate or be released.

[0096] Rough surfaces including surface features can be prepared by physical vapor deposition methods that include, for example, evaporation and sputtering.

[0097] In evaporative methods, a substrate can be placed in a high vacuum chamber at room temperature with a crucible containing the material to be deposited. A heating source can be used to heat the crucible causing the material to evaporate and condense on all exposed cool surfaces of the vacuum chamber and substrate. Typical sources of heating include, for example, e-beam, resistive heating, RF-inductive heating. The process typically can be performed on one side of the substrate at a time. In some systems, the substrate can be heated during deposition to alter the composition/stress of the deposited material.

[0098] In sputtering methods, a substrate can be placed in a vacuum chamber with a target (a cathode) of the material to be deposited. A plasma is generated in a passive source gas {e.g., Argon) in the chamber, and the ion bombardment is directed towards the target, causing material to be sputtered off the target and condense on the chamber walls and the substrate. A strong magnetic field can be used to concentrate the plasma near the target to increase the deposition rate. The ejection of atoms or groups of atoms from the surface of the cathode of a vacuum tube can be the result of heavy-ion impact. Sputtering methods can be used to deposit a thin layer of metal on a glass, plastic, metal, or other surface in a vacuum. [0099] Chemical vapor deposition (CVD) methods can also be used to prepare rough surfaces. CVD methods pertain to the growth of thin solid films on a crystalline substrate as the result of thermochemical vapor-phase reactions. CVD methods include, for example, low-pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD).

[00100] LPCVD can be performed in a reactor at temperatures up to about 900 0C. A deposited film is a product of a chemical reaction between the source gases supplied to the reactor. The process typically can be performed on both sides of the substrate at the same time.

[00101] PECVD can be performed in a reactor at temperatures up to about 400 0C.

The deposited film is a product of a chemical reaction between the source gases supplied to the reactor. A plasma is generated in the reactor to increase the energy available for the chemical reaction at a given temperature. The process typically can be performed on one side of the substrate at a time.

[00102] In the present methods, multidimensionally rough surfaces can be prepared as disclosed herein.

[00103] Surface energy gradients can be designed by preparing surfaces having varying degrees of roughness. For example, chemically homogeneous surfaces of varying roughness can be prepared by photolithographic techniques. To prepare a surface roughness gradient, for example, substantially parallel strips of surfaces can be prepared and positioned so that fluid droplets in contact with the surface will contact at least two strips along the surface roughness gradient. Surface features are typically at least one order of magnitude smaller than the fluid droplet size. The strips can be selected such that each strip has a successively greater surface roughness. A path that is substantially perpendicular to the strips, therefore, constitutes a gradient of surface roughness. In such a system, the fluid droplet sequentially contacts strips of increasing roughness as it moves from strips of lower roughness to strips of greater roughness, thereby successively minimizing its contact angle with the surface as roughness increases. 3. SURFACE TENSION DRIVEN MICROFLUIDIC SYSTEMS

[00104] At the microscale, surface tension becomes a relatively large force, as compared to other forces such as gravity or structural stiffness. In mechanical devices, surface tension begins to dominate other forces when physical features are shrunk to micrometers. Electrocapillary and electrowetting actively use surface tension at the microscale. Electrowetting is an electrically-induced change of a material's wettability.

[00105] Surface tension driven microfluidic systems employ surface tension to generate motion in fluid droplets. For example, hydrophobic and hydrophilic interactions of the fluid droplet with the system surface drive the droplet from regions of greater hydrophobicity (lower hydrophilicity) to regions of lower hydrophobicity (greater hydrophilicity) along a gradient of successively decreasing hydrophobicity (increasing hydrophilicity).

4. TORTUOUS SOLID-LIQUID-GAS CONTACT LINE

[00106] Fractally rough surfaces generally provide a highly involved and intricate interface with fluid droplets in contact with the surface. The contact angle at the interface between the fractally rough surface with hydrophobic surface coating and the fluid droplet can be high, often approaching the theoretical maximum of a 180░ apparent contact angle. Accordingly, fractally rough surfaces possess a smaller level of contact angle hysteresis when superhydrophobic than well-ordered surfaces or surfaces that are rough at the microscale but not at the nanoscale.

5. RATIO OF SURFACE AREA TO VOLUME

[00107] Generally, at the microscale, for example in microfluidic devices, the ratio of surface area to volume of a given liquid is extremely high compared to the ratio of surface area to volume at normal scales. Accordingly, surface properties and interactions begin to dominate other properties and interactions.

6. FLUID DROPLET

[00108] The liquids as disclosed herein can be in the form of drops or droplets which represent discreet self contained units of the liquid. The drops and droplets can be any size, such as the sizes disclosed herein. The word "drop" or "droplet," when applied to a fluid, can include any discrete portion of fluid, including a free standing drop or portion on a surface, a portion of fluid in a capillary, channel, or similar partially confined space, and fluid portions within a porous medium.

7. CONTACT ANGLE

[00109] The contact angle between a fluid droplet and a surface generally refers to the pure water equilibrium contact angle. Advancing angles generally follow Cassie-Baxter wetting with a constant fraction of the surface wetted for a particular roughness, while receding angles generally follow Wenzel wetting. Due to contact with the surface, Wenzel wetting creates the condition for water drop movement. The photowetting driving force is proportional to roughness.

8. CONTACT ANGLE HYSTERESIS

[00110] The contact angle hysteresis is the difference between the advancing contact angle and the receding contact angle in resistance to motion of the fluid droplet. If the contact angle hysteresis is larger than the light induced contact angle change, contact hysteresis occurs, and movement of the fluid is slowed or stopped.

[00111] This hysteresis effect can be caused by the interaction of the receding edge with the surface. For example, attractive interactions between the surface and the fluid at the receding edge can retard motion of the fluid droplet. Hysteresis can make the driving force smaller and hence slow the speed of movement. Hysteresis can be overcome by using very rough surfaces in combination with surface modification by hydrophobic molecules. At a constant velocity the driving force equals the drag force; hence, the smaller the drag force the lower the velocity, a small difference means a slower velocity.

9. REDUCED CONTACT ANGLE HYSTERESIS

[00112] Superhydrophobic rough surfaces provide a reduced contact angle hysteresis when used in surface-tension driven micro fluidic applications. See, Lafϊtma, A. & Quere, D., Superhydrophobic states. Nature Materials 2, 457-460 (2003); Bico, J., Marzolin, C. & Quere, D. Pearl drops. Europhysics Letters 47, 220-226 (1999); Shin, J.-Y., Kuo, C-W., Chert, P. & Moth C-Y. Fabrication of tunable superhydrophobic surfaces by nanosphere lithography, Chemistry of Materials 16, 561-564 (2004). One reason for the small degree of hysteresis is the very low solid-surface free energy resulting from the hydrophobic molecular coating. See Chibowski, E., Surface free energy of a solid from contact angle hysteresis. Advances in Colloid and Interface Science 103, 149-172 (2003).

[00113] Fractally-rough surfaces are particularly interesting for microfluidic applications as there are indications that these surfaces possess a smaller level of contact angle hysteresis than well-ordered ones. See, e.g., Shin, J. -Y., Kuo, C-W., Chert, P. & Moth C-Y. Fabrication of tunable superhydrophobic surfaces by nanosphere lithography, Chemistry of Materials 16, 561-564 (2004); Ramos, S. M. M., Charlaix, E. & Benyagoub, A., Contact angle hysteresis on nano -structured surfaces, Surface Science 540, 355-362 (2003). This phenomenon can be due to the instability of the three-dimensional, tortuous solid- liquid- gas contact line in randomly rough surfaces as compared to that in well-ordered two- dimensional rough surfaces.

[00114] Accordingly, photoresponsive monolayer coatings on fractally rough, superhydrophobic surfaces can exhibit contact angle magnification and lowered contact angle hysteresis. Using this approach, contact angle amplification and hysteresis reduction were improved by as much as a factor of two.

[00115] In an alternative aspect, the fluid droplet can comprise a liquid other than water. For example, the fluid droplet can be a nonpolar liquid such as an oil or an organic solvent. In this aspect, the fractally rough silicon nanowire-bearing surfaces can be used as suitably rough surfaces. Likewise, the disclosed spiropyrans can be used as a photosensitive variable hydrophobicity agent in this aspect.

[00116] However, in order to minimize solid-surface free energy and interactions with the nonpolar droplet, and therefore minimize contact angle hysteresis for the nonpolar fluid droplet, a hydrophilic (polar) surface coating can be used.

[00117] Exemplary hydrophilic coating materials can include ethylene glycol, ethylene glycol derivatives, polyethylene glycol, polyethylene glycol derivatives, polyvinylpyrrolidone, polyvinylpyrolidinone derivatives, and the like. [00118] Hydrophilic surfaces can also be prepared by contacting silicon surfaces with diluted sulfuric acid, nitric acid, or hydrofluoric acid, thereby producing a top layer consisting of hydroxyl) moieties on the oxide surface. In this aspect, a nonpolar fluid droplet placed upon a suitably rough surface that has been coated with a photosensitive variable hydrophobicity agent can be induced to move by exposure to an ultraviolet- visible light gradient. However, in this aspect, the nonpolar fluid droplet is induced to move in the direction of increasing hydrophobicity. That is, the droplet would move in a direction opposite to that which would be moved by a water droplet in the same ultraviolet- visible light gradient.

C. DEVICES

[00119] In one aspect, the invention relates to a digital magneto fluidic device comprising a hydrophobic surface; a magnetically active fluid droplet in contact with the surface; and a magnetic field coupled with at least a portion of the droplet. In one aspect, the hydrophobic surface is a superhydrophobic surface. It is understood that the devices can be used in combination with the methods.

1. SURFACES

[00120] In one aspect, the hydrophobic surface comprises at least two regions of differing hydrophobicity. In a further aspect, the hydrophobic surface comprises a wettability gradient. See Lu et al., "Low-density polyethylene (LDPE) surface with a wettability gradient by tuning its microstructures," Macromolecular Rapid Communications, 2005, 26 (8), 637- 642. In a further aspect, the hydrophobic surface comprises at least two different superhydrophobic materials having differing superhydrophobicities. In a yet further aspect, the hydrophobic surface comprises at least two superhydrophobic materials having differing roughnesses.

[00121] Generally, the hydrophobic surface can be any hydrophobic surface known by those of skill in the art and can be prepared by any method known to those of skill in the art. As an example, the hydrophobic surface can comprise poly(te/t-butyl acrylate)-block- poly(dimethylsiloxane)-6/ocú-poly(tert-butyl acrylate) (PtBA-δ-PDMS-δ-PtBA). See Han et al, "Diverse Access to Artificial Superhydrophobic Surfaces Using Block Copolymers," Langmuir, 2005, 21, 6662-6665.

[00122] As a further example, the hydrophobic surface can comprise superhydrophobic isotactic polypropylene. See Erbil et al, "Transformation of a Simple Plastic into a Superhydrophobic Surface," Science, 2003, 299, 1377-1380.

[00123] As a further example, the hydrophobic surface can comprise superhydrophobic boehmite (AlOOH) or superhydrophobic silica (SiO2). Such surfaces can be prepared by sublimation of aluminum acetylacetonate according to the procedure of Nakajima et al, "Transparent Superhydrophobic Thin Films with Self-Cleaning Properties," Langmuir, 2000, 16, 7044-7047.

[00124] As a further example, the hydrophobic surface can comprise a superhydrophobic fluorine-containing nanocomposite coating prepared from a sol gel prepared from tetraethoxysilane, lH,lH,2H,2H-perfluorooctyltriethoxysilane, and silica. See Pilotek et al, "Wettability of Microstructured Hydrophobic Sol-Gel Coatings," Journal of ,SoZ-Ge/ Science and Technology, 2003, 26, 789-792.

[00125] As a further example, the hydrophobic surface can comprise polytetrafluoroethylene (PTFE) coated mesh film. See Feng et al, "A Superhydrophobic and Super-Oleophilic Coating Mesh Film for the Separation of Oil and Water," Angew. Chem. Int. Ed., 2004, 45, 2012-2014.

[00126] As a further example, the hydrophobic surface can comprise fluorinated dislocation-etched aluminum. See Qian, B. et al, "Fabrication of Superhydrophobic Surfaces by Dislocation-Selective Chemical Etching on Aluminum, Copper, and Zinc Substrates," Langmuir, 2005, 21, 9007-9009.

[00127] As a further example, the hydrophobic surface can comprise a multiplicity of carbon nanotubes. See Lau et al, "Superhydrophobic Carbon Nanotube Forests," Nano Letters, 2003, 3(12), 1701-1705. [00128] As a further example, the hydrophobic surface can comprise a multiplicity of carbon nanotubes coated with polytetrafluoroethylene (PTFE). See Lau et al., "Superhydrophobic Carbon Nanotube Forests," Nano Letters, 2003, 3(12), 1701-1705.

[00129] As a further example, the hydrophobic surface can comprise a multiplicity of carbon nanotubes coated with a zinc oxide thin film. See Huang et al., "Stable

Superhydrophobic Surface via Carbon Nanotubes Coated with a ZnO Thin Film," J. Phys. Chem., 2005, 109, 11A6-11W.

[00130] As a further example, the hydrophobic surface can comprise a multiplicity of superhydrophobic amphiphilic poly( vinyl alcohol) (PVA) nano fibers. Such surfaces can be prepared using the template -based extrusion method of Feng et al. , "Creation of a

Superhydrophobic Surface from an Amphiphilic Polymer," Angew. Chem. Int. Ed., 2003, 42, 800-802.

[00131] As a further example, the hydrophobic surface can comprise anode oxidized aluminum. See Shibuichi et al., "Super Water- and Oil-Repellant Surfaces resulting from Fractal Structure," Journal of Colloid and Interface Science, 1998, 208, 287-294.

[00132] As a further example, the hydrophobic surface further can comprise a superhydrophobic coating comprising residues of lH,lH,2H,2H-perfluorooctyltrichlorosilane or lHJH^H^H-perfluorodecyltrichlorosilane. See Shibuichi et al., "Super Water- and Oil- Repellant Surfaces resulting from Fractal Structure," Journal of Colloid and Interface Science, 1998, 208, 287-294.

[00133] As a further example, the hydrophobic surface can comprise a superhydrophobic micropatterned polymer film having micro- or nano-scale surface concavities. See Wang et al., "Phase-Separation-Induced Micropatterned Polymer Surfaces and Their Applications," Adv. Fund. Mater., 2005, 15, 655-663.

[00134] As a further example, the hydrophobic surface can comprise a superhydrophobic porous poly(vinylidene fluoride) membrane. See Peng et al., "Porous Poly(Vinylidene Fluoride) Membrane with Highly Hydrophobic Surface," Journal of Applied Polymer Science, 2005, 98, 1358-1363. [00135] As a further example, the hydrophobic surface can comprise superhydrophobic microstructured zinc oxide. See Wu et al., "Fabrication of Superhydrophobic Surfaces from Microstructured ZnO-Based Surfaces via a Wet-Chemical Route," Langmuir, 2005, 21, 2665- 2667.

