Controlled Lateral Spreading and Pinning of Oil Droplets Based on

In addition, for oil repellent superoleophobic surfaces based on re-entrant geometries, controlling the lateral pinning is a useful backup mechanism, ...
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Controlled Lateral Spreading and Pinning of Oil Droplets Based on Topography and Chemical Patterning Ville Jokinen,*,† Lauri Sainiemi,†,‡ and Sami Franssila*,† †

Department of Materials Science and Engineering, Aalto University, School of Chemical Technology, P.O. Box 16200, FIN-00076 Aalto, Finland ‡ Division of Pharmaceutical Chemistry, University of Helsinki, Fabianinkatu 26, 00100 Helsinki, Finland ABSTRACT: Geometric pinning sites can be used to control the lateral spreading and pinning of oils on surfaces. The geometric pinning effect combined with lithographic surface chemistry patterning allows controlling the shapes of oil droplets. We study the confinement effect on test structures of various protruding and intruding geometries, and employ scanning electron microscopy analysis to study the shape of the meniscus at the edges of the chemical patterns. Nanopillar and micropillar topographies are compared, revealing that it is a necessity for accurate oil patterns that the length scale of the roughness is smaller than the resolution of the surface chemistry pattern. We also find that there exists a critical, geometrydependent threshold contact angle, below which the geometric confinement does not work, as olive oil with a static advancing contact angle of 57° accurately replicated the chemical pattern on top of nanopillar topography, but hexadecane with a static advancing contact angle of 50° penetrated the pinning sites and wetted the whole surface.

’ INTRODUCTION Hydrophobic and oleophobic surfaces can be fabricated by controlling the topography and the chemical composition of the surface. Hydrophobicity is usually taken to mean contact angles of over 90°, meaning that the spreading of liquids is energetically unfavorable on such surfaces. On the other hand, superhydrophobic surfaces are anti adhesive toward liquids and have very high contact angles, low contact angle hysteresis, and, consequently, small sliding angles. Superhydrophobic surfaces in particular have received a lot of attention, because of both their prevalence in nature1 and their technological applications.2 Water droplets sitting on a superhydrophobic surface can be in either the Cassie state3 or the Wenzel state,4 depending on the parameters of the surface as well as the way the water is introduced to the surface. In the Cassie state, the liquid does not completely penetrate the topography of the surface, but instead rests on a composite surface comprised partly of the surface material, and partly of air. The contact angles of droplets in the Cassie state are always higher than the inherent contact angle of the material. In the Wenzel state, the liquid penetrates the whole topography of the surface, resulting in more liquidsolid interaction as compared to the planar and Cassie cases. Because of this, droplets in the Wenzel state show increased hydrophilicity for inherently hydrophilic materials and increased hydrophobicity for inherently hydrophobic materials. Fabricating a superhydrophobic surface is in principle relatively straightforward, and consists of combining micro- or nanoscale roughness to an inherently hydrophobic surface chemistry.58 On such surfaces, the Cassie state of a water droplet is at least metastable, and the composite surface can have very low adhesion to the droplet as the air parts do not contribute toward r 2011 American Chemical Society

adhesion at all. On the other hand, superoleophobic surfaces are significantly harder to fabricate, since inherently oleophobic (θ > 90° on a smooth surface) surface chemistries are unavailable,6,9,10 which means that the Wenzel state just lowers the contact angle further, while the Cassie state is inherently unstable on most geometries. However, Tuteja et al. demonstrated,10 on the basis of earlier works,11 that a re-entrant surface geometry, where the meniscus needs to expand in order to advance downward from the re-entrant site, can render the Cassie state of an oil droplet metastable, when combined with a surface chemistry with a surface energy as low as possible. On the basis of this effect, multiple groups have presented superhydrophobic and superoleophobic surfaces6,10 that exhibit limited adhesion to oil droplets, based on the metastable composite Cassie state. Compared to superoleophobicity, less attention has been given to the question of structured surfaces that prevent lateral spreading of oils. The relationship between surfaces, on which the lateral spreading of oils is energetically unfavorable, and superoleophobic surfaces, on which oil droplets adhere poorly, is more complex than the relationship between the corresponding cases with water. In the case of water, superhydrophobic surfaces automatically also prevent lateral spreading due to the inherently hydrophobic surface chemistry. However, the surface chemistry of superoleophobic surfaces based on re-entrant geometry is actually slightly oleophilic, which means that it is not clear whether oils will spread laterally or be pinned in case the liquid falls to the Wenzel state at some spot due to some perturbation. Received: February 8, 2011 Revised: April 26, 2011 Published: May 06, 2011 7314

