DEVICE AND METHOD FOR GAS MAINTENANCE IN MICROFEATURES ON A SUBMERGED SURFACE

Abstract

A microstructured surface with microfeatures formed thereon and defining spaces between the microfeatures includes least one electrode of an electrode pair in the spaces, wherein electrodes of the pair are electrically connected to one another. The at least one electrode located in the space is configured to generate a gas in between the microfeatures when an electrolyte solution penetrates into the microfeatures. Importantly, the electrodes are not connected to any external power source. Because the microstructured surface is self-powered in replenishing the gas lost in a submerged condition, no additional provision to supply energy or regulate the replenishment is necessary for implementation and use.

Claims

1. A microstructured surface comprising: a plurality of microfeatures disposed on a substrate and defining a space located between inner surfaces of the plurality of microfeatures, wherein the inner surfaces between the plurality of microfeatures are at least partially hydrophobic; a gas generator comprising a first electrode of an electrode pair electrically connected to a second electrode of the electrode pair, the first electrode disposed within the space located between inner surfaces of the plurality of microfeatures and configured to generate a gas by a spontaneous electrochemical reaction when part of the space located between the microfeatures is filled with an electrolyte solution while the microstructured surface is submerged in the electrolyte solution.

2. The microstructured surface of claim 1, wherein the first electrode of the electrode pair is located on the substrate between microfeatures.

3. The microstructured surface of claim 1, wherein the first electrode of the electrode pair is located on the inner surface of at least one of the microfeatures.

4. The microstructured surface of claim 1, wherein the first electrode and the second electrode of the electrode pair have respective standard electrode potentials that generate gas by spontaneous electrochemical reaction when contacted by the electrolyte solution, and the difference between the standard electrode potential of the electrolyte and the standard electrode potential of the second electrode is, in magnitude, larger than an overpotential for gas generation at the first electrode, and wherein the overpotential at the first electrode is, in magnitude, smaller than the overpotential at the second electrode.

5. The microstructured surface of claim 1, wherein the first electrode of the electrode pair comprises one of a trace, track, porous layer, mesh, or wire.

6. The microstructured surface of claim 1, wherein the first electrode comprises nickel and the second electrode comprises magnesium.

7. A microstructured surface comprising: a plurality of microfeatures disposed on a substrate and defining a space located between inner surfaces of the plurality of microfeatures, wherein the inner surfaces between the plurality of microfeatures are at least partially hydrophobic; a gas generator comprising a first electrode and a second electrode of an electrode pair electrically connected to one another, the first electrode and the second electrode disposed within the space located between inner surfaces of the plurality of microfeatures and configured to generate a gas in between the microfeatures by spontaneous electrochemical reaction when part of the space is filled with an electrolyte solution while the microstructured surface is submerged in the electrolyte solution.

8. The microstructured surface of claim 7, wherein the electrode pair is located on the substrate between microfeatures.

9. The microstructured surface of claim 7, wherein the electrode pair is located on the inner surface of at least one of the microfeatures.

10. The microstructured surface of claim 7, wherein the electrodes of the electrode pair have respective standard electrode potentials that generate gas by spontaneous electrochemical reaction when contacted by the electrolyte solution, and the difference between the standard electrode potential of the electrolyte and the standard electrode potential of the second electrode is, in magnitude, larger than an overpotential for gas generation at the first electrode, and wherein the overpotential at the first electrode is, in magnitude, smaller than the overpotential at the second electrode.

11. The microstructured surface of claim 7, wherein the electrode pair comprise one of a trace, track, porous layer, mesh, or wires.

12. The microstructured surface of claim 7, wherein the first electrode comprises nickel and the second electrode comprises magnesium.

13. A method of forming a microstructured surface comprising: depositing electrodes on a surface of a substrate; securing a mold against the surface of the substrate containing the electrodes so as to form a tight contact between the mold and electrodes, the mold containing a plurality of cavities therein; applying pressure between the mold and the substrate to force material from the substrate into the plurality of cavities to form a plurality of microfeatures; and separating the mold from the substrate.