[00136] As a further example, the hydrophobic surface can comprise conductive superhydrophobic microstructured zinc oxide. See Li et al., "Electrochemical Deposition of Conductive Superhydrophobic Zinc Oxide Thin Films," J. Phys. Chem. B, 2003, 107, 9954- 9957.

[00137] As a further example, the hydrophobic surface can comprise a superhydrophobic block copolymer of polypropylene and poly(methyl methacrylate). See Xie et al., "Facile Creation of a Bionic Superhydrophobic Block Copolymer Surface," Adv. Mater., 2004, 76, 1830-1833.

[00138] As a further example, the hydrophobic surface can comprise a superhydrophobic block copolymer of fluorine-end-capped polyurethane (FPU) and poly(methyl methacrylate) (PMMA). See Xie et al., "Facile Creation of a Super-

Amphiphobic Coating Surface with Bionic Microstructure," Advanced Materials, 2004, 16 (4), 302-305.

[00139] As a further example, the hydrophobic surface can comprise superhydrophobic low-density polyethylene (LDPE). See Lu et al., "Low-Density Polyethylene (LDPE) Surface With A Wettability Gradient By Tuning Its Microstructures," Macromolecular Rapid Communications, 2005, 26 (8), 637-642; Lu et al., "Low-density polyethylene (LDPE) surface with a wettability gradient by tuning its microstructures," Macromolecular Rapid Communications, 2005, 26 (8), 637-642.

[00140] As a further example, the hydrophobic surface can comprise a superhydrophobic film deposited by microwave plasma-enhanced chemical vapor deposition (MPECVD) of trimethyltrimethoxysilane (TMMOS) and carbon dioxide. See Wu et al, "Mechanical Durability Of Ultra- Water-Repellent Thin Film By Microwave Plasma- Enhanced CVD," Thin Solid Films, 2004, 457 (1), 122-127. [00141] As a further example, the hydrophobic surface can comprise a superhydrophobic polystyrene microsphere/nanofiber composite film (PMNCF). See Jiang et al., "A lotus-leaf-like superhydrophobic surface: A porous microsphere/nanofiber composite film prepared by electrohydrodynamics," Angew. Chem. Int. Ed., 2004, 43(33), 4338-4341.

[00142] As a further example, the hydrophobic surface can comprise a superhydrophobic coating comprising residues of 2-(3-

(triethoxysilyl)propylaminocarbonylamino)-6-methyl-4[lH]pyrimidinone. See Han et al., "Fabrication of Superhydrophobic Surface from a Supramolecular Organosilane with Quadruple Hydrogen Bonding," J. Am. Chem. Soc, 2004, 126, 4796-4797.

[00143] As a further example, the hydrophobic surface can comprise a superhydrophobic calcium carbonate and poly(7V-isopropyl acrylamide) hierarchical structure. See Zhang et al., "Fabrication of Superhydrophobic Surfaces from Binary Colloidal Assembly," Langmuir, 2005, 21, 9143-9148.

[00144] As a further example, the hydrophobic surface can comprise superhydrophobic electrospun polystyrene trichomelike structures. See Gu et al., "Artificial silver ragwort surface," Applied Physics Letters, 2005, 86, 201915.

[00145] As a further example, the hydrophobic surface can comprise a superhydrophobic copolymer comprising poly((3-trimethoxysilyl)propyl methacrylate-r- polyethylene glycol methyl ether methacrylate) (poly(TMSMA-r-PEGMA)). See Suh et al, "Control Over Wettability of Polyethylene Glycol Surfaces Using Capillary Lithography," Langmuir, 2005, 21, 6836-6841.

[00146] As a further example, the hydrophobic surface can comprise microscale features produced by sol-gel etching. See Smoukov et al., "Cutting into Solids with Micropatterned Gels," Advanced Materials, 2005, 17, 1361-1365.

[00147] Superhydrophobic surfaces that combine hydrophobic molecular coatings with surface roughness are generally characterized by either well-ordered microstructures, see, e.g., Lafitma, A. & Quere, D., Superhydrophobic states. Nature Materials 2, 457-460 (2003); Bico, J., Marzolin, C. & Quere, D. Pearl drops. Europhysics Letters 47, 220-226 (1999), or by random fractal geometry, see, e.g., Onda, T., Shibuichi, S., Satoh, N. & Tsujii, K., Super- water-repellent fractal surfaces, Langmuir 12, 2125-2127 (1996); Shibuichi, S., Yamamoto, T., Onda, T. & Tsujii, K., Super water- and oil-repellent surfaces resulting from fractal structure, Journal of Colloid and Interface Science 208, 287-294 (1998). Rough fractal surfaces are particularly interesting due to the extremely high degree of roughness that they possess.

[00148] The liquid contact angle on a solid surface is a function of the interfacial energy and roughness. The dependence of the apparent solid-liquid contact angle on surface roughness in terms of flat-surface contact angle can be described by the Cassie model, see Cassie, A. B. & Baxter, S., Wettability of porous surfaces, Transactions of the Faraday Society 40, 546-551 (1944), and the Wenzel model, see Wenzel, R. N., Resistance of solid surfaces to wetting by water, Industrial and Engineering Chemistry Research 28, 988-994 (1936).

[00149] Wenzel, see Wenzel, R.N., Resistance of solid surfaces to wetting by water,

Industrial and Engineering Chemistry Research, 1936, 28: p. 988-994, and Cassie, see Cassie, A.B. and Baxter, S., Wettability of porous surfaces, Transactions of the Faraday Society 1944. 40: p. 546-551, developed approaches to model the rough surface contact angle, θr , using average roughness characteristics of the surface. Wenzel approached the problem by assuming that the liquid filled every part of the rough surface in the region of its contact. Cassie, on the other hand, assumed that the features on the surface would lift up the liquid in the region of contact, leading to the formation of a composite surface.

[00150] Both Cassie and Wenzel types of wetting represent local energy minima for drops on rough surfaces. See, Patankar, N. A., On the modeling of hydrophobic contact angles on rough surfaces, Langmuir 19, 1249-1253 (2003). Drops gently deposited onto superhydrophobic rough surfaces resulted in extremely high contact angles which were well represented by the Cassie model, whereas drops that were allowed to fall onto the surface from a height gave lower contact angles that were better represented by the Wenzel model. See, He, B., Patankar, N. A. & Lee, J., Multiple equilibrium droplet shapes and design criterion for rough hydrophobic surfaces, Langmuir, 19, 4999-5003 (2003).

[00151] Contact angles on rough surfaces can transition from Cassie to Wenzel behavior when pressure is applied to the drop. See, Bico, J., Marzolin, C. & Quere, D., Pearl drops. Europhysics Letters 47, 220-226 (1999). It was found that when using smaller drops (~5μL), visible irradiation of the coated nanowire surface resulted in advancing contact angles of greater than 170░. Larger drops (~15 μL) produced advancing contact angles of 157░. The weight of the larger drops can force the liquid into the depressions in the surface, making the Wenzel model applicable under these conditions.

a. CASSIE MODEL

[00152] Cassie' s model is based on the assumption that the liquid does not fill the crevices of the rough surface, but rests on a composite surface composed of the solid material and air.

b. WENZEL MODEL

[00153] In contrast, Wenzel's model hypothesizes that the liquid completely fills the depressions in the rough surface over the projected area of solid- liquid contact.

[00154] In the Wenzel model, the apparent contact angle on the fractal surface, θ/ may be expressed as,

where θ is the contact angle on a flat surface with identical chemistry and D is the fractal dimension of the surface between the upper and lower scale limits, L and 1, respectively. This indicates that if the flat surface contact angle is changed from a value θ \ to θ 2 by the action of an external stimulus such as light, the apparent contact angle change on the fractal surface - θ/2 ) may be expressed by

/ \ Z)- 2

[00155] Since the term [L/ /) is always >1 for a rough surface, this indicates that the use of a superhydrophobic rough surface will always amplify the magnitude of the stimulus-induced contact angle change relative to the smooth surface (until the theoretical limit of a 180░ contact angle is reached). Thus, by combining photoswitchable surface chemistry with control of surface morphology, it is possible to amplify the photo-induced changes in the water contact angle.

[00156] Each of these theories leads to different values of θr , and it has been experimentally demonstrated that both the Cassie angle, Q^ , and the Wenzel angle, θ™ , can be formed on the same surface depending on the means of formation of the droplet. See He, B., Patankar, N.A., and Lee, J., Multiple equilibrium droplet shapes and design criterion for rough hydrophobic surfaces, Langmuir, 2003. 19: p. 4999-5003. It has also been experimentally confirmed in different systems, that the Cassie and Wenzel contact angles each represent local energy minima separated by energy barriers corresponding to partial wetting of the roughness features. See Id.; Bico, J., Marzolin, C. and Quere, D., Pearl drops, Europhysics Letters, 1999. 47(2): p. 220-226.

[00157] The global energy minimum for this system was calculated to be the smaller of the two contact angles, θ^ &nάθ^ . Experimental measurements on PDMS microstructures showed some quantitative agreement with this calculation. Differences between the experimental and calculated values were attributed to the 30░ contact angle hysteresis present on even "flat" PDMS surfaces.

[00158] The interfacial energies of spiropyran-coated surfaces can be changed solely by altering the wavelength of irradiation. See Rosario, R. et al., Photon-modulated wettability changes on spiropyran-coated surfaces, Langmuir 18, 8062-8069 (2002). The maximum difference obtained between water contact angles under UV and Visible light was of the order of 13░.

[00159] Selection of a particular surface roughness allows amplification of the magnitude of the light-induced contact angle changes on spiropyran-coated surfaces. The roughness characteristics are defined by the geometry of surface features. Figure 2 shows an example of how the selection of a particular rough surface can increase the light-induced contact angle change. In this example a 5░ light-induced change is predicted to be amplified into a 17░ change in contact angle on the rough surface. c. DETERMINATION OF OPTIMAL SURFACE MORPHOLOGY

[00160] The apparent contact angle on non-porous Euclidian rough surfaces is given by the Wenzel equations. See Wenzel, R. N., Industrial and Engineering Chemistry Research, 1936, 28, 988.

cos θw = r cos θs (Eqn. 3)

[00161] The roughness coefficient r is defined as the ratio of the actual solid- liquid interfacial area to the projected solid- liquid interfacial area, and θw and θs are the solid- liquid contact angles on the rough surface and smooth surface, respectively. The effect of r is to enhance the inherent wetting behavior of the surface (by increasing the contact angle >90░, and decreasing the contact angle <90░).

[00162] However, for fractal surfaces, the term r is very large and can even be infinite for a mathematically ideal fractal surface. Additionally, if the fractal behavior extends to the molecular scale, fluids having different molecular dimensions would experience different solid-liquid interfacial areas. Thermodynamic models for the equilibrium contact angle, which take into account both the fractal nature of the surface and the relative dimensions of the different fluid molecules, have been developed. See, e.g., Hazlett, R. D., Journal of Colloid and Interface Science, 1990, 137, 527. The equilibrium contact angle is given by

COS θ fractal = cosθs (Eqn. 4)

[00163] Here, / ≡ (σ2 / σi) and T ≡ {γS2lγsι) . σ refers to the area of the interfacial tension, D is the fractal dimension, and the subscripts s, I, and 2 refer to the surface, liquid, and vapor, respectively, OR is a reference area that represents the scale that would yield the Euclidean area if the fractal nature and dimension held to this scale, such that

πR2 sin2 θ = CσR ι D/2 (Eqn. 5) where R is the radius of the drop.

[00164] The first term within the correction factor in Eqn. 5 can either depress or elevate the contact angle depending on the relative sizes of the fluid molecules and their wetting tendencies. The second term is a measure of the extent of the fractal nature of the surface and is always greater than 1. When the lower limit of fractal behavior is larger than the areas of the fluid molecules, then the fluid molecules are able to probe all the irregularities on the surface and Eqn. 4 reduces to

cos θw, fractal = f —Lγ-2 cos θs (Eqn. 6)

where L and 1 are the upper and lower limits of fractal behavior. The correction term here is analogous to the roughness correction term of the Wenzel equation and quantifies the ratio of the actual solid surface area to the projected surface area. For example, and alkyl ketene dimmer fractal surface was found to possess a correction factor of

(L/ 1) = (34 / 0.2) ' s 4.43 , with the fractal limits being expressed in microns. See Onda, T., Shibuichi, S., Satoh, N. & Tsujii, K., Super-water-repellent fractal surfaces, Langmuir 12, 2125-2127 (1996).

[00165] In the case of rough porous surfaces, Cassie's equation describes the equilibrium contact angle on a composite surface, which retains pockets of air underneath the sessile drop. See Cassie, A. B. & Baxter, S., Wettability of porous surfaces, Transactions of the Faraday Society 40, 546-551 (1944).

cos Qc = /cos θι + /2 (Eqn. 7) where/} and/ are the ratios of projected areas of the solid surface-liquid and air surface- liquid interfaces, respectively, to the total projected area, θc is the Cassie contact angle and θι is the solid-liquid contact angle. Figure 3 shows a drop of liquid sitting on a fractally rough composite surface made up of solid and air. From the figure, fι = a /(a + b) = f and /2 = 6 /(a + b) = (1 - /) . Substituting these ratios into Eqn. 7, the equation becomes

cos θc = f cos 0j + / - 1 (Eqn. 8)

[00166] Since the surface is fractally rough, in this case, θι is equivalent to the Wenzel contact angle on fractally rough surfaces as given by Eqn. 6. Therefore, the Cassie equation can be extended to uniformly heterogeneous fractal surfaces by substituting Eqn. 6 into Eqn.

8.

cos# I, C, fractal = J f \ — r cosθs + f -l (Eqn. 9) [00167] This is the equivalent fractal form of Cassie's equation. While/can be calculated for well-defined Euclidean surfaces, fractal surfaces are not amenable to this quantitative treatment.

[00168] Both Wenzel and Cassie equations represent local energy minima in drop conformation. For fractal surfaces, the Wenzel contact angle is always lesser or equal to the Cassie contact angle. The equilibrium drop shape with the lower value of apparent contact angle on rough Euclidean surfaces will have lower energy. See Patankar, N. A., On the modeling of hydrophobic contact angles on rough surfaces, Langmuir 19, 1249-1253 (2003). Extending this result to fractal surfaces, the Wenzel contact angle represents the global energy minimum of the system.