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Langmuir This question is decided by the geometry of the surface, but since the oil is now coming from the side instead of the top, the surface would need to also contain re-entrant geometries in the lateral orientation. This distinction has received little attention in the literature, and even the terminology is a little bit troublesome. What should a surface whose advancing contact angle with an oil is above 90°, but which is not superoleophobic, be called? On one hand, such a surface could be termed oleophobic, in which case the terminology for applications, such as capillary filling of microchannels or controlled lateral spreading, would be consistent between hydrophobicity and oleophobicity: closed hydrophobic/oleophobic channels do not fill spontaneously with water/oil, and water/oil does not spontaneously laterally spread on hydrophobic/oleophobic areas. However, a potential problem arises due to the geometry dependency of oleophobicity, as many superoleophobic surfaces might then not be oleophobic in the sense of preventing lateral spreading, but this is an inconsistency only if oleophobicity and superoleophobicity are considered as direction-independent surface properties. In this work, we have fabricated lithographically patterned surface chemistries on top of micro- and nanopillar topographies, and have chosen to call the high surface energy domains, on which oils spontaneously spread, oleophilic, and low surface energy domains, on which the spreading is prevented by a pinning mechanism, oleophobic. In practice, surfaces with patterned oleophilic/ oleophobic domains for controlled spreading would be useful, since applications where the key issue is confining the lateral spreading of water droplets rather than anti-adhesion have been presented,8,12 but few, if any, such reports exist for oils.

’ EXPERIMENTAL SECTION A random array of nanopillars was fabricated by a maskless deep reactive ion etching process explained in detail in previous publications.13 The nanopillars had a pyramid shape, and were roughly 500 nm high and 100 nm wide (see Figure 1a,b). The micropillar surfaces were fabricated by anisotropic cryogenic deep reactive ion etching of silicon (Oxford Instruments Plasmalab System 100; pressure, 10 mTorr; temperature, 120 °C; RF power, 3 W; ICP power, 1000 W; SF6 flow, 40 sccm; and O2 flow, 6 sccm) using a photoresist mask patterned by optical lithography. Two geometries were tested: 9 μm square pillars in a rectangular lattice with 4.5 μm spacing etched 4.2 μm deep, and 10 μm square pillars in a rectangular lattice with 10 μm spacing etched 9.2 μm deep. For surface chemistry patterning, the whole surface is first coated with a low surface energy, plasma-deposited fluoropolymer (Oxford Instruments Plasmalab80plus, pressure 250 mtorr, RF power 50 W, CHF3 flow 100 sccm and treatment time 5 min). Nominal thickness of the coating on a smooth surface was 40 nm. The surface was then selectively protected by a lithographically patterned photoresist layer, and an oxygen plasma step (Oxford Instruments Plasmalab 80plus; pressure 250, mTorr; RF power, 50 W; O2 flow, 45 sccm; Ar flow, 5 sccm; treatment time, 5 min) is used to remove the fluoropolymer and oxidize the underlying silicon on the exposed areas. With the parameters used, both plasma treatments were isotropic enough to treat the entire surface, including the sidewalls and the bottom of the substrate (see Figure 1a; the contrast between the fluoropolymer-coated area and the oxidized area is clearly visible). The lithography step on top of the nanopillars is explained in a previous publication8 and is based on a thick layer of resist that is able to cover the entire topography.14 In this study, the same thick resist lithography process was also used for patterning substrates with micropillar topographies. The test liquids used in the study were deionized water, olive oil, hexadecane (Merck, Hohenbrunn, Germany), isopropanol, and a solgel

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Figure 1. (a) Oxidized and fluoropolymer-coated silicon nanopillars. (b) Geometry of the pillars. (c) Unfilled intruding oleophobic test patterns. (d) Oil-filled protruding oleophilic test patterns. (e,f) Snapshots of the filling process; note the fractal nature of the imbibition front, the lagging droplet spreading front, and the mask defect causing a nonwetted spot inside the droplet. The scale bar (bottom left) in cf is 200 μm. polymer Ormocer (Microresist Technology, Berlin, Germany). Contact angles on planar and structured surfaces were measured by the sessile droplet method using a Cam-101 goniometer (KSV instruments, Helsinki, Finland). On planar surfaces, liquid droplets were gently applied on the surface, and an image was taken after the droplet spontaneously stopped advancing on the surface, leading to static advancing contact angle values. The reported values are averages of three measurements. The oil pinning experiments were performed by pipetting an oil droplet on oleophilic test patterns of various shapes. The test structures consisted of large 2 cm 2 cm square-shaped reservoirs, the perimeter of which had the various test geometries in both protruding (the described shape is filled by oil) and intruding (the described shape in devoid of oil, while the surrounding is filled by oil) polarities. Plastic photomasks with a minimum line width of ∼20 μm were used in the lithography for surface property patterning. The experimental procedure was to pipet 510 μL of the test liquid into the center of the test structures and to observe the contact line after it had settled on a mechanical equilibrium. Typical waiting times were hours, but we also observed the surfaces over time frames of months in order to verify the equilibrium state. For scanning electron microscopy (SEM) (Supra 40, Carl Zeiss SMT) of the meniscus pinning, the solgel polymer was introduced to the test structures as explained above. After waiting for the imbibition to reach equilibrium overnight, the solgel was cured by exposure to UV-light, and observed under SEM. We verified experimentally that no meniscus restructuring happens during the curing process itself.