14. The method of claim 13, wherein pressure is applied at an elevated temperature.

15. The method of claim 13, further comprising removing any substrate material covering the electrodes after separating the mold from the substrate.

16. The method of claim 13, further comprising coating the plurality of microfeatures with a hydrophobic layer.

17. The method of claim 16, wherein the hydrophobic layer comprises polytetrafluoroethylene (PTFE).

18. The method of claim 13, further comprising connecting the electrodes on the substrate to another electrode formed from a different material.

19. A water-borne vehicle or watercraft having the microstructured surface of claim 1 on at least a portion of a water-contacting surface.

20. A water-borne vehicle or watercraft having the microstructured surface of claim 7 on at least a portion of a water-contacting surface.

21. A pipe having the microstructured surface of claim 1 on an interior surface thereof.

22. A pipe having the microstructured surface of claim 7 on an interior surface thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 illustrates a SHPo surface according to the prior art. FIG. 1 illustrates the transition of a SHPo surface from a non-wetted state (top) to a wetted state (bottom).

[0016] FIGS. 2A-2C illustrates one embodiment of a microstructured surface wherein one electrode of an electrode pair is disposed within the space located between adjacent microfeatures. A cross-sectional view taken along the line A-A of FIG. 3A is shown. FIG. 2A illustrates the microstructured surface in a dewetted state. FIG. 2B illustrates the microstructured surface in a partially wetted state (e.g., one of the spaces between adjacent microstructures is filled with electrolyte). FIG. 2C illustrates the microstructured surface in a de-wetting state wherein gas that is generated at the one electrode pushes the electrolyte fluid out of the space as indicated by the arrows.

[0017] FIGS. 3A and 3B illustrate two different electrode configurations of the embodiment of FIG. 2. FIG. 3A illustrates a wire pattern embodiment. FIG. 3B illustrates a mesh pattern embodiment.

[0018] FIGS. 4A-4C illustrates another embodiment of a microstructured surface wherein an electrode pair is disposed within the space located between adjacent microfeatures. A cross-sectional view taken along the line A-A of FIG. 5A is shown. FIG. 4A illustrates the microstructured surface in a dewetted state. FIG. 4B illustrates the microstructured surface in a partially wetted state (e.g., one of the spaces between adjacent microstructures is filled with electrolyte). FIG. 4C illustrates the microstructured surface in a de-wetting state wherein gas that is generated at the electrode pair pushes the electrolyte fluid out of the space as indicated by the arrows.

[0019] FIGS. 5A and 5B illustrate two different electrode configurations of the embodiment of FIG. 4. FIG. 5A illustrates a wire pattern embodiment. FIG. 5B illustrates a mesh pattern embodiment.

[0020] FIGS. 6A-6E illustrates a method of fabricating a microstructured hydrophobic surface according to one embodiment.

[0021] FIG. 7 illustrates a perspective view of a microstructured hydrophobic device according to one embodiment. In this embodiment, the microstructured surface includes a series of trenches and electrodes are configured to have a pattern of wires.

[0022] FIGS. 8A-8D illustrate a panel of photographs of a microstructured hydrophobic device illustrating operation of one embodiment. The vertical black lines are the electrodes. Horizontal black lines are the gas that is formed in the spaces between microfeatures (which appear clear, i.e., the black lines disappear where they are wetted). FIG. 8A illustrates the microstructured hydrophobic device with the surface in a dewetted state. FIG. 8B illustrates the microstructured hydrophobic device with the surface in a partially wetted state. FIG. 8C illustrates the microstructured hydrophobic device with the surface in a de-wetting state where gas is actively created and pushing out the electrolyte. FIG. 8D illustrates the microstructured hydrophobic device with the surface in a de-wetted state resembling that of FIG. 8A.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0023] FIG. 2 illustrates a microstructured hydrophobic surface 2 according to one embodiment. The microstructured surface 2 includes a substrate 4 having a plurality of microstructures or microfeatures 6 disposed thereon. The microfeatures 6 are generally oriented substantially orthogonal to the substrate 4. The use of the term microfeatures 6 means, when used herein, microscopic physical features; they are commonly in micrometer scale but can be in smaller or larger scale. The microfeatures 6 may be arranged in an array or random configuration. Microfeatures 6 can include gratings, trenches, ridges, posts, pillars, holes, random structures, or porous materials. The substrate 4 may be formed from any number of materials but, as explained in the method, may include a polymer if molding techniques are used to form the microstructured surface 2. The microfeatures 6 have a height in which they project relative to a surface of the substrate 4. The height may vary but typically is tens of microns (e.g., height=50 m) for drag-reduction applications and may be reduced to the single micron range (e.g., height=5 m) for anti-fouling applications. The microfeatures 6 are separated from one another by a distance. The pitch is center-center distance between the nearest neighboring microfeature 6. A liquid solution containing electrolyte 8 is disposed on the microstructured surface 2. When the space between neighboring microfeatures 4 is filled with a gas 10, a meniscus is formed that may sag or bulge depending on the pressure difference between the liquid and the gas.