[00169] At intrinsic contact angles of >90░, the apparent contact angles (Wenzel and

Cassie) increase as a function of the roughness of the surface as represented by the fractal dimension, D, until the physical limit of an apparent 180░ contact angle is reached. The magnification of any light-induced contact angle change, as a function of D, has a maximum at the roughness that first produces an apparent 180░ contact angle on the more hydrophobic surface. Therefore, the degree of fractal surface roughness that produces the maximum magnification of light-induced contact angle changes, Doptmmι, can be predicted by

optimal t In( /Z T / / / i\) 1 In r[Z T / I / I~]\ ^ 1 '

d. FRACTALLY ROUGH SURFACES

[00170] Cross sectional images of fractally rough oxidized silicon nano wire -bearing surfaces were obtained using SEM (Figure 3). A box counting fractal analysis was performed on trace curves of the cross sectional SEM images, and the cross sectional fractal dimension of the surface, Dcross, was determined to be 1.54 between the lower and upper limits of fractal behavior of 74 nm and 202 nm, respectively. The three-dimensional fractal dimension of the surface was estimated to be D ~ Dcross + 1 = 2.54. See, Vicsec, T., Fractal growth phenomena (World Scientific, Singapore, 1989).

[00171] The use of these fractal roughness parameters in the Wenzel model for contact angles on fractal surfaces (Eqn. 1) gave an excellent fit with the experimental contact angles on the rough surface as shown by the dashed line in Figure 4. This demonstrates that the large water drops filled the crevices in the nanowire structure, as described by the Wenzel model.

2. DROPLETS

[00172] It is understood that the fluid droplets can comprise any fluid known to those of skill in the art. It is also understood that the droplet can comprise a magnetically active fluid. In one aspect, the magnetically active fluid droplet comprises an aqueous fluid. In further aspects, the aqueous fluid comprises at least one of water, sea water, saliva, blood, semen, plasma, urine, lymph, serum, tears, vaginal fluid, sweat, plant or vegetable extract fluid, or cell or tissue culture media, or a mixture thereof. In yet further aspects, the magnetically active fluid droplet further comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof.

a. ADDITIVES

[00173] In one aspect, the magnetically active fluid droplet further comprises ampholytes. Generally, ampholytes are chemical species of bifunctional amphoteric (both acid and basic) buffer molecules which form a pH gradient when an electric field is applied across a medium. Examples of ampholytes are glycine, lysine, ornithine, and serine; but other materials can be used.

[00174] In a further aspect, the magnetically active fluid droplet further comprises at least one of a chemically active agent, a chemical labeling agent, or a radioactive agent or a mixture thereof. Suitable chemically active agents include EDTA, carboxylic acids, amines, oxidizants, chemiluminescent reactants, and reductants A chemically active agent can be provided at a chemically effective amount. Suitable chemical labeling agents include fluorescein derivatives , rhodamine derivatives, BODIPY derivatives, eosin derivatives, and nanodots. A chemical labeling agent can be provided at an effective labeling amount. Suitable radioactive agents include radioactive isotopes of europium, iodine, phosphorous, and sulfur. A radioactive agent can be provided at a radioactively effective amount; that is, the agent can be provided in an amount sufficient for detection or sufficient to provide a desired amount of radiation. [00175] In a further aspect, the magnetically active fluid droplet comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof. Suitable pharmaceutically active agents include hormones, steroids, NO, antiviral agents, and antibiotics. A pharmaceutically active agent can be provided at a pharmaceutically effective amount. Suitable biologically active agents include biotin, DNA, RNA, antibodies, proteins, peptides, and enzymes. A biologically active agent can be provided at a biologically effective amount.

[00176] In one aspect, the magnetically active fluid droplet comprises at least one of a paramagnetic material, a diamagnetic material, or a ferromagnetic material or a mixture thereof. Suitable paramagnetic materials include particles of iron oxide, cobalt iron oxide, magnesium iron oxide, nickel, ruthenium, and cobalt. Suitable diamagnetic materials include kaolin, bentonite, barium sulfate, copper, silver, and gold particles. Suitable ferromagnetic materials include iron, iron oxide, cobalt, nickel, iron boron, and mixtures of iron oxides with copper, magnesium, and nickel oxides. In one aspect, the magnetically active fluid droplet comprises an aqueous solution or suspension of at least one of iron, nickel, or cobalt or a mixture thereof. In a further aspect, the magnetically active fluid droplet comprises an aqueous suspension of paramagnetic carbonyl iron particles.

[00177] In further aspects, at least one of a paramagnetic material, a diamagnetic material, or a ferromagnetic material or a mixture thereof is present in the droplet at a concentration of from about 0.05% (w/v) to about 5% (w/v), from about 0.1% (w/v) to about 10% (w/v), from about 0.5% (w/v) to about 5% (w/v), from about 1% (w/v) to about 10% (w/v), or from about 0.1% (w/v) to about 1% (w/v).

[00178] In a further aspect, the device can further comprise an electric field coupled with at least a portion of the droplet.

[00179] In one aspect, the particles, for example paramagnetic particles, can comprise functionalization. By functionalization, it is meant that the particles can bear chemically- or biologically-active moieties at the surface of the particle. Such moieties can be associated with the particles surface by for example covalent, noncovalent, hydrophobic, hydrophilic, hydrogen-bonding, or van der Waals interactions. For example, the functionalization can comprise at least one of a molecular recognition moiety, an optical tag, an acidic moiety, a basic moiety, a cationic moiety, and anionic moiety, a hydrophilic moiety, a hydrophobic moiety, or a stimulus-responsive molecule or a mixture thereof.

b. CONTACT ANGLE

[00180] In one aspect, the magnetically active fluid droplet has a contact angle with the superhydrophobic surface. The contact angle between the magnetically active fluid droplet and the superhydrophobic surface can be, for example, at least about 120░, at least about 130░, at least about 140░, at least about 150░, at least about 155░, at least about 160░, or at least about 165░. In various aspects, the contact angle between the magnetically active fluid droplet and the superhydrophobic surface can be from about 120░ to about 180░, from about 130░ to about 180░, from about 140░ to about 180░, from about 150░ to about 180░, from about 155░ to about 180░, from about 160░ to about 180░, from about 165░ to about 180░, from about 140░ to about 160░, from about 150░ to about 170░, or about 160░. In one aspect, the contact angle is magnified relative to a smooth surface. In a further aspect, the magnetically active fluid droplet can have a contact angle hysteresis that is decreased relative to a smooth surface. In one aspect, the magnetically active fluid droplet is in motion across the surface of the superhydrophobic surface, thereby creating an advancing edge contact angle and a receding edge contact angle.

c. UNDER A MAGNETIC FIELD

[00181] Under the influence of the magnetic field, the particles form chain- like clusters. Without wishing to be bound by theory, it is believed that the simplest way to understand this system is to consider that the permanent magnet generates a spatially nonuniform magnetic field on the region were the drop is located. This magnetic field magnetizes the paramagnetic particles that aggregate into cylindrical clusters that follow the magnetic field lines. When the magnet is displaced, the clusters move and drive the motion of the drop, as shown in Figure 5.

[00182] It is typically very difficult to make paramagnetic-particles containing water drops perch steadily on a superhydrophobic surface. Journet, C, et al., Carbon angle measurements on superhdrophobic carbon nanotube forests: effect of fluid pressure. Europhysics letters, 2005. 71(1): p. 104-109. Several methods for making drops were evaluated: spray, pipette, syringe, and capillary. Very small drops (lμl sprayed drops or smaller) dry very quickly, leaving a small agglomerate of particles over the surface. Aqueous drops with paramagnetic particles with sizes in the range of 5-30μl can be placed and stabilized on a superhydrophobic surface by the magnetic force on the paramagnetic particles exerted by a permanent magnet just below the surface. The drops were made using pipettes with plastic tips. Water drops have a higher affinity for the pipette tip than for the surface, and do not fall onto the surface even when the tip is so close to the surface that the drop bottom is in contact with the surface. If the drop contains paramagnetic particles and a magnet is placed below the surface, when the drop bottom is touching the surface the drop can be separated from the pipette tip because the drop is being held by the force exerted on the particles by the magnet. If the drop is about one millimeter away from the surface, the force on the particles may make the drop fall on the surface. If the magnetic force on the particles is strong enough, the particles are pulled out of the drop to the surface. Another technique is to place a small spot of magnetic particles on the surface (or pulled out from a different drop) can be used to make a water drop overcome its affinity for the plastic tip, thus attracting it to this point on the surface due to capillary action followed by pinning.

3. MAGNETIC FIELD

[00183] Generally, the surfaces and droplets are used in connection with a magnetic field. In one aspect, the magnetic field has a strength of at least about 0.05 nN, at least about 0.1 nN, at least about 0.2 nN, at least about 0.3 nN, at least about 0.4 nN, at least about 0.5 nN, at least about 0.6 nN, at least about 0.7 nN, at least about 0.8 nN, at least about 0.9 nN, at least about 1 nN, about 0.1 nN, about 0.2 nN, about 0.3 nN, about 0.4 nN, about 0.5 nN, about 1 nN, about 2 nN, about 5 nN, or about 10 nN.

[00184] While it is understood that the field can be produced by any method known to those of skill in the art, the magnetic field, in one aspect, is produced by a permanent magnet or an electromagnet. The field can be stationary or can be moving. In one aspect, the magnetic field is rotating.

[00185] The effect of a magnetic field on drops with varied size and particle concentrations was studied, when the magnet was placed at different positions with respect to the surface and drop. Drops with a high concentration of particles were typically deformed by the action of a magnet above or on the side of the drop. In some case, when the magnet was placed very close to the drop, the particles overcame their hold by the drop's surface tension and were pulled out before the drop moved. However, when the magnet was placed under the surface, particles inside the drop aggregated in chains and inclined towards the opposite side of the magnet, thereby following the magnetic field intensity lines. Drop movement following the magnet movement from below in linear and circular patterns was observed, for particle concentrations from 0.1%wt/wt, magnetic field intensity of approximately 0.2 kGauss to approximately 2.5 kGauss - for example from about 0.25 kG to about 1.5 kG or from about 0.5 kG to about 1.0 kG - and with speeds up to about 7cm/sec along a 2 cm path.

D. METHODS

[00186] Generally, the methods relate to methods for moving and controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields. It is understood that the methods can be used in combination with the devices.

1. LINEAR MOVEMENT

[00187] In one aspect, the method of inducing linear movement of a fluid droplet on a surface comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; coupling a magnetic field with at least a portion of the droplet; and varying the magnetic field intensity across the surface. In a further aspect, the magnetic field has an intensity sufficient to overcome friction between the magnetically active fluid droplet and the hydrophobic surface but insufficient to overcome the surface tension of the magnetically active fluid droplet.

[00188] In one aspect, the magnetic field has an intensity of about 0.InN. In a further aspect, the magnetic field has an intensity of about InN. The magnetic field can be varied, for example, so as to produce a droplet speed of about 0.5 cm/s, about 1 cm/s, about 2 cm/s, about 3 cm/s, about 4 cm/s, about 5 cm/s, about 6 cm/s, or about 7 cm/s. The maximum attainable speed remains to be determined, since the maximum speed in this experiment was limited by the maximum magnet speed. It is understood that the speed can theoretically be higher by achieving higher magnet speeds. In a further aspect, the method can further comprise the step of rotating the magnetic field, thereby subjecting the droplet to a rotational force vector.

[00189] In a further aspect, the hydrophobic surface further can comprise at least one stimulus-responsive molecule. In such an aspect, the fluid droplet can be controlled by either magnetic stimulus or by light stimulus or both.

[00190] The movement of water drops on horizontal surfaces was due to the application of a magnetic field that aligns paramagnetic particles, attracts them to the magnet, and moves the drop of water in the process. Alignment of paramagnetic particles as a chain inside the droplet moves the droplet due to the rigidity of the chain. The chain distorts the shape of the drop at the bottom because the particles are attracted to the magnet and when the magnet moves the chain moves with it pushing against the "skin" or contact line of the drop due it surface tension (Figure 6). Unlike the body force of gravity, it is believed that the imposed force can be communicated at the contact line formed by air- liquid-solid phases. The water drop movement in the system occurs in the Cassie-Baxter, mostly non-wetted mode. With the larger-sized droplets studied, without wishing to be bound by theory, it appears that the drop is sliding which likely occurs due to the lack of significant frictional resistance since the wetted contact area is low. Mahadevan, L. and Y. Pomeau, Rolling droplets. Physics of fluids, 1999. 11(9): p. 2449-2453.

[00191] It was observed that the drop size does not affect the intensity of the magnetic field that is required to move drops on the hydrophobic surface, which indicates that frictional resistance is extremely low. These results are in accordance with molecular studies that predict roughness from the nano to the micro scale at the solid-liquid interface can greatly enhance slippage, probably due to the existence of bubbles at a nano-scale at the liquid-solid interface that influence slippage. Cottin-Bizonne, C. B., Jean-Louis; Bocquet, Lyderic; Charlaix, Elisabeth, Low-friction flows of liquid at nanopatterned interfaces. Nature

Materials, 2003(2): p. 237-240. From measurements of the magnetic field intensity required to move the drop, it is roughly estimated that H is proportional to l/(particle concentration)Λb, where the exponent b is in the [1/3, 2] interval.

[00192] On inclined superhydrophobic surfaces, smaller water drops actually roll down faster than larger drops which slide down the inclined surface. Quere, D. and D. Richard, Viscous drops rolling on a tilted non-wettable solid. Europhysics letters, 1999. 48(3): p. 286- 291; Mahadevan, L. and Y. Pomeau, Rolling droplets. Physics of fluids, 1999. 11(9): p. 2449- 2453. Experiments were conducted to detect whether the drops in this system "slide" or "roll." Based on observation of hydrophobic powders placed on top of 2 mm drops, it is believed that drops of this size "slide" across the surface since the powders are not swirling even when the drops move at relatively high speeds. These drops are relatively large and they should slide when placed on an inclined superhydrophobic surface according to theoretical predictions. Quere, D. and D. Richard, Viscous drops rolling on a tilted non-wettable solid. Europhysics letters, 1999. 48(3): p. 286-291. The drop size dynamics transition point can be interpreted as depending in part upon the wetting transition between Cassie-Baxter and

Wenzel wetting modes. Cassie-Baxter wetting assumes that the drop does not penetrate the valleys caused by the roughening of the surface, while Wenzel wetting assumes that the drop does penetrate completely. However and more importantly, for smaller droplets where surface tension forces dominate over gravitational forces and when viscous effects dominate over inertia, drop rolling may occur possibly even when wetting is between the Cassie-Baxter and Wenzel regimes.

[00193] In order to investigate the mechanism by which the paramagnetic particles act on the drop and to determine how the particles were distributed inside the drop while it moved, a camera was mounted on top of the surface to record particle distribution inside a moving drop. The aggregated particle chains inside the drop were regularly distributed on the bottom of the drop as it was moving, apparently sliding with it. (See Figure 7)

2. COALESCENCE

[00194] One method relates to controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields; specifically, droplets can be combined or coalesced. In one aspect, the method further comprises the steps of positioning an additional fluid droplet in contact with the hydrophobic surface; varying the magnetic field intensity so as to move the magnetically active fluid droplet substantially toward the additional fluid droplet; and contacting the magnetically active fluid droplet with the additional fluid droplet with a force sufficient to overcome surface tension of the magnetically active fluid droplet or the additional fluid droplet, thereby coalescing the droplets. [00195] In a further aspect, the second fluid droplet comprises a magnetically active fluid. In a further aspect, the second fluid droplet comprises at least one of a biologically active agent or a pharmaceutically active agent or a mixture thereof. In a further aspect, the second fluid droplet comprises particles. In a further aspect, the second fluid droplet comprises paramagnetic particles. In a further aspect, the paramagnetic particles comprise functionalization. In a further aspect, the functionalization comprises at least one of a molecular recognition moiety, an optical tag, an acidic moiety, a basic moiety, a cationic moiety, and anionic moiety, a hydrophilic moiety, a hydrophobic moiety, or a stimulus- responsive molecule or a mixture thereof.

[00196] In a yet further aspect, one or more of the droplets can optionally further comprise one or more reactive components, for example, at least one of a biologically active agent, a pharmaceutically active agent, a chemically active agent, a chemical labeling agent, or a radioactive agent or a mixture thereof. Coalescing the droplets consequently mixes the components of the drops. This aspect, therefore, provides procedures for carrying out chemical reactions using digital micro fluidic methods. For example, a first droplet further comprising a first reactive component (e.g., an activated carboxylic acid) can be coalesced with a second droplet further comprising a second reactive component (e.g., an amine) using the digital microfluidic methods. Upon coalescence, the combined droplet comprises both the first and second reactive components, allowing them to react and form a product (e.g., an amide). It is contemplated that additional additives (e.g., catalysts, buffers, or indicators) can also be added to the droplets to facilitate the reactions. It is also contemplated that such digital microfluidic methods can be used in automated processes, for example, in automated peptide synthesis, in automated oligonucleotide synthesis, in automated combinatorial synthesis, or in automated analytical methods.

3. IMMOBILIZATION

[00197] One method relates to controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields; specifically, droplets can be immobilized or "pinned" on the surface. In one aspect, a method of immobilizing a fluid droplet on a surface comprises the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; and coupling a stationary magnetic field with at least a portion of the droplet. In a further aspect, the hydrophobic surface is a superhydrophobic surface. In a further aspect, the fluid droplet comprises a magnetically active fluid.

[00198] In a further aspect, a method of immobilizing a fluid droplet on a surface comprises the steps of positioning a fluid droplet in contact with a surface having a more hydrophobic region and a less hydrophobic region; and contacting the droplet with the less hydrophobic region. In one aspect, the more hydrophobic surface is a superhydrophobic surface. In a further aspect, the fluid droplet comprises a magnetically active fluid.

[00199] Drops containing magnetic particles can be placed and moved on the superhydrophobic surfaces, but in order to work with drops that do not contain magnetic particles a surface defect is typically present. The surface defect can be created by physical damage or damage to the hydrophobic chemical coating. Physical damage can be created using a sharp point such as a small needle, while the chemical coat can be removed using a laser pulse. In either case, the abrupt change in contact angle in the damaged region pins a water drop that is dropped from above this region. It has been demonstrated that the movement of a water drop containing paramagnetic particles towards a water drop held by pinning and the subsequent coalescence of the drops. This can also be accomplished using two or more water drops containing paramagnetic particles using two or more magnetic fields in order to place and/or move the drops towards each other.

[00200] While drops without paramagnetic particles that are pinned due to a surface defect cannot be moved, when combined with a drop containing paramagnetic particles or when a drop with paramagnetic particle is placed on a surface defect a magnetic field can be used to force the drop out of the defect. The utility of this action is the ability to combine the two types of drops and then continue to move the combined drop for further processing. Thus, for example, an aqueous solution to be analyzed can be combined with other drops sequentially for sample pretreatment reasons and subsequently moved to another location for analysis. Depinning takes place with only a very small amount of water left behind on the defect. Visual evidence indicates that the amount of water left on the defect depends on the size of the defect. Such depinning action with essentially all of the liquid being removed from the pinned location has not been previously described in the literature using any type of force. Without wishing to be bound by theory, it is believed that this action is performed using the surface tension of the drop as a "lever" and the paramagnetic particles as a "fulcrum."

4. DISPENSING DROPLETS

[00201] One method relates to controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields; specifically, droplets can be dispensed. In one aspect, the invention relates to a method of dispensing a fluid droplet from a reservoir comprising the steps of positioning a fluid within a reservoir having an opening; increasing the pressure within the reservoir, thereby dispensing at least a droplet of the fluid. In a further aspect, the fluid is a magnetically active fluid.

[00202] In a further aspect, the invention relates to a method of dispensing a fluid droplet from a reservoir comprising the steps of positioning a magnetically active fluid within a reservoir having an opening; coupling a magnetic field with at least a portion of the fluid; and moving the magnetic field substantially away from the reservoir, thereby dispensing at least a droplet of the fluid. In one aspect, the reservoir comprises a substantially enclosed chamber.

[00203] The ability to hold water drops using surface defects as well as the ability to split water drops containing paramagnetic particles indicates the following capabilities based on electrospray and electrospinning technology as well as the well-known phenomena of droplet formation due to liquid jet instabilities. See, e.g., Fouillet, Y, Achard, J-L, Microfluidique discrete et biotechnologie, C. R. Physics 5(2004) 577-588. This can be important since microfluidic systems with integrated dispensing, flow, and analysis are highly desirable.

[00204] Using a simple geometric design popular in electrospray technology applied to microfluidics, drops with paramagnetic particles can be dispensed from a reservoir through the use of magnetic fields. Figure 8 illustrates the barrier separating the hydrophilic reservoir from the superhydrophobic substrate, and shows the progression from a meniscus to an elongated drop and finally a liberated drop.

[00205] For drops that do not contain paramagnetic particles, pressure can be used to force the liquid to form a neck and a surface defect to pin the drop on the superhydrophobic substrate followed by a release of pressure. For this dispensing technique, it can be necessary to have an enclosed reservoir in order to build up sufficient pressure to force the water to enter onto the superhydrophobic surface. As shown in Figure 9, the water drop grows relatively uniformly round in order to minimize the surface touching the superhydrophobic surface. As the radius becomes large enough to reach the surface defect shown as a star in Figure 9, the pressure is then slowly released in order to form a neck that leads to instability followed by breakage leaving behind a drop. Based on the methods herein for combining a water drop with a drop containing paramagnetic particles, this drop can be processed and subsequently analyzed.

5. SPLITTING

[00206] One method relates to controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields; specifically, a droplet can be divided or split into two or more smaller droplets. In one aspect, the invention relates to a method of dividing a fluid droplet comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; coupling a first magnetic field with at least a first portion of the droplet; coupling a second magnetic field with at least a second portion of the droplet; and varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet. In a further aspect, the hydrophobic surface is a superhydrophobic surface.

6. DIGITAL ISOELECTRIC FOCUSING

[00207] One method relates to controlling droplets of fluids on hydrophobic surfaces through the use of magnetic fields; specifically, droplets can be manipulated so as to segregate materials dissolved or dispersed within the droplet and subsequently split to divide the materials. In one aspect, the invention relates to a digital isoelectric focusing method comprising the steps of providing a magnetically active fluid droplet comprising ampholytes, a first protein having a first isoelectric point, and a second protein having a second isoelectric point different from the first isoelectric point; positioning the droplet in contact with a hydrophobic surface; coupling an electric field with the droplet, thereby generating a pH gradient within the droplet; allowing the first protein to migrate along the pH gradient to the first isoelectric point; allowing the second protein to migrate along the pH gradient to the second isoelectric point; coupling a first magnetic field with at least a first portion of the droplet, wherein the first portion comprises the first isoelectric point; coupling a second magnetic field with at least a second portion of the droplet wherein the second portion comprises the second isoelectric point; and varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet. In a further aspect, the hydrophobic surface is a superhydrophobic surface. In one aspect, the providing step is performed before the electric field is coupled with the droplet.

[00208] The methods extend digital magnetofluidics to separate proteins within a drop, based upon current practices in the use of an electric field and a group of molecules known as ampholytes in order to generate a pH gradient within a single drop. When a pH gradient is established, proteins dissolved in a droplet migrate to a particular zone in the gradient based on each of their isoelectric point. This well established process is known as isoelectric focusing (IEF) and can be done in a gel phase or in free solution.

[00209] The present invention, however, extends IEF by splitting a droplet using magnetic fields once the proteins have undergone focusing, along the longitudinal axis where the electric field is applied. Once split, one part of the former drop is enriched with a fraction of proteins above a particular isoelectric point and the other part is enriched with proteins below a particular isoelectric point. Since drops can be split and combined with the methods, this process can be repeated if further separation to more completely isolate a particular group of proteins based on isoelectric point is desired.

[00210] The choice of ampholytes can be important in IEF and DIEF. There are a number of commercially available ampholytes and special mixtures useful in particular situations. Ampholytes are well known to those of skill in the art and can be obtained commercially. Suitable ampholyte mixtures are typically low molecular weight species of different isoelectric points. The isoelectric point range can be varied by changing the chemical structure of the ampholytes. Depending on the number of different ampholytes employed and their specific isoelectric point, the pH steps and range can be altered [00211] A schematic diagram of the proposed process is provided in Figures 10 and 11.

A protein solution can be provided in a droplet. See Figure 10. In a further aspect, another protein solution drop can be added after the electric field is applied. The droplet is subjected to an electric field, and the protein(s) migrate within the droplet as a function of isoelectric point. See Figure 11.

E. COMBINATION METHODS

[00212] It is also contemplated that the digital magnetofluidic devices and methods can be used in combination with methods employing other forces, for example, gravity or light- driven methods.

1. GRAVITY METHODS

[00213] In one aspect, the devices and methods can be used in combination with gravity-based methods. For example, gravity can be used to create a force across a surface at an angle other than substantially perpendicular to a gravitational field. The vector of the gravitational field can combine with a force created by a magnetofluidic vector to produce a net force on a fluid droplet.

2. LIGHT-DRIVEN MICROFLUIDIC METHODS

[00214] In one aspect, the devices and methods can be used in combination with light- driven methods as disclosed in Rosario, R., et ah, "Lotus Effect Amplifies Light-Induced Contact Angle Switching," J. Phys. Chem. B, 2004, 108, 12640-12642. For example, a hydrophobicity gradient can be created by a functionalized surface in response to a light frequency gradient to create a net force across a hydrophobic surface.

[00215] In one aspect, the methods can be used in combination with a device comprising a surface, wherein the surface has roughness, a hydrophobic layer, and a photoresponsive molecule. In a further aspect, the methods can be used in combination with a device comprising a surface, wherein the surface has roughness, a hydrophobic layer, and an isomerization molecule which can be isomerized into a first and a second form, wherein the first and second forms have different effects on the wetting of the surface by a fluid. In a further aspect, the methods can be used in combination with a device comprising a fractally rough, hydrophobic surface, and a liquid droplet, wherein the liquid droplet has a contact angle with the surface, and wherein the advancing contact angle under a first condition is lower than the receding contact angle under a second condition. In a further aspect, the methods can be used in combination with a device comprising a surface, wherein the surface has roughness, a hydrophobic layer, and a stimulus inducible molecule, wherein the stimulus inducible molecule causes a contact angle change when stimulated, producing a stimulus induced contact angle change. In one aspect, the methods can be used in combination with a hydrophobic surface that has roughness and a hydrophobic layer. In a further aspect, the roughness is a well ordered microstructure. In a yet further aspect, the roughness is a well ordered nanostructure. In a yet further aspect, the roughness is a random fractal geometry.

[00216] In a yet further aspect, the superhydrophobic surface comprises a nanoscale structure. The nanoscale structure can be grown by, for example, one or more of a vapor- liquid-solid technique, a chemical or physical vapor deposition onto patterned substrates, dry plasma deposition of pattered substrates, wet etching of a patterned substrate, or deposition of separately fabricated nanostructured materials. In a further aspect, the separately fabricated nanostructured materials are nanodots or nanowires.

a. NANOWIRES

[00217] In a further aspect, the nanoscale structure comprises a nanowire. In a yet further aspect, the nanowire comprises at least one magnetically active material or at least one magnetically inactive material. In a further aspect, the nanowire comprises silicon, zinc oxide, alumina, silicon dioxide, titanium, tungsten, tantalum, iron, nickel, or alloy nanowire or a mixture thereof. In a further aspect, the nanowire comprises a silicon nanowire. In a further aspect, the nanowire is in one or more of a random array of nanowires, an ordered array of nanowires, or a hierarchically patterned array of nanowires. In one aspect, the device comprises a nanowire having a diameter of from about 1 nm to about 100 micrometers, from about 10 nm to about 100 micrometers, from about 10 nm to about 200 nm, from about 20 nm to about 500 nm, from about 20 nm to about 100 nm, or from about 20 nm to about 50 nm. b. HYDROPHOBIC LAYER

[00218] In one aspect, the device comprises a hydrophobic layer comprising a hydrocarbon. In a further aspect, the hydrophobic layer comprises a perfluorinated hydrocarbon. In a further aspect, the hydrophobic layer further comprises at least one stimulus-responsive molecule. In various aspects, the stimulus can comprise at least one of light, heat, pH, a biologically active molecule, or solution chemistry or a combination thereof.

c. PHOTORESPONSIVE MOLECULES

[00219] Photoresponsive molecules, or stimulus inducible/responsive molecules, or variable hydrophobicity molecules, can be used to create a hydrophobicity gradient in response to a light frequency gradient to create a net force across a hydrophobic surface.

[00220] In one aspect, the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form and second form have different effects on the wetting of the surface. In a further aspect, the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form is more hydrophilic than the second form. In a further aspect, the stimulus-responsive molecule comprises an isomerization molecule which can be isomerized between a first form and a second form, wherein the first form is more polar than the second form. In a further aspect, the stimulus- responsive molecule has predominantly a polar form when exposed to light having a first wavelength. In a further aspect, the stimulus-responsive molecule has predominantly a nonpolar form when exposed to light having a second wavelength.