[0024] The height of the microfeatures 6 should be high enough such that the sagging meniscus does not touch the substrate 4. This height is also a function of the pitch of the microfeatures 6. It should be understood that a microstructured surface 2 includes a surface that contains microfeatures 6 made of or whose surface is made of a material that water or liquid of interest does not wet, i.e., forms a contact angle larger than 90 on its surface. This means the liquid is not restricted to water and may be of any kind, including water, oil, or solvent, despite the use of hydrophobic as described herein.

[0025] The microfeatures 6 may be formed from the same or different material from the substrate 4. For instance, in some embodiments, the microfeatures 6 may be formed from a polytetrafluoroethylene (PTFE) or the like. The microfeatures 6 may be made mostly from a material that is particularly hydrophobic. Alternatively, at least some portions of the microfeatures 6 may be coated with a hydrophobic material such as PTFE or the like. In some embodiments, the top surfaces of the microfeatures 6 may not be hydrophobic so that only the inner surfaces between the microfeatures 6 are hydrophobic. During typical applications, a fluid of electrolyte 8 that is electrically conductive is disposed on the microstructured surface 2. A typical electrolyte 8 may include seawater or water having dissolved ionic species therein.

[0026] With reference to FIG. 2, the microstructured surface 2 includes a first electrode 12 and a second electrode 14 that are in electrical communication with one another, for example, connected to one another via a conductor 16. Conductor 16 may be a wire, electrical line, trace, tracks, vias, or the like. The first electrode 12 and the second electrode 14 together makes a built-in or self-powered gas generator that is configured to generate a gas 10 in between the microfeatures 6. Importantly, the gas 10 is generated on the electrode 12 without the aid of any applied power source. Instead, the first electrode 12, the second electrode 14, and the electrolyte 8 are made from materials having distinctly different standard electrode potentials (SEP) (or standard reduction potentials) so that when electrolyte 8 fills the space between microfeatures 6 the two electrodes 12, 14 and the electrolyte 8 form an electrochemical cell (or more specifically a galvanic cell) that spontaneously generates a gas 10 on electrode 12. For the spontaneous reaction to occur, the overall potential of the galvanic cell should overcome the overpotential needed for gas generation on the first electrode 12. The overpotential is the potential difference (i.e., voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox (reduction/oxidation) event is observed experimentally. For example, bubble formation of gas 10 causes overpotential. Also, the magnitude of overpotential needed for gas generation on the first electrode 12 should be smaller than that on the second electrode 14, so that gas is generated on electrode 12 rather than on electrode 14. Any electrolytes 8 that would generate gas in half-reaction could be used here.

[0027] The following criteria set forth the conditions required for the operation of the microstructured surface 2 submerged in electrolyte 8 having a first electrode 12 used for gas generation that is located in the space 13 between microfeatures 6 and a second electrode 14 that may or may not be located in the space 13 as explained herein. Gas generation does not appreciably occur at the second electrode 14 but rather occurs at the first electrode 12 in this description. For the first condition, the SEP of the second electrode 14, denoted SEP.sub.2, should be different from the SEP of electrolyte 8, denoted SEP.sub.3 as seen in Equation 1 below:

SEP.sub.2SEP.sub.3 Eq. 1

[0028] This condition (Equation 1) is required to ensure that there is a reaction between the second electrode 14 and the electrolyte 8.