[00221] In one aspect, the stimulus-responsive molecule is a photochrome. In a further aspect, the photochrome isomerizes under two different wavelengths of light. In a further aspect, the photochrome comprises an organic molecule. In a further aspect, the photochrome is covalently attached to the surface. The photochrome can be, for example, one or more of a spiropyran, an indolinospiropyran, a spirooxazine, a benzo-naphthopyran, a naphthopyran, an azobenzene, a fulgide, a diarylethene, a dihydroindolizine, a photochromic quinone, a perimidinespirocyclohexadienone, or a dihydropyrene or a combination thereof. (1) SPIROPYRANS

[00222] Spiropyrans are a class of organic photochromes that undergo a reversible transition from a closed, nonpolar form to a highly polar, open form when irradiated with higher energy, shorter wavelength light (e.g., ultraviolet (UV) light (e.g., 366nm)). (Figure 12).

[00223] Irradiation with lower energy, longer wavelength light (e.g., visible (VIS) light

(e.g., 450-550 nm)) converts the molecule back to its closed, nonpolar form. Visible light irradiation of the spiropyran coating yields a relatively hydrophobic surface (higher contact angle) that can be reversibly converted into a more hydrophilic surface (lower contact angle) with UV light irradiation. The reversible switching of contact angles using UV and visible light for these molecular monolayers on smooth glass surfaces is due to the photon-modulated conversion of the spiropyran molecules between open and closed forms. See, Rosario, R. et ah, Photon-modulated wettability changes on spiropyran-coated surfaces, Langmuir 18, 8062- 8069 (2002).

(2) DlHYDROINDOLIZINES

[00224] Dihydroindolizines are a class of organic photochromes that undergo a reversible transition from a closed, nonpolar, form to a highly polar, open form when irradiated with higher energy, shorter wavelength light (e.g., ultraviolet (UV) light). (Figure 13).

(3) DlTHIENYLETHENE

[00225] Dithienylethenes are a class of organic photochromes that undergo a reversible transition from an open, nonplanar form to a closed, planar form when irradiated with higher energy, shorter wavelength light (e.g., ultraviolet (UV) light). (Figure 14).

(4) DlHYDROPYRENE

[00226] Dihydropyrenes are a class of organic photochromes that undergo a reversible transition from a closed, planar form to an open, nonplanar form when irradiated with higher energy, shorter wavelength light (e.g., ultraviolet (UV) light). (Figure 15). F. KITS

[00227] Disclosed herein are kits that are drawn to reagents that can be used in practicing the methods disclosed herein. The kits can include any reagent or combination of reagent discussed herein or that would be understood to be required or beneficial in the practice of the disclosed methods. For example, the kits could include primers to perform the amplification reactions discussed in certain embodiments of the methods, as well as the buffers and enzymes required to use the primers as intended.

G. COMPOSITIONS WITH SIMILAR FUNCTIONS

[00228] It is understood that the compositions disclosed herein have certain functions. Disclosed herein are certain structural requirements for performing the disclosed functions, and it is understood that there are a variety of structures which can perform the same function which are related to the disclosed structures, and that these structures will ultimately achieve the same result.

H. METHODS OF MAKING THE COMPOSITIONS AND DEVICES

[00229] The compositions disclosed herein and the compositions necessary to perform the disclosed methods can be made using any method known to those of skill in the art for that particular reagent or compound unless otherwise specifically noted.

[00230] Disclosed are processes for making the compositions and devices as well as making the intermediates leading to the compositions. There are a variety of methods that can be used for making these compositions, such as synthetic chemical methods and standard molecular biology methods. It is understood that the methods of making these and the other disclosed compositions are specifically disclosed.

I. METHODS OF USING THE COMPOSITIONS

[00231] The disclosed compositions can be used in a variety of ways as research tools. The compositions can be used, for example, in screening protocols to isolate molecules that possess desired functional properties. [00232] The disclosed compositions can be used as discussed herein as either reagents in micro arrays or as reagents to probe or analyze existing microarrays. The disclosed compositions can be used in any known method for isolating or identifying single nucleotide polymorphisms. The compositions can also be used in any known method of screening assays, related to chip/micro arrays. The compositions can also be used in any known way of using the computer readable embodiments of the disclosed compositions, for example, to study relatedness or to perform molecular modeling analysis related to the disclosed compositions.

J. EXAMPLES

[00233] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in 0C or is at ambient temperature, and pressure is at or near atmospheric.

1. PREPARATION OF SUPERHYDROPHOBIC SURFACES

[00234] Superhydrophobic surfaces were prepared using vapor-liquid-solid (VLS) growth systems to create high aspect ratio Si nanowires with various diameters, spacing, and lengths. In order to create the hydrophobic effect, a perfluoronated hydrocarbon coating was covalently applied to the entire nanowire surface. The resultant superhydrophobic nanowire surfaces do not follow a simple geometric pattern and exhibit fractal, multidimensional, random roughness, with contact angles near 180 degrees. Dailey, J. W., et al., Vapor-liquid- solid growth of germanium nanostructures on silicon. Journal of applied physics, 2004. 96(12): p. 7556-7567. The VLS growth technique employs small dots of gold that act as catalytic seeds for growing a high density of nanowires on a surface (Figure 16). During evaporation of a few monolayers of Au on a clean Si or glass surface, the Au self assembles into nanodots. In the subsequent VLS synthesis the Au dots form a eutectic liquid with Si from which liquid-mediated growth of single crystal Si nanowires occurs. The nanowire diameters are set by the Au dot diameters, with one-dimensional growth occurring as the AuSi eutectic dot rides along at the free end of the growing wire. The growth rate is linear in time and pressure, and the length of the nano wires is thus easily controlled by fixing the growth time. Typically, the Au dots at the end of the nanowires account for only a very small area. If desired, they can be chemically removed after growth to eliminate any effect they may have on interfacial properties.

2. PREPARATION OF MAGNETICALLY ACTIVE FLUID DROPLETS

[00235] A small amount of paramagnetic particles was added to water drops, which were placed and held over a superhydrophobic surface sample by a permanent magnet located below the surface (Figure 1). Milli-Q water was used to prepare aqueous solution with different particle concentrations ranging from 0.1% to 5%. The spherical paramagnetic carbonyl iron particles used were acquired from Lord Corporation. The particles are highly polydisperse in size (ranging from 0.2 to 4.0 μm) and they have a high saturation magnetization (211 emu/g). The magnetic field was generated by a NdFeB bar magnet located below the superhydrophobic surface. Drop movement was studied by recording images with two CCD cameras provided with zoom systems (Navitar 12x). One camera was located at one side of the drop and the other camera above the drop

3. EFFECT OF COMPOSITION AND VISCOSITY OF DROPLETS

[00236] The movement of blood droplets was observed and recorded using a digital camera on a superhydrophobic surface. These blood droplets, at first, appeared to roll off. Then, after observing several droplets, the droplets behaved differently — appearing to stick. Simply lifting one end of the material, however, allowed the droplets to slide off (See Figure 17).

[00237] It is possible that blood viscosity may have prevented the droplets from sliding off. Notably, the heparinized blood appeared to coagulate during the course of the blood droplet study. It will be interesting to investigate less viscous blood solutions in order to understand how viscosity affects droplet movement on these superhydrophobic surfaces. [00238] This blood droplet study was followed by observations of urine droplet movement. See Figure 18. These urine droplets behaved like water — easily sliding off the surface.

[00239] Saliva droplets were also observed on the superhydrophobic surface. See Figure 19. These droplets were very difficult to deliver using a pipette due to their viscosity and overall stickiness. Once deposited, the saliva droplets tended to stick, but could be moved by elevating one end of the surface. At an angle, the saliva droplets rolled off the surface.

4. COALESCENCE OF DROPLETS

[00240] Coalescence of two drops was achieved by placing a 6 microliter water drop without particles deliberately on a surface defect to hold it. Another 4 microliter water drop containing paramagnetic particles was displaced over the surface by the action of the magnet towards the pure water drop, until they were close enough to touch and coalesce. After coalescence, the combined drop was pulled out of the surface defect by the magnet. (See Figure 20)

5. SPLITTING OF DROPLETS

[00241] In a drop splitting experiment, a drop was loaded with a high concentration of paramagnetic particles and two magnets were placed below the surface of the drop. The drop spread under the influence of the separating magnets until it split (See Figure 21).

6. MOVEMENT OF LIQUID BY LIGHT-INDUCED CHANGES

[00242] A polished silicon wafer bearing random silicon nanowires with diameters of

20-50 nm was prepared by a vapor-liquid-solid technique. See, e.g., Wagner, R. S. in Whisker Technology (ed. Levit, A. P.) 47-119 (Wiley-Interscience, New York, 1970). (Figure 22) The air-oxidized silicon surface was treated with tert-butyldiphenylchlorosilane and perfluorooctyltrichlorosilane, followed by 3-aminopropyldiethoxymethylsilane, to which a photochromic spiropyran molecule was later attached by a published technique. See, Rosario, R. et ah, Photon-modulated wettability changes on spiropyran-coated surfaces, Langmuir 18, 8062-8069 (2002). [00243] After surface derivatization with spiropyran-containing monolayers on silicon

(Si) nanowire and adjacent smooth silicon surfaces, multiple measurements of advancing and receding water contact angles under UV and visible irradiation were performed using the sessile drop method. Direct comparisons of water contact angles on adjacent polished and nanowire areas are shown in Figure 23. The combination of the surface roughness and the hydrophobic coating resulted in significantly higher contact angles on the nanowire surface compared to the smooth surface.

[00244] The average advancing contact angle on the smooth surface was 12░ lower under UV irradiation than under visible irradiation (Figure 4). On the nanowire-bearing surface, this light-induced contact angle change increased to 23░ (Figure 4). The increase in the light-induced contact angle changes on the nanowire-bearing surface confirmed that roughness has the effect of amplifying stimulus-induced contact angle changes relative to smooth surfaces by nearly a factor of two.

[00245] Under visible irradiation of the spiropyran-coated surfaces, the water contact angle hysteresis was measured to be 37░ on the smooth surface, whereas on the nanowire- bearing surface a significantly lower value of only 17░ was observed.

[00246] In a control experiment on the smooth spiropyran-coated surface, the advancing water contact angle under UV irradiation (110░) was higher than the receding water contact angle under visible irradiation (85░). This does not fulfill the criterion for liquid motion, and it was found that water drops on the smooth surface could not be moved using light.

[00247] In contrast, on the spiropyran-coated nanowire-bearing surface, the advancing water contact angle under UV irradiation (133░) was lower than the receding water contact angle under visible irradiation (140░). Accordingly, when an ultraviolet light- visible light gradient was applied across water drops sitting on the nanowire-bearing surface, the drops moved towards the UV end of the gradient.

[00248] Control experiments performed on drops sitting on nanowire-bearing surfaces coated with the hydrophobic layer, but without the spiropyran, did not result in any drop motion. Therefore, it can be concluded that the motion of the water droplets on the photoresponsive, nanowire-bearing surface was due to the roughness-magnified light-induced switching of surface energy by the spiropyrans coupled with the lower contact angle hysteresis of the superhydrophobic surface.

[00249] Thus, it is demonstrated that surface roughness can be an effective tool for the amplification of stimulus-induced contact angle switching. The degree of amplification due to roughness was predicted using a Wenzel model. The combination of roughness- amplification of contact angle change with the reduced contact angle hysteresis of the nanowire-bearing, photoresponsive surfaces resulted in advancing contact angles under UV irradiation that were lower than the receding angles under visible irradiation. This for the first time permitted water drops on the nanowire surface to be moved solely using gradients of UV and visible light.

[00250] This result can lead to the development of photonic control of water movement in microfluidic devices. Additionally, since the fluid driving force in electrowetting, (see, e.g., Lahann, J. et al, A reversibly switching surface, Science 299, 371-374 (2003); Schneemilch, M., Welters, W. J., Hayes, R. A. & Ralston, J., Electrically induced changes in dynamic wettability. Langmuir 16, 2924-2927 (2000)) and thermowetting (see, e.g., Yakushiji, T. & Sakai, K., Graft architectural effects on thermoresponsive wettability changes of poly(N-isopropylacrylamide)-modifϊed surfaces, Langmuir 14, 4657-4662 (1998); Liang, L., Ski, M., Viswanathan, V. V., Peummg, L. M. & Young, J. S., Temperature-sensitive polypropylene membranes prepared by plasma polymerization, Journal of Membrane Science 177, 97-108 (2000)) microfluidic systems is also the stimulus-induced difference between advancing and receding contact angles, these findings can enhance fluidic motion and control in these systems.

7. HYDROPHOBIC/SPIROPYRAN COATING PROCEDURE.

[00251] Both nanowire and flat silicon oxide samples were cleaned using a 1 : 1 volume ratio of methanol/concentrated hydrochloric acid solution, followed by extensive washing in deionized water, yielding a nanowire surface contact angle of about 0░ and a flat surface contact angle of 19░. The samples were then treated with a toluene solution of tert- butyldiphenylchlorosilane and perfluorooctyltrichlorosilane in the ratio of 10:1, giving a nanowire surface contact angle of about 175░ (i.e., Cassie-Baxter) and a flat surface contact angle of 106 ▒ 2░. This was followed by treatment with a toluene solution of (3- aminopropyl)diethoxymethylsilane and curing at 140 0C, yielding a nanowire surface contact angle of about 175░ and a flat surface contact angle of 103 ▒ 3░. The silane-treated nanowire and flat silicon oxide samples were then incubated in an ethanolic solution of a photochromic spiropyran acid (1 mM) in the presence of the coupling agent l-ethyl-3-(3-

(dimethylamino)propyl)carbodiimide (10 mM), washed sequentially with ethanol and water, and dried under vacuum, producing a nanowire surface contact angle of about 174░ and a flat surface contact angle of 107 ▒ 8░ under visible irradiation.

a. GROWTH OF NANOWIRES

[00252] Silicon nanowires were prepared by a vapor-liquid-solid (VLS) growth technique, using small dots of gold that act as catalytic seeds for growing a high density of nanowires on silicon substrates (Figure 3). During evaporation of a few monolayers of Au on a clean Si or glass surface, the Au self assembles into nanodots. In the subsequent VLS synthesis the Au dots form a eutectic liquid with Si from which liquid-mediated growth of single crystal Si nanowires occurs. The nanowire diameters are set by the Au dot diameters, with one-dimensional growth occurring as the AuSi eutectic dot rides along at the free end of the growing wire. The growth rate is linear in time and the length of the nanowires is thus easily controlled by fixing the growth time. The Au dots at the end of the nanowires account for only a very small area. Typical VLS silicon nanowire growth conditions for these studies were 400 to 650 0C with disilane gas pressures of 3-500 mTorr, resulting in nanowire diameters of 20 - 100 nm and lengths of 1 - 3 μm.

b. UV-OZONE TREATMENT

[00253] In one approach to the study of the effect of surface chemistry changes on nanowire surfaces without altering the surface geometry, a UV-ozone cleaner (Jelight Company Inc., model 42) was used. This apparatus contains a UV source and a chamber with adjustable oxygen flow and pressure. Atomic oxygen is generated when molecular oxygen and ozone are dissociated by UV light. Any organic coating on the nanowires reacts with atomic oxygen, forming volatile molecules that desorb from the surface. The process is known not to damage delicate structures in semiconductor processing. A nanowire coating can thus be removed to different degrees, leading to a continuous variation in hydrophobicity, by varying the treatment time while conducting the cleaning at room temperature.

c. CONTACT ANGLE MEASUREMENTS

[00254] Advancing and receding contact angle measurements were performed using a Rame-Hart Model 250 standard automated goniometer. For measuring the advancing angle on flat surfaces, 5 microliters of deionized water was dropped onto the sample from a microsyringe bearing a needle with a hydrophobic tip. For superhydrophobic surfaces, a larger drop of about 15-20 microliters was used because smaller drops easily rolled off the surface. This led to a small degree of measurement error since the drop was not fully spherical. An image of the drop was taken shortly after the drop was deposited in order to avoid measurement error due to drying. For receding angles, the microsyringe needle was used to draw some of the water out of the drop. The software automatically generates tangent measurements on the drop profiles. Usually four measurements were taken on different parts of the sample surface in order to characterize the overall properties of the surface.

8. MANIPULATION OF DROPLETS ON SUPERHYDROPHOBIC SILICON SURFACES

[00255] Silicon superhydrophic surfaces (SHSs) were prepared using vapor-liquid- solid (VLS) growth systems to create high aspect ratio Si nanowires covalently coated with a perfluorinated hydrocarbon. The resulting superhydrophobic nanowire surfaces do not follow a simple geometric pattern but rather exhibit fractal, multidimensional, random roughness, with high contact angles. [J. W. Dailey, J. Taraci, T. Clement, D. J. Smith, J. Drucker, and S. T.

Picraux, J. Appl. Phys. 96, 7556 (2004)] The resulting SHSs contained nanowires with diameters ranging from 20 to 50 nm and with a height of approximately 2 μm. The separation distance between nanowires was between 60 and 100 nm. The contact angle θc ranged from 145░ to 160░ for particle-containing water drops on these Si nanowire surfaces [Figure 24(a)]. Drop volume was varied from 5 to 35 μl and prepared from aqueous suspensions with different particle concentrations (0.1%— 5% in weight). Spherical, paramagnetic, carbonyl iron particles were supplied by Lord Corporation. These particles are moderately polydisperse in size (diameter d ranging from 0.2 to 4.0 μm) and have a high saturation magnetization (211 emu/g). The magnetic field was generated by a cylindrical NdFeB bar magnet located below the SHS. The drop motion was recorded with a digital charge-coupled device (CCD) camera using a Navitar 12x zoom system positioned at the side or above the SHS.

[00256] The effect of the magnet motion on drops with different sizes and particle concentrations was studied. Drop movement was observed following the magnet motion for drops with particle concentrations down to 0.1 wt % and speeds up to 7 cm/s, both in straight and circular paths (see Electronic Physics Auxiliary Publication Service (EPAPS) document No. E-APPLAB-89-303629 for additional materials. Video 1 : controlled movement of a 6 μl drop on a linear path. Video 2: controlled movement of a 35 μl drop on a circular path (top view). Video 3: coalescence of two drops. A magnet is used to move a 4 μl drop containing paramagnetic particles towards a 6 μl drop lacking particles pinned to a surface defect. The resultant drop is then moved away from the surface defect by displacing the magnet. Video 4: splitting and coalescence of a drop (top view) using two magnets. This document can be reached via a direct link in the online article's HTML reference section or via the EPAPS homepage (http://www.aip.org/pubservs/epaps.html)]. videos 1 and 2). The main characteristics of drop dynamics can be summarized as follows: (a) Hydrophobic powders placed on top of 2 mm diameter drops were not observed to move during drop displacement. This observation indicates that the drop slides rather than rolls on the SHS. (b) Side views during drop motion show that particle clusters appear on the lateral drop surface [see Figure 24(b)]. The magnet drives the clusters which pull the drop surface due to capillary forces and strongly distort the drop shape. There is clear evidence of dynamically induced contact angle hysteresis, with the advancing contact angle θa being larger than the receding contact angle θr [Figure 24(b)]. (c) Top views during drop motion show that the particle clusters which are visible from above are regularly distributed in a pattern that is reminiscent of the Rosensweig instability [Figure 24(b)] [D. Salin, Europhys. Lett. 21, 667 (1993)]. Also, they appear to slide with the drop, (d) In terms of the magnet-to-drop distance, there is a magnetic field threshold, B^ below which no motion of the drop occurs. In one aspect, magnet velocity can affect this threshold, (e) Within the range measured here (Figure 25), the drop size does not affect the threshold field required to move drops on these SHSs, which indicates that frictional resistance is extremely low. These results are in accordance with molecular studies that predict that roughness from the nano-to the microscale at the solid-liquid interface can greatly enhance slippage, probably due to the existence of nanoscale bubbles at the liquid- so lid interface. [J.-L. Cottin-Bizonne, L. Bocquet, and E. Charlaix, Nat. Mater. 2, 237 (2003)] (f) 5th increases upon decreasing the particle concentration; if there are fewer magnetic particles in the drop, higher magnetic field intensities are needed to start the drop motion (Figure 25). This increase is expected because the total force exerted by the magnetic field is roughly proportional to the magnetic dipole moment of the clusters, which in turn is proportional to the number of magnetic particles present in the cluster.

[00257] Coalescence and/or drop splitting are essential processes in microfluidic applications. Coalescence of two drops was achieved on SHSs using an approach where a moving, particle-laden drop is moved into a second, standing drop lacking particles and deliberately held at a surface defect. Movement of the resultant drop away from the defect was then demonstrated using a magnet (EPAPS, video 3). A drop was split into two smaller drops by progressively separating two magnets held below the surface (EPAPS, video 4).

[00258] When there is no external magnetic field, the particles do not have a permanent magnetic dipole moment and simply sediment to the bottom of the drop. The permanent magnet generates a spatially nonuniform magnetic field and magnetizes the paramagnetic particles that, due to the induced magnetic dipolar interaction, aggregate into cylindrical clusters that follow the magnetic field lines. [S. Melle, M. A. Rubio, and G. G. Fuller, Phys. Rev. Lett. 87, 115501 (2001); E. M. Furst and A. P. Gast, Phys. Rev. E 61, 6732 (2000)] The particles are always kept inside the drop by capillary forces. [The surface tension force (γ=70 mPa s) on a particle (d ~ 2 m) keeping it inside the drop is πdγ ~ 100 nN. The magnetic force exerted by the magnet on the particle is ~ 1 nN for magnet-particle distances higher than about 3 mm (i.e., the depth of the glass slide)] When the magnet is displaced, the clusters move and drive the motion of the drop. At first, the clusters slide inside the drop following the motion of the magnet until they reach the contact line. When the first clusters reach the contact line, the competition between the capillary (F) and magnetic forces (F") makes the clusters start to climb along the drop surface. Now F" acts along the cluster axis (forming an angle a with the vertical direction) while F=(πγD)/[cos(a-aa)] acts along the local normal to the drop surface (forming an angle αa with the vertical direction). Here, D is the diameter of the cluster. The horizontal component of F" is responsible for drop motion. The vertical component of F" will twist the drop surface at the contact line and distort the drop surface, increasing the advancing contact angle (decreasing αa= π-θa). This creates a difference between the contact angles at the advancing and receding segments of the contact line, leading to a retention force Fr=2γrJ that opposes drop motion. [C. G. L. Furmidge, J. Colloid Sci. 17, 309 (1962)] Here r=R cos((9c-π/2) is the radius of the circle of contact between the drop and the SHS, and J=cos (9a-cos θr. Thus the drop will move when the horizontal component of the magnetic force compensates the capillary and retention forces. In Figure 24(b), the 5 contact angle difference and the inclination of the clusters can be appreciated. Calculations based on the actual measured values of the angles (θc ~ 147░, θa ~ 160░, θr ~ 136░, and a ~ 44░) show that the magnetic force modulus needed to balance the capillary and retention forces is within an order of magnitude of the range achievable in the present experimental setup.

i o 9. PREPARATION AND USE OF POLYETHYLENE SUPERHYDROPHOBIC SURFACES

[00259] Superhydrophobic low density polyethylene (LDPE) surfaces were prepared by adapting the method of Lu et al. [Lu, X., C. Zhang, and Y. Han, Low-Density Polyethylene Superhydrophobic Surface by Control of Its Crystallization Behavior. Macromolecular Rapid Communications, 2004. 25(18): p. 1606 - 1610.]. LDPE was chosen as the substrate for its

15 inherent hydrophobicity, low cost, and flexibility. A 1.59 mm thick commercial-grade LDPE sheet was purchased from McMaster-Carr (Los Angeles, CA). LDPE crystals were grown from LDPE pellets (Sigma- Aldrich, St. Louis, MO) with a melt index of 2.50 g/min at 190 0C. The pellets were dissolved in xylene (isomers plus ethylbenzene, reagent grade, Sigma- Aldrich) and methyl ethyl ketone (MEK) (reagent grade, J. T. Baker, Phillipsburg, NJ). The

20 LDPE sheet was cut into 61 x 99 mm rectangles to fit into an aluminum solvent-casting fixture. The rectangular samples were lightly abraded and cleaned with acetone before being clamped on the fixture. The LDPE pellets, xylene, and MEK were used without additional preparation. Xylene solvent and LDPE pellets (at a concentration of 30 mg/mL) were placed in a flask and immersed in a water bath at 92 0C. After the LDPE had fully dissolved

25 (approximately 35 minutes), MEK (a non-solvent for LDPE) was added to the flask at a ratio of 55:45 xylene:MEK. The addition of a non-solvent to the solvent-plastic solution was shown by Erbil et al [Erbil, H. Y., et al., Transformation of a Simple Plastic into a Superhydrophobic Surface, in Science. 2003. p. 1377-1380.] to increase surface roughness and aqueous drop contact angle for solvent-cast polypropylene. A similar result is achieved

30 for solvent-cast LDPE using MEK as the non-solvent. The addition of the MEK caused the solution to immediately turn cloudy due to crystallization, but after several minutes of gentle swirling the solution cleared. At this point the water bath temperature was decreased to 80 0C, and the solution was held at that temperature for an additional 90 minutes to promote controlled crystallization. Next, 5 mL of solution was carefully withdrawn and pipetted into the fixture (resulting in a surface concentration on the LDPE sheet of 0.090 mL of solution/cm2, or 2.7 mg of crystals/cm2). The fixture was gently rocked to level the solution, and then placed in an enclosed box and kept in a fume hood to dry overnight.

[00260] Aqueous drops containing paramagnetic iron particles (6-9 μm, Fluka) were pipetted onto the Si NW and LDPE superhydrophobic surfaces. The iron particles were coated with polysiloxane to prevent oxidation [Pu, H., F. Jiang, and Z. Yang, Studies on preparation and chemical stability of reduced iron particles encapsulated with polysiloxane nano-films. Materials letters, 2006. 60(1): p. 94-97.]. Drop movement was accomplished by displacing a cylindrical NdFeB bar magnet, which the drop being positioned directly below the superhydrophobic surface (for horizontal movement) (Figure 29a) or on the opposite face of the surface (for vertical and upside down movement) (Figure 29b-c).

[00261] Contact angle measurements of water drops on both silicon nanowire and low density polyethylene super hydrophobic surfaces were made by the sessile drop method, adjusting the drop volume from the top of the drop, with a Rame-Hart (Mountain Lakes, NJ) NRL Contact Angle Goniometer, model 100-00.

[00262] Silicon nanowire (Si NW) superhydrophobic surfaces were prepared using vapor-liquid-solid (VLS) growth systems, as disclosed herein, to create high aspect ratio nanowires of various diameters, spacing, and lengths. The nanowire substrates were rendered hydrophobic by covalently applying a perfluorinated hydrocarbon coating to the entire surface. This combination of topography and hydrophobic coating resulted in surfaces where drop contact angles measurements gave advancing contact angles close to 180░ (Figure 27a), with no detectable difference between advancing and receding contact angles. The Si NW superhydrophobic surfaces are macroscopically smooth, however, they exhibit multidimensional, random roughness at a small scale (Figure 27b). The smallest feature length scale is about 20 nm which corresponds to the nanowire diameters. The next largest length scale is the separation distance between nanowires, which ranged from 60 to 100 nm. The largest roughness length scale is represented by the nanowire height, of approximately 2μm. These different feature/roughness lengthy scales give rise to the mechanism for water repellency in Lotus leaf- like structures which is the presence of air pockets underneath the liquid [Otten, A. and S. Herminghaus, How Plants Keep Dry: A Physicist's Point of View. Langmuir, 2004. 20(6): p. 2405 -2408.; Quere, D., Fakir droplets. Nature Materials, 2002. 1(1): p. 14-15.].

[00263] Si NW superhydrophobic surfaces have shown to be excellent substrates for movement of drops even for solutions with protein concentrations similar to those found in blood. It has also been observed that drops of whole blood, serum, plasma, urine, and saliva are repelled by Si NW superhydrophobic surfaces. While such surfaces show excellent promise as a platform for biological fluid micro fluidics, broadening the types of materials that could support digital magneto fluidics is also desirable. LDPE is intrinsically hydrophobic, and a more commercially relevant material. Polymer surfaces are in general attractive because of their flexibility and the ease in which complex structures can be formed. Through solvent crystallization, LDPE superhydrophobic surfaces can be made very quickly and inexpensively in large slabs.

[00264] Contact angles on LDPE superhydrophobic surfaces are approximately 148░ when measured on a static drop, 152░ for an advancing drop and 138░ for a receding drop. LDPE solvent crystallization produces floral-like structures with multidimensional roughness. Macroscopically, LDPE superhydrophobic surfaces appear very rough. The largest feature length scale (approximately lOOμm) is due to cracks that appear between the crystallized areas (Figure 28 b). At the complex floral structure level, features of lμm and lOμm are also found, and significant roughness at the nanoscale-level should also be present.

[00265] The mechanism for water repellency of superhydrophobic surfaces is given by

Wenzel's law [Wenzel, R.N., Surface roughness and contact angle (letter)[J]. J. Phys. Colloid Chem, 1949. 53(1): p. 466-1.], cosΘ*= r cos Θ; where Θ* is the resulting contact angle for a surface with a given roughness parameter r, and the contact angle Θ is the value measured for a smooth surface. The value of r is the ratio of the real surface area divided by the projected flat surface area [Otten, A. and S. Herminghaus, How Plants Keep Dry: A Physicist's Point of View. Langmuir, 2004. 20(6): p. 2405 -2408.]. From values of water on pristine low density polyethylene films of 93░ [Svorcik, V., et al., Modification of surface properties of high and low density polyethylene by Ar plasma discharge. Polymer degradation and stability, 2006. 91(6): p. 1219-1225.] and an estimated Wenzel contact angle of 147 ░, r can be estimated as approximately 16 for the LDPE surfaces.