[0029] The second condition is that the half-reaction of electrolyte 8 should produce gas 10 under standard pressure and temperature. This condition is to ensure that the reaction will generate gas.

[0030] The third condition is that the difference in the theoretical standard cell potentials between the second electrode 14 and electrolyte 8 (SEP.sub.2SEP.sub.3) should be, in magnitude, larger than the overpotential of half-reaction of electrolyte 8 on the first electrode 12, denoted OP.sub.3/1 as seen in Equation 2 below:

|SEP.sub.2SEP.sub.3|>|OP.sub.3/1|Eq. 2

This condition (Equation 2) is to ensure that gas 10 can be generated on the first electrode 12.

[0031] For the fourth condition, the overpotential of half-reaction of electrolyte 8 on the first electrode 12, denoted OP.sub.3/1, should be, in magnitude, smaller than the overpotential of half-reaction of electrolyte 8 on the second electrode 14, denoted OP.sub.3/2 as seen in Equation 3 below:

|OP.sub.3/1|<|OP.sub.3/2|Eq. 3

[0032] This condition (Equation 3) is to ensure the gas 10 is generated mainly on the first electrode 12 rather than on the second electrode 14. All four conditions should be satisfied for the gas generation to operate properly.

[0033] Note that the above conditions can be divided into two scenarios, depending on whether the gas generation is by reduction or oxidation reaction. If the gas generation on the first electrode 12 is by reduction reaction (e.g., 2H.sup.++2e.sup..fwdarw.H.sub.2), then the SEP of the second electrode 14, SEP.sub.2, should be smaller than the SEP of the electrolyte, SEP.sub.3 as seen by Equation 4 below.

SEP.sub.2<SEP.sub.3 Eq. 4

[0034] In addition, the half-reaction of the electrolyte 8 should produce the gas under standard pressure and temperature. Further, the theoretical standard electrode potential between the second electrode 14 and the electrolyte 8 (SEP.sub.2SEP.sub.3) should be smaller (i.e., more negative) than the overpotential of half-reaction of the electrolyte 8 on the first electrode 12, OP.sub.3/1 as seen by Equation 5 below.

SEP.sub.2SEP.sub.3<OP.sub.3/1 Eq. 5

[0035] The overpotential of half-reaction of the electrolyte 8 on the first electrode 12, OP.sub.3/1, should be larger (i.e., less negative) than the overpotential of half-reaction of the electrolyte 8 on the second electrode 14 OP.sub.3/2 as seen by Equation 6 below.

OP.sub.3/1>OP.sub.3/2 Eq. 6

[0036] Alternatively, if the gas generation on the first electrode 12 is by oxidation reaction (e.g., 4OH.sup.+4e.sup..fwdarw.O.sub.2+2H.sub.2O), the SEP of the second electrode 14 SEP.sub.2 should be larger than the SEP of the electrolyte 8 SEP.sub.3 as seen by Equation 7 below.

SEP.sub.2>SEP.sub.3 Eq. 7

[0037] The half-reaction of the electrolyte 8 should produce a gas under standard pressure and temperature. The theoretical standard electrode potential electrode potential between the second electrode 14 and the electrolyte 8 (SEP.sub.2SEP.sub.3) should be larger (i.e., more positive) than the overpotential of half-reaction of the electrolyte 8 on the first electrode 12, OP.sub.3/1 as seen by Equation 8 below.

SEP.sub.2SEP.sub.3>OP.sub.3/1 Eq. 8

[0038] The overpotential of half-reaction of the electrolyte 8 on the first electrode 12, OP.sub.3/1, should be smaller (i.e., less positive) than the overpotential of half-reaction of the electrolyte 8 on the second electrode 14 OP.sub.3/2 as seen by Equation 9 below.