[00266] Drops of sizes up to 4μl, with iron-microparticle concentration of 5% can be moved through digital micro fluidics in 3 dimensions (Figure 29). Up to now, digital microfluidics systems have been confined to motion in a 2-D channel or a 2-D surface. Yet, 3-D movement is important because more compact devices can be fabricated and more sophisticated subsystems for detection and processing would be possible. Three dimensional motion may also allow for more flexibility when employing multiple sensor elements since they do not have to be integrated with the substrate as would be the case for a planar configuration. Several qualitative observations can be made. First, Si NW superhydrophobic surfaces provide very little resistance to movement; and thus drops can be moved on these surfaces at speeds of up to 7cm/sec [Egatz-Gomez, A., et al., Discrete Magnetic Microfluidics. Applied Physics Letters, 2006. 89(3): p. 034106.]. Secondly, LDPE surfaces provide higher resistance to movement and drops can be moved only when higher concentrations of magnetic particles are added (5-10%) or smaller drops are used. Thirdly, LDPE superhydrophobic surfaces are more robust than Si NW surfaces and continue to give consistent performance over repetitive drop movement cycles. Finally, superparamagnetic particles do not seem to get caught in the LDPE surface features, whereas there is visual and contact angle evidence indicating that they become embedded in Si NW surfaces.

[00267] It is also believed that the LDPE surfaces can be yet better designed to reach the same performance characteristics seen in the Si NW surfaces. Erbil et al. note [Erbil, H. Y., et al., Transformation of a Simple Plastic into a Superhydrophobic Surface, in Science. 2003. p. 1377-1380.] that polypropylene superhydrophobic surfaces can yield higher contact angles. Also, better control of LDPE crystal growth [Lu, X., C. Zhang, and Y. Han, Low- Density Polyethylene Superhydrophobic Surface by Control of Its Crystallization Behavior. Macromolecular Rapid Communications, 2004. 25(18): p. 1606 - 1610.] should also increase contact angles. It is also understood that a higher contact angle can result in easier drop movement with plastic surfaces requiring lower concentrations of magnetic particles. 10. ELECTROCHEMICAL MEASUREMENTS IN DISCRETE DROPLETS

[00268] Since electrochemistry holds great promise for assays of ultrasmall environments [Wang, J., Analytical Electrochemistry (3rd Edition), Wiley, New York, 2006.], it is important to add it to the emerging arsenal of detection strategies in digital magnetic microfluidics. Moreover, an added benefit lies in capitalizing on the wide range of specialized microcells that have been designed for reliable electroanalysis of microliter samples [Wang, J., Analytical Electrochemistry (3rd Edition), Wiley, New York, 2006.]. While electrochemical detection has been widely used for continuous-flow microchannel systems [Wang, J., Analytical Electrochemistry (3rd Edition), Wiley, New York, 2006; J. Wang, Electroanalysis 2005,17, 1133.], there are no previous reports known of electrochemical measurements in droplet-based digital micro fluidic systems.

[00269] In developing such open droplet-based electrochemical microfluidic systems, attention was given to the ability to record both the sample and background voltammograms. The electrode rinse step (between successive sample drops) was also automated in a manner analogous to electrochemical measurements in continuously flowing streams. As illustrated in Figure 31, three different aqueous microdroplets - serving as the "Sample," "Blank," and "Wash" solutions (containing 2% w/v polysiloxane-coated iron microparticles) - were moved magnetically in air, towards and away from a three-electrode assembly, by moving different external magnets below the superhydrophobic surface. The inset of Figure 31 shows a typical 30 μl "Sample" drop (along with the paramagnetic particles) contacting the electrode assembly. The replacement of the surrounding oil phase - common in earlier digital microfluidic schemes [Y. Fouillet and J. L. Achard, Digital Comptes Rendus Physique 2004, 5, 577.; 3] - with air, facilitates such repetitive voltammetric measurements, as it circumvents the formation of a surface-fouling oil layer on the sensing electrode. Rapid square -wave voltammetry was used here for direct magnetofluidic measurements in successive droplets (in view of its high speed and sensitivity), while chronoamperometry was employed for monitoring biocatalytic reactions in connection with the merging of corresponding substrate and enzyme droplet solutions.

[00270] Superhydrophobic surfaces were prepared using a vapor-liquid-solid (VLS) growth process to create high-aspect-ratio Si nanowires [J.W. Dailey, J. Taraci, T. Clement, DJ. Smith, J. Drucker, S.T. Picraux, J. Applied Physics 2004 96, 7556.]. Briefly, liquid Au droplets were deposited on Si surfaces and used to catalyze the heteroepitaxial growth of Si nanowires in an ultra-high vacuum, chemical vapor deposition chamber. It is also understood that superhydrophobic polyethylene surfaces, as disclosed herein, can also be employed. In these experiments, the Si nanowire surfaces exhibit multidimensional, random roughness with diameters ranging from 20 to 50 nm and with a height of ~2 μm. The separation distance between nanowires was between 60 and 100 nm. The nanowire substrates were rendered hydrophobic by covalently applying a perfluorinated hydrocarbon coating to the entire surface. This combination of nanoscale topography and hydrophobic coating resulted in surfaces where drop contact angles approached 180░.

[00271] Aqueous drops containing paramagnetic iron particles (6-9 μm, Fluka) were pipetted onto the superhydrophobic surface. The iron particles were coated with polysiloxane (to prevent their oxidation), following the procedure described by Pu et al. [H. Pu, F. Jiang, Z. Yang, Materials Letters 2006, 60, 94.]. Drops containing these particles could then be directed to move on the nanowire surface using a magnetic field generated by a cylindrical NdFeB bar magnet, which was positioned directly below the superhydrophobic surface. Drops were moved both into and out of the electrode assembly using the magnetic field; where up to three drops were displaced simultaneously on a 2.5 cm x 2.5 cm substrate in response to the movement of three corresponding bar magnets positioned below the surface.

[00272] Square -wave voltammetry and chronoamperometry were carried out using a μAutolab III modular electrochemical system (Eco Chemie, Ultrecht, Netherlands) that was driven by GPES software (Eco Chemie) and connected to a personal computer. The three- electrode assembly consisted of a platinum- wire (0.25 mm diameter) or a carbon-paste working electrode, an Ag/AgCl-wire (0.25 mm diameter) reference electrode, and a platinum- wire counter electrode. The electrode assembly was immersed into the 30 μl drops using a holder clamp.

[00273] The Pt and Ag/ AgCl wires were each soldered to a copper wire. Each pair of wires was covered with heat-shrink tubing insulator (Radio Shack Inc.); additional heat- shrink tubing was then used to cover both wires and to join them together. Nail polish was used to seal the wires at the end of the tubing, exposing 3 mm of bare wires to the solution droplets. [00274] The carbon paste electrode (used for glucose measurements) consisted of graphite powder (GP; grade no. 38, Fisher Scientific, Tustin, CA), 5 wt % platinum-on-active carbon (Sigma-Aldrich Inc., St. Louis, MO), and mineral oil (Sigma-Aldrich Inc.). The GP/Pt-on-carbon/mineral oil biocomposites were prepared by hand mixing to yield a final composition of 57% GP, 27% Pt-on-carbon, 15% mineral oil. Mixing proceeded for 30 minutes. Then, a portion of the resulting carbon paste was packed tightly into the end of a 7- cm long Teflon tube (i.d. 0.559-mm, o.d. 1.068-mm; Cole-Parmer Instrument Co., Vernon Hills, IL). The paste filled the tip to a height of 1 mm and electrical contact was made with a 0.48 mm diameter copper wire. The paste surface was smoothed on weighing paper (VWR Scientific Products, West Chester, PA).

[00275] All solutions were prepared using deionized water (PURELAB system, R >

18.2 MΩ.cm). Dopamine, β-D(+)glucose, glucose oxidase (GOx; 146000 U/g, Type X-S: From Aspergillus niger), and sodium hydroxide were purchased from Sigma-Aldrich Inc. Carbonyl-iron microparticles (>99.9% iron, 6-9 μm diameter) were purchased from Fluka (Switzerland). Iron-polysiloxane composites were prepared by hydrolysis-condensation polymerization of tetraethylorthosilicate (Sigma-Aldrich Inc.). Iron particles (20 g) were added to a mixture of tetraorthosilicate (40 ml) and ethyl alcohol (160 ml) and stirred. Next, 10 ml of ammonium hydroxide (25 wt %; Sigma-Aldrich Inc.) was slowly added to the mixture, which was then stirred for 24 h at room temperature. Coated particles were washed three times with ethyl alcohol, four times with deionized water, and dried at 60 0C in a vacuum oven for 24 h. The particle coating was estimated to be 60 nm in thickness using scanning electron microscopy. A pH 7.0 phosphate buffer solution (PBS) was prepared using sodium phosphate (dibasic and monobasic), purchased from EMD (Gibbstown, NJ). NaOH was used to adjust the buffer pH. Stock solutions of dopamine (100 mg/ml), glucose oxidase (4.84 U), and glucose (IM) in buffer were prepared daily.

[00276] Experiments were conducted using three different droplets (containing 2% w/v paramagnetic particles), acting as the "Blank," "Sample," and "Wash" solutions (Figure 31). The movement of all three drops was simultaneously controlled using the magnetic fields generated by three separate permanent magnets positioned below the superhydrophobic surface. In a typical experiment, the electrode assembly was first exposed to the 30 μl

"Blank" drop, followed by a "Sample" drop, and then a "Wash" drop. The "Blank" drop was scanned immediately after it was moved into the electrode, which occurred over an 11.5 second period. This "Blank" drop was then moved out of the electrode as the "Sample" drop was moved towards the electrode. The switch from "Blank" to "Sample" droplet took ~20 seconds. The "Sample" drop was then immediately scanned over an 11.5 second period, followed by its removal and movement of the "Wash" drop towards the electrode assembly. After 20 seconds, the electrode was inserted into the "Wash" drop for one minute to remove residual dopamine. Finally, the "Wash" drop was moved out of the electrode assembly as the "Blank" drop was moved towards the electrodes for the next electrochemical measurement. This sequence of droplet movement, both into and out of the electrode assembly, was repeated multiple times using "Blank," "Sample," and "Wash" drops.

[00277] Electrochemical measurements of dopamine were carried out using 30 μl phosphate -buffer (0.1M, pH 7.0) droplets and rapid square -wave voltammetry (SWV), with an initial potential of -0.45 V, a final potential of 0.7 V, an amplitude of 25 mV, a frequency of 25 Hz, and a step potential of 4mV. In some cases, a subtractive SWV operation was performed by subtracting the "Background" drop signal. A baseline smoothing operation was applied to the sample response.

[00278] Chronoamperometric detection of glucose was performed at room temperature under quiescent conditions. Experiments were conducted using a 15μl "Reagent" drop containing glucose oxidase (4.84 U) and 15μl "Sample" drops containing varying concentrations of glucose (0, 2 , 6, 10 mM). All drops contained 2% w/v of paramagnetic particles and were directed to move using a NeFeB magnet positioned directly below the surface. The glucose drop was first moved into the electrode assembly, followed by the glucose oxidase drop, which resulted in a combined 30μl droplet. The electrochemical measurement of the hydrogen-peroxide product was then performed, by stepping the potential from 0.0 to +0.65 V and sampling the current over a 4 min period following the potential step (following an initial 3 min period at open circuit). Subsequently, the combined glucose/glucose oxidase drop was moved away from the electrode assembly.

[00279] The utility of this new droplet-based electrochemical magnetomicrofluidic system is illustrated below for voltammetric measurements of dopamine and for chronoamperometric bioassays of glucose. The detection of dopamine can be significant for managing various neurological disorders, since this important neurotransmitter provides a communication link between neurons. The optimization and control of relevant experimental variables allows reliable electrochemical measurements of the model dopamine analyte within multiple microdrops in rapid succession and with minimal cross talk. Such minimization of carry-over effects between successive drops is accomplished by intermittent exposure of the electrode assembly to the "Wash" droplet, which serves as a rinse step (e.g. Figure 31). The minimal carry-over effects between successive 'Sample' drops is illustrated in Figure 30 where square-wave voltammograms obtained upon exposing the electrode assembly alternately to the 'Sample' (b) and 'Blank' (a) droplets (along with the intermittent 'Wash' step) are displayed. Highly reproducible dopamine sample peaks, with no observable carry over between the 'Sample' and 'Blank' droplets are observed. A relative standard deviation of 9.4% was calculated for 8 such measurement cycles, reflecting the high reproducibility of the magnetic movement and the efficiency of the rinse/cleaning step.

[00280] Several studies were performed for optimizing relevant experimental variables of the new electrochemical magneto fluidic operation. The dopamine peak current was nearly independent of the sample volume (over the 10-30 μl range) and of the amount of paramagnetic particles within the "Sample" drop (over the 2-4% w/v range; not shown). All subsequent analytical work was conducted using 30 μl droplets containing 2% w/v iron particles.

[00281] Quantitative evaluation of electrochemical measurements with magnetically- controlled droplet movement is based on the correlation between the voltammetric peak current and the analyte concentration. Figure 32 displays square -wave voltammograms for sequential exposures to 30 μl droplets of ascending dopamine concentrations (5-25 mg/1). Well defined current signals, proportional to the analyte concentration, are observed (a-e). The resulting calibration plot of peak current versus concentration (inset, upper right) is highly linear, with a slope of 0.05471 μA/(μg/ml) (correlation coefficient, 0.997). Such linearity indicates again that carry-over effects between successive droplets are negligible.

[00282] Thus far we examined the ability to perform rapid voltammetric measurements in magnetically moving open droplets. An attractive feature of the digital magnetomicrofluidic system is its ability to manipulate (merge or split) microliter droplets, including splitting of sample droplets or merging of sample and reagent ones. An enzymatic assay of glucose was used for demonstrating this capability within the droplet-based electrochemical microfluidic system. The growing global demands for managing diabetes make glucose the most commonly-tested analyte in clinical diagnostics. The new droplet- based glucose bioassay consisted of coalescing 15 μl enzyme (GOx) and substrate droplets, followed by chronoamperometric measurements of the hydrogen-peroxide product in the merged droplet. The photographs in Figure 33 illustrate the sequence of events in this digital microfluidic enzymatic assay. These events include movement of the glucose sample droplet towards the electrode assembly (A), movement of the enzyme droplet (B) and its merger with the sample drop (C), chronoamperometric measurements of the peroxide product (D) and removal of the droplet from the electrode assembly (E, F).

[00283] Figure 34 displays chronoamperograms obtained for such microscale bioassays of "Sample" droplets containing increasing glucose concentrations (2(b), 6(c), and 10(d) mM), along with the response for a "blank" droplet (a). Well-defined chronoamperometric signals are observed for the oxidation of the peroxide product upon stepping the potential to +0.65 V. As indicated from the corresponding calibration plot, the anodic current increases nearly linearly with the substrate concentration. These data clearly indicate the ability of an electrochemical microfluidic system to manipulate microliter sample and reagent solutions in order to measure biological or chemical species and indicates great promise for a wide range of diagnostic applications.

[00284] Accordingly, in one aspect, the invention relates to a method of digital microelectrochemical detection comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface; contacting an electrode with the droplet; and measuring an electrochemical property. In a further aspect, the method can further comprise the steps of coupling a magnetic field with at least a portion of the droplet; and varying the magnetic field intensity across the surface. The droplet can further comprise an electrochemically active species, for example, glucose or a derivative thereof or dopamine or a derivative thereof. In a yet further aspect, the contacting step further comprises contacting a reference electrode. In a still further aspect, the electrochemical property can be the current or the potential of the droplet. In one aspect, the method is chronoamperometric detection.

[00285] In a further aspect, the invention relates to a method of digital microelectrochemical reaction comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface, the droplet comprising at least one electrochemically active species; contacting an electrode with the droplet; and applying electrochemical energy, thereby oxidizing or reducing the at least one electrochemically active species. In a further aspect, the method further comprises the steps of coupling a magnetic field with at least a portion of the droplet; and varying the magnetic field intensity across the surface. In a yet further aspect, the electrochemical energy comprises a voltage potential or a current.

[00286] In one aspect, the invention also relates to a digital isoelectric focusing method comprising the steps of positioning a magnetically active fluid droplet in contact with a hydrophobic surface, the droplet comprising: ampholytes, a first protein having a first isoelectric point, and a second protein having a second isoelectric point different from the first isoelectric point; contacting an electrode with the droplet, thereby generating a pH gradient within the droplet; allowing the first protein to migrate along the pH gradient to the first isoelectric point; allowing the second protein to migrate along the pH gradient to the second isoelectric point; coupling a first magnetic field with at least a first portion of the droplet, wherein the first portion comprises the first isoelectric point; coupling a second magnetic field with at least a second portion of the droplet wherein the second portion comprises the second isoelectric point; and varying the first magnetic field intensity so as to move the first portion substantially away from the second magnetic field with a force sufficient to overcome surface tension of the magnetically active fluid droplet, thereby dividing the first portion of the droplet from the second portion of the droplet.

11. POLYSILOXANE-C OATED IRON PARTICLES

[00287] Carbonyl iron particles (Sigma-Aldrich Inc., St Louis, MO) with sizes ranging from 6 to 9μm were used. These microparticles exhibit high magnetic saturation and are commonly used in experimental studies and technological applications on magneto- rheological fluids [U.S. pat: R.I. Vardarajan (1947) 2,417,590 and J.M. Ginder, L.D. Elie, L.C. Davis, 5,549,837]. In simple terms, when an external magnetic field H is applied, a net magnetic dipole moment aligned with the external field is induced in the particles. This magnetic moment is m=(4π/3)α5M, with M=χH, where M is the magnetization of the particle, a is its radius, and χ is the magnetic susceptibility. Therefore, the induced magnetic dipoles of the microparticles align with the external magnetic field lines, causing the formation of clusters. These particles are usually regarded as paramagnetic or superparamagnetic, because their magnetization curve (M-H curve) has a small or null hysteresis, little or no magnetic remanence, and their magnetic response is linear for applied magnetic fields of small intensity [S. Melle, M. Lask, and G. G. Fuller, Langmuir 21 (2005) 2158-2162].

[00288] The carbonyl iron microparticles were coated with polysiloxane following the procedure described by Pu et al. [H. Pu, F. Jiang, Z. Yang, Mat. Lett. 60 (2005) 94.] to prevent oxidation. Iron-polysiloxane composites were prepared by hydrolysis-condensation polymerization of tetraethylorthosilicate (Sigma-Aldrich Inc., St. Louis, MO). Iron particles (20 g) were added to a mixture of tetraorthosilicate (40 ml) and ethyl alcohol (160 ml) and stirred. Next, 10 ml of ammonium hydroxide (25 wt %; Sigma-Aldrich Inc., St. Louis, MO) was slowly added to the mixture, which was then stirred for 24 h at room temperature. Coated particles were washed three times with ethyl alcohol, four times with deionised water, and dried at 60 0C in a vacuum oven for 24 h. Figure 35 (a) shows a SEM image of polysiloxane-coated carbonyl iron microparticles Based on this and other images, the particle coating was estimated to be 60 nm in thickness. Figure 35(b) shows the field dependent magnetization of the samples, characterized at 300 K using a Quantum Design vibrating sample magnetometer (VSM). This magnetometer was equipped for the Physical Property Measurement System (PPMS) with an applied magnetic field of 1OkOe in order to reach saturation values. Both uncoated and the coated microparticles exhibited negligible hysteresis as described by others using iron carbonyl particles [H. Pu, F. Jiang, Z. Yang, Mat. Lett. 60 (2005) 94.]. Notably, the polysiloxane coating only slightly affected the magnetic properties of the particles, reducing the magnetic saturation value of the carbonyl iron microparticles from approximately 225emu/g to 191emu/g and increasing the coercive force from approximately 1.3Oe for the uncoated microparticles to 6.5Oe for the polysiloxane-coated microparticles. The change in microparticle magnetic properties, also, did not visibly affect the magnetically-controlled drop movement observed in this study.

[00289] Drops (20 μl) from several biological fluids, containing 2% polysiloxane- coated iron particles were pipetted onto the superhydrophobic silicon nanowire surface and observed. These fluids included deionised water supplemented with 8% bovine serum albumin (BSA; Rockland Immunochemicals Inc., Gilbertsville, PA), fetal bovine serum (VWR, West Chester, PA), and whole bovine blood supplemented with the anti-coagulant, K3EDTA (Innovative Research Inc., Southield, MI). A NdFeB cylindrical (6.44 mm diameter and 12.7 mm length) rare earth magnet (Magcraft, NSN0718/N40) with residual flux density (Br) of 12800 Gauss, coercive force (Hcb) of 11900 Oe, and maximum energy product (BHmax) of 40 MGOe, was positioned directly below the superhydrophobic surface and moved by hand. The directed movement of these drops was recorded using an Optura 20 Canon digital camcorder (Canon, Lake Success, NY). Snapshots from the videos were analyzed to obtain an approximate value of the maximum drop velocity. Static and dynamic contact angles were determined using the contact angle tool in Image J (National Institutes of Health, http://rsb.info.nih.gov/ij).

[00290] Drops of deionized water containing a suspension of coated carbonyl iron microparticles were added to the superhydrophobic surface and directed to move by a NeFeB magnet. In this study, drop movement was studied as a function of size and particle concentration. Drops followed the magnet motion with particle concentrations as small as 0.1% in weight, and along a 2 cm linear path at speeds up to 7 cm/sec, and in a circular path. Figure 36 shows still images from a typical video, where a paper grid with boxes of a tenth of an inch in width is given as a reference. Water drops moved effortlessly and responded quickly to magnet displacement. After observing this phenomenon, one could argue that the drop is sliding across the surface. However it could also be argued that the particles are moving over the surface of the water drop in a "tank-treading" like motion, or in a better analogy, as a pet hamster would move when placed inside of a plastic play ball. When a small piece of Styrofoam is placed on top of a water drop, the Styrofoam remains virtually motionless as the drop moves across the superhydrophobic nanowire surface. Thus we conclude that the water drop slides along the surface due to the combined effects of a superhydrophobic surface and the very low amount of contact area between the rounded water drop and the nanowire surface. A first order analysis of this type of drop motion is derived below.

[00291] When the magnet is displaced, the clusters follow the magnet motion, sliding inside the drop, until they arrive at the contact line. When the more advanced cluster arrives at the contact line, the competition between capillary force and magnetic force causes the cluster to start climbing along the drop surface as depicted in Figure 37. The magnetic force acts along the cluster axis, while the capillary force acts along the normal to the drop surface. The vertical component of the magnetic force will then deform the drop surface at the contact line towards the superhydrophobic surface. Hence, aa

decreases and the advancing contact angle, θa, increases. Consequently, the magnetic force builds a difference between the contact angles at the advancing and the receding parts of the contact line. This contact angle difference opposes the drop motion with a force that can be expressed as 2γrJ [C. G. L. Furmidge, J. Coll. Sci. 17 (1962) 309.], where /is the surface tension, r=Rcos(θc-π/2) is the radius of the circle of contact between the drop and the superhydrophobic surface with R being the radius of the drop, and J=cos#Ω-cos#r where θr is the receding contact angle. At equilibrium, the total horizontal force balance on the drop can be written as follows:

Fm sinα (cos#a -cos0r ) = 0

j '

where F m is the modulus of the magnetic force on the cluster, a is the angle that the magnetic force forms with the vertical axis, and D is the diameter of the cluster. For completeness, the vertical force balance reads:

Fm cosα cosα = 0 cos(α -αβ)

[00292] If there is no difference between the advancing and receding contact angles, the force balance equations simply state that the system would be in equilibrium provided a= aa. Instead, when a contact angle difference is produced, the capillary force that opposes the contact line motion introduces a threshold that can only be overcome when the inclination of the clusters, a, is larger than the advancing contact angle, aa. This is precisely what would happen in the situation depicted in Figure 37, where the vertical components of magnetic and capillary force compensate, while the magnetic force horizontal component is visibly higher than the corresponding capillary component. When this excess magnetic force horizontal component is higher than the contact angle difference term, drop motion will occur. From this point of view, large values of a would be more convenient in order to overcome the opposing force due to the contact angle difference. [00293] It is possible that the first cluster arriving at the contact line would not be able to overcome the capillary retention force. Then the magnet displacement with respect to the drop will keep increasing, making the first cluster climb along the drop surface and driving more clusters to the contact line. Due to the magnetic field spatial structure, the clusters arriving at the contact line would have a larger inclination, which will then create a comparatively stronger contribution to the horizontal component of the magnetic force than the vertical component. Consequently, the advancing contact angle would be slightly perturbed, while the horizontal force will increase significantly and drop motion will occur. This is precisely what happens in experiments, as shown in Figure 36, where the contact angle difference and the inclination of the clusters can be appreciated.

[00294] A full numerical check of the above expressions is not possible, however, because precise values concerning the size and the number of the clusters would be needed. This possibility is precluded by the lens effect of the drop surface. Nevertheless, calculations based on the actual measured values of the angular variables of the problem (θc -147░, θa~l60░, θr -136░, and α~44░) show that the order of magnitude of the magnetic force modulus needed to balance the capillary and retention forces is within the range achievable in our experimental setup.

[00295] Another interesting aspect is the role of the magnet velocity in the initiation of drop motion. Actually, the magnet displacement entrains the cluster structure in its motion. Before the threshold condition for drop motion is achieved, the drop is standing and the clusters move through it with possibly large velocities. In order to make some estimations, the clusters appearing in the experiments can be approximated as circular cylinders of, say, diameter D = 100 μm. These clusters may move within the standing drop at speeds V up to 10 cm/s. These values (D = 100 μm, V = 0.1 m/s, and using the density and the viscosity of water) yield a Reynolds number: Re=pVD/μ~ 10. This means that inertial effects of the fluid may be important at these high magnet speeds.

[00296] Fluid drops surrounded by another fluid can show multiple types of motion depending on drop shape, drop size, and viscosity contrast. For instance, slipping, sliding, rolling, and tank-treading motions have been described [S.R. Hodges, O.E. Jensen, and J.M. Rallison, J. Fluid Mech., 512 (2004) 95.] in the case of drops slowly moving under gravity body force on inclined plates. In such a case, the shape of the drop is controlled by the Bond number, Bo=ΔpgR2/γ, where Ap is the density difference between both fluids, g is the acceleration due to gravity, and R is the radius of the drop. Low Bo values correspond to drops with virtually undeformed spherical shape, medium Bo values correspond to drops considerably flattened in contact with the plate, and high Bo values correspond to pancake shaped drops. It is known that flattened and pancake drops with higher viscosities than the surrounding fluid should exhibit a sliding motion. In the experiments reported here, we are dealing with flattened water drops in air (e.g. a high viscosity contrast condition). [S. R. Hodges, O.E. Jensen, and J.M. Rallison, J. Fluid Mech., 512 (2004) 95.] Therefore, sliding motion is expected. The above mentioned analysis pertains to Stokes flow induced by a gravity body force and with buoyancy as the drop deforming force. However, motion is caused by forces applied to the drop surface; the deformation of the drop is due to this same surface force, not to flow.

[00297] Drop motion experiments with an aqueous solution of BSA and serum were also conducted. Both of these biological fluids were moved, though differences in contact angles and in motion were observed. Table 1 shows that while the static advancing angle is very similar for water, BSA solution, and serum, the dynamic angles can be very different. Before conducting these experiments, the behavior of these solutions was studied, as well as saliva, urine, plasma, and whole blood on superhydrophobic surfaces without magnetic particles. An overall conclusion from those experiments is that viscous biological fluid drops have a high contact angle on superhdyrophobic surfaces, but sometimes do not roll off the surface. Without wishing to be bound by theory, it is believed that viscoelastic fluids create a higher amount of friction than water presumably due to their ability to deform around the tops of the nanowires. When drops of BSA solution or serum using magnetic particles are moved under the influence of a magnetic field, they appear to move more sluggishly than water.

[00298] Coalescence and/or splitting of drops are also steps with practical utility in micro fluidics. Coalescence of two drops as seen in Figure 38 has been achieved by moving two particle-laden BSA-containing drops towards each other using two magnets. The resulting coalesced drop can also be moved with the magnet. Drop splitting can also be achieved for a drop with a higher concentration of paramagnetic particles (5%), by means of two magnets that are placed below the superhydrophobic surface. As seen in Figure 39, by placing two magnets with poles oriented in the same direction under one BSA containing drop and progressively separating the two magnets, the drop was deformed until it split. In this figure, the resultant drops are unequal in size. Interestingly, when water drops are split the resultant drops appear nearly identical in size. It is typically more difficult to create two drops of equal sizes in BSA solution than with water, possibly due to the viscoelastic properties of the BSA solution.

[00299] Figure 40 shows a sequence of still images from a video where a drop is moved to a microelectrode system for dopamine analysis in aqueous solution. Measurements can be taken to determine the dopamine concentration of a drop. The drop coating protects the iron particles from any unwanted reactions that would cause a false reading or create impurities within the drop. The use of a magnetic field to move the drop also serves to pin the drop on the surface, thus preventing capillary action from wicking the drop up the microelectrode assembly.

Table 1

Comparison of contact angles for water and biological solutions for drops controlled on a superhydrophobic surface using paramagnetic particles and an external magnetic field.

[00300] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

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