OP.sub.3/1<OP.sub.3/2 Eq. 9

[0039] Specifically, according to one embodiment, if the electrolyte 8 is a neutral or alkaline water solution (e.g., seawater), the standard electrode potential is about 0.83 V from 2H.sub.2O+2e.sup.H.sub.2(g)+2OH.sup.. If the electrolyte 8 is an acid water solution, the standard electrode potential is 0 V from 2H.sup.++2e.sup.H.sub.2(g). Once the electrolyte 8 is chosen, possible materials combinations for the first electrode 12 and the second electrode 14 could be obtained, satisfying the constraints stated above regarding standard electrode potential and over-potential. For a specific example, the first electrode 12 may be made from a metal or metal alloy containing nickel, the second electrode 14 may be made from a metal or metal alloy containing magnesium, and the electrolyte 8 may be seawater (although other electrolytes may also be used such as fresh water, acid solutions, basic solutions, and the like). In this case, the gas 10 that is produced is hydrogen at the first electrode 12. Table 1 below lists various materials and SEP and overpotential values that have been tested with successful results.

TABLE-US-00001 TABLE 1 |OP| (H.sub.2 evolution Name Material Half reaction SEP at 25 C.) First Nickel N/A (inert) N/A 0.28 V Electrode Second Magnesium Mg.sup.2+(aq) + 2e.sup. .Math. Mg(s) 2.372 V ~1 V Electrode Electrolyte Seawater 2H.sub.2O(l) + 2e.sup. .Math. H.sub.2(g) + 2OH.sup.(aq) 0.8277 V N/A

[0040] Where and when at least a portion of the space between microfeatures 6 are filled with the electrolyte 8 and contact the gas generating electrode, gas 10 (e.g., hydrogen) is automatically generated by the spontaneous electrochemical reaction (i.e., galvanic reaction) that takes place. The microfeatures 6 are designed and spaced such that the gas 10 that is generated on the electrode 12 is trapped and grows within the localized space, gap, or well 13 formed between adjacent microfeatures 6. Once the volume of generated gas 10 has grown in size to the top (e.g., upper surface) of the microfeatures 6 the gas 10 proceeds to extend laterally in between the microfeatures 6 across the microstructured surface 2. In this regard, the microstructured surface 2 can be restored from a partially or fully wetted state back into a non-wetted state. In one embodiment, the first electrode 12 is disposed on or embedded in the substrate 4. FIGS. 2A-2C illustrates the first electrode 12 located on the substrate 4 within the space 13 formed between adjacent microfeatures 6. In this embodiment, the first electrode 12 is located on a bottom surface of space or gap 13 located between the microfeatures 6. Alternatively, the first electrode 12 may be located on the sidewall surface of the microfeatures 6. Thus, the first electrode 12 can be located in any location between the microfeatures 6, wherein between microfeatures 6 refers to the spaces 13 and inner surfaces between microfeatures 6 (excluding the top or exposed surfaces of the microfeatures 6). Note that in the embodiment of FIGS. 2A-2C, the second electrode 14 is not located between the microfeatures 6. Instead, the second electrode 14 is located outside of the internal space 13 created by the microfeatures 6.

[0041] The gas 10 that is generated by the first electrode 12 is trapped and grows in the localized space or well 13 formed between adjacent microfeatures 6. It should be noted that gas generation is self-regulated in this embodiment. In particular, the electrochemical circuit is closed and gas generation occurs only if the microstructured surface 2 becomes partially or fully wetted. Once the electrolyte 8 invades the space 13 between adjacent microfeatures 6 the circuit is closed between the first electrode 12 and the second electrode 14 and the electrochemical reaction proceeds, thereby liberating gas 10 within the spaces 13 located between the microfeatures 6. After sufficient gas generation, the electrochemical circuit then becomes open and the electrochemical reaction stops. This feature is particularly advantageous because there is no need for any sensing and controlling circuitry to switch the gas generator (i.e., electrode) on or off; in other words, the gas generation is self-regulated to respond to presence of electrolyte 8 in between the microfeatures 6.

[0042] The gas generation occurs spontaneously without connecting the microstructured surface 2 to any external power source (e.g., battery); in other words, the gas generation is self-sufficient. However, since the gas generation is the product of an electrochemical reaction (i.e., powered electrically albeit using internal energy) rather than pure chemical reaction (i.e., no electric power involved), one may call the disclosed method of gas generation not only self-sufficient but also self-powered. In addition and related to the above difference, it is worth noting that the gas generator (i.e., electrode) located in between the microfeatures 6 is not a reactant in the current self-powered, electrochemical reaction. Self-powering and self-regulating the gas generation, the microstructured surface 2 has the built-in ability to maintain the gas 10 trapped in between its microfeatures 6.

[0043] FIG. 3A illustrates a top view of one configuration of a microstructured surface 2. The view of FIGS. 2A, 2B, and 2C is taken along the cross-sectional line A-A in FIG. 3A. In this embodiment, the first electrodes 12 are formed as a pattern of substantially parallel wires located between and under the microfeatures 6. The microfeatures 6 are formed as a grating or the like that extend across the surface of the substrate 4. The first electrode wires 12 run along or across the spaces 13 formed between adjacent microfeatures 6. FIG. 3B illustrates a top view of another configuration of a microstructured surface 2. In this embodiment, the first electrodes 12 are formed as a mesh pattern that is located between and under the microfeatures 6.

[0044] FIGS. 4A-4C illustrates another embodiment of a microstructured surface 2. In this embodiment, both the first electrode 12 and the second electrode 14 are placed in the space 13 formed between the microfeatures 6. Again, the first electrode 12 and the second electrode 14 are electrically connected directly or via a conductor 16. The first electrode 12, the second electrode 14, and the electrolyte 8 are made from materials having distinctly different standard electrode potentials so that when electrolyte 8 fills the spaces 13 of the microfeatures 6 the two electrodes 12, 14 and the electrolyte 8 form an electrochemical cell (more specifically, a galvanic cell) that spontaneously generates a gas 10 on the first electrode 12. For the spontaneous reaction to occur, the overall potential of the galvanic cell should overcome the overpotential needed for gas generation on the first electrode 12. Also, the magnitude of overpotential needed for gas generation on the first electrode 12 should be lower than that on the second electrode 14 so that gas is generated on electrode 12 rather than on electrode 14. Any electrolytes 8 that would generate gas in half-reaction could be used here. The same criteria of four conditions described above apply to this embodiment as well.

[0045] FIG. 5A illustrates a top view of one configuration of a microstructured surface 2 of the embodiment of FIG. 4. The view of FIGS. 2A, 2B, and 2C is taken along the cross-sectional line A-A in FIG. 3A. In this embodiment, one or both of the first electrodes 12 and the second electrodes 14 are formed as a pattern of substantially parallel wires between and under the microfeatures 6. FIG. 5B illustrates a top view of another configuration of a microstructured surface 2. In this embodiment, one or both of the first electrodes 12 and the second electrodes 14 are formed as a mesh pattern between and under the microfeatures 6. Again, while FIG. 4 illustrates the first electrode 12 and the second electrode 14 being located at the bottom surface of the substrate 4 between the microfeatures 6, one or both of the electrodes 12, 14, may be located on the sidewall surfaces of the microfeatures 6.

[0046] FIG. 6A-6E illustrates a process of fabricating a microstructured surface 2 according to one embodiment. This process relies on a molding technique such as embossing or imprinting. First, the first electrode 12 or first and second electrodes 12, 14 are placed between substrate 4 and mold 7 as seen in FIGS. 6A, 6B. The electrodes 12, 14 may be a porous layer, a mesh, or a collection of wires, among others. The feature size (e.g., width of the wires) of the electrodes 12, 14 should be smaller than the openings (e.g., cavity diameter, trench width) on the mold 7 so that the electrodes 12, 14 do not substantially block the recesses. The substantial coverage of the recess openings would hinder the filling of the recesses during the molding step shown in FIG. 6C. During the molding step of FIG. 6C, under applied pressure and optionally at an elevated temperature as well, the material or its top portion of substrate 4 flows around (or through the openings of) the electrodes 12, 14 and fill into the recesses on mold 7. After the mold 7 is full as shown in FIG. 6D, mold 7 is separated from substrate 4, as shown in FIG. 6E. At this point, the electrodes 12, 14 may be completely exposed or covered with a thin residual layer of the material from substrate 4. This thin residual material may be removed by one of many etching methods known to those skilled in the art, such as plasma etching, chemical etching, or the like if necessary. The device may function as intended even with the electrodes 12, 14 not fully exposed, as electrochemical gas generation can still occur if the residual layer is thin enough. The substrate 4 may be thick, thin, rigid, or flexible, and may be made of one material or multiple layers of different materials. The material of microfeatures 6 maybe the same as or different from the material of substrate 4, although FIGS. 6A-6E illustrates the same material being used for the substrate 4 and microfeatures 6.

[0047] An important aspect for the disclosed fabrication method is how to realize the electrodes (e.g., electrodes 12, 14) exposed or almost exposed in between microfeatures 6 while forming microfeatures 6 by embossing/imprinting. As shown in FIG. 6C and 6D, the electrodes are in a tight contact with mold 7 during the molding step. This tight contact can prevent the substrate 4 material from seeping onto the electrode surfaces, while flowing into the recesses or cavities of mold 7. After de-molding, the electrode surfaces that were in tight contact with mold 7 are exposed in between microfeatures 6 with no or little residual material on top, as shown in FIG. 6E. Usually, embossing/imprinting microfeatures 6 with functional materials in between the microfeatures 6 would require separate fabrication steps of molding the microfeatures 6 and placing the functional materials in between the molded microfeatures 6. The current invention completes the process with only a single or nearly single molding step and still does not leave much residual material on the functional materials, keeping the simplicity of traditional embossing/imprinting techniques while obtaining the functional materials exposed in between the microfeatures 6. Although developed for the gas-retaining device above, this processing method is useful for other purposes as well, such as microfluidic devices for biochemical applications.

[0048] FIG. 7 illustrates a perspective view of the microstructured surface 2 in one exemplary embodiment. In this embodiment, the microfeatures 6 are gratings and the spaces 13 are formed as trenches located between adjacent microfeatures 6. The electrodes 12, 14 are illustrated as wires that extend generally transverse with respect to the microfeatures 6. Alternatively, the electrodes 12, 14 may be aligned parallel with the microfeatures 6.

[0049] FIGS. 8A-8D illustrate photographs of experimental results of a microstructured surface 2 operating in the configuration of FIG. 2. While fully submerged in water (i.e., seawater), the FIG. 8A image show the spaces 13 between microfeatures 6 filled with gas (shown as black or dark lines perpendicular to the microfeatures 6. In the state of FIG. 8A, the microstructured surface 2 is in a dewetted state. FIG. 8B shows a substantial portion of the spaces 13 between microfeatures 6 filled with water (i.e., much of the microstructured surface 2 in a wetted state). In the state of FIG. 8B, the microstructured surface 2 is in a partially wetted state. FIG. 8C illustrates the water in the spaces being replaced by the gas 10 generated within the spaces 13 (i.e., the microstructured surface 2 being dewetted). FIG. 8C thus illustrates the de-wetting state whereby gas 10 is used to push our or expel the water contained in the spaces 13. FIG. 8D shows that nearly all the water in the spaces 13 is replaced by the gas 10, i.e., the microstructured surface 2 has returned to a dewetted state. The surface was not connected to any power source during the experiment. The gas in the microfeatures was maintained in a self-regulated and self-powered fashion.

[0050] The microstructured surfaces 2 described herein may be used on an exterior surface of a water-borne vehicle or watercraft (e.g., boat, ship, or the like) that comes into contact with the water. In one particular embodiment, the choice of materials for electrodes 12, 14 is selected for particularly suitability for use in salt-water contact applications (e.g., for water-borne vehicles or watercraft that travel in seawater). The microstructured surfaces 2 may also be used in other applications. For example, the microstructured surfaces 2 may be located on the inner surface of a pipe or conduit.

[0051] While embodiments have been shown and described, various modifications may be made without departing from the scope of the inventive concepts disclosed herein. The invention(s), therefore, should not be limited, except to the following claims, and their equivalents.