’ RESULTS AND DISCUSSION Oleophobic Nanopillars. We observed that fluoropolymercoated silicon nanopillars (Figure 1a,b) in a random array can prevent the lateral spreading of oils of moderate surface tension, such as olive oil. Olive oil (surface tension, ∼32 mN/m) had a 7315

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Figure 2. SEM analysis of meniscus pinning on nanopillar topography. (a) Triangle-shaped oleophilic protrusion filled with polymer. (b) Top view of the pinned meniscus. (c) Another top view of the pinned meniscus. (c) Sideview of the pinned meniscus.

contact angle of 57° on a smooth fluoropolymer-coated surface, while the contact angle between olive oil and fluoropolymercoated nanopillars was 93°, which means that the nanopillar geometry makes the surface oleophobic. The rolling angles of the droplets were large, so the surface was not superoleophobic. This contact angle behavior is different from what would be predicted by the Wenzel model, which always predicts lowered contact angles for rough surfaces with inherently oleophilic chemistries. Quantitatively, the Wenzel model predicts that cos(θW) = r cos(θ), where r is the roughness factor relating the total surface area of the rough surface to the total surface area of a smooth surface. From Figure 1b, we estimate that the pillar width is 100 nm, spacing is 100 nm, and the height is 500 nm, which, assuming a regular rectangular lattice, gives a roughness factor of r ≈ 5. Inserting this into the Wenzel formula gives cos(θW) > 1, which means that the contact angle should be zero according to the Wenzel model, instead of slightly oleophobic as found in the experiments. This demonstrates that the droplet is indeed in a pinned state, since the most stable equilibrium contact angle should indeed correspond to the Wenzel angle15 as the wavelength of the roughness on our surface is orders of magnitude smaller than the size of the oil droplet. Controlled Oil Spreading. The oleophobic nanopillars can be used for controlled oil spreading similar to what was previously reported for water droplets.8 The approach is based on lithographically patterning the surface properties of the nanopillars to be either a low surface energy fluoropolymer or a high surface energy oxygen plasma-treated silicon (Figure 1a). The resulting surface domains, on top of the nanopillar topography, are then oleophobic and oleophilic, respectively. Our test structures consist of a central oleophilic reservoir, where the oil droplet is applied, surrounded by an oleophobic area. The perimeter of the oleophilic area was filled with test patterns of intruding and protruding triangles, diamonds, lines, and text, of various dimensions (Figure 1c,d). Figure 1e,f shows optical microscopy images of olive oil droplets spreading on the test patterns. The spreading

proceeded in two stages, so that the oil first fills the oleophilic domains by capillary imbibititon, followed by a slower process where the oil droplet itself spreads. Both the imbibition and the droplet spreading stopped at the oleophilic/oleophobic boundary. The oil droplets replicated all of the test patterns to resolution similar to that found on the photomask. For example, Figure 1c shows intruding lines and diamonds, while Figure 1d shows protruding diamonds and triangles. Neither imbibition nor droplet spreading to the oleophobic domains were observed in a time frame of 1 month. Pinning Sites on Nanopillars. Bico et al. have developed a model for imbibition and wetting of structured surfaces by partially wetting liquids.16 The approach is based on surface energy minimization considering a two-stage filling process, where capillary imbibition precedes the movement of the droplet edge. For the capillary imbibition, they derive a critical contact angle cos θc = (1  jS)/(r  jS), below which capillary imbibition lowers the overall surface energy. From Figure 1b, we again estimate the roughness factor r ≈ 5 and the solid fraction remaining dry jS ≈ 0.05, giving a critical contact angle of θc ≈ 80°. Therefore, capillary imbibition would be expected to take place since the inherent contact angle of the oil on the fluoropolymer was only 57°, but in the experiments the fluoropolymer coated areas remained dry. The model of Bico et al. has definite experimental support in the case of lower contact angles,16,17 and we suggest that the discrepancy here is due to contact line pinning to a metastable state at certain pinning sites between the pillars. The complications caused by metastable states were already discussed by Bico et al.,16 and meniscus pinning was recently discussed in the context of hysteresis of water droplets on a partially wetting micropillar surface.18 We utilized SEM to study the shape of the meniscus pinned to the boundary of the oleophilic and oleophobic domain. A solgel polymer with a contact angle of 55° on a smooth fluoropolymer surface was pipetted to the test structures, left to spread overnight, and cured. The SEM micrographs, shown in Figure 2, 7316

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Figure 3. Oil droplets on surfaces with 9 μm pillars (ac) and 10 μm pillars (df, h). (a) Oil imbibing outside the circular oleophilic test pattern. (b) Meniscus pinning on two 270° corners. (c) Structural defects assisting the pinning on two specific corners. (d) Protruding oil lines and triangles. (e) Protruding diamonds and intruding diamonds (on two different test structures). (f) Meniscus pinning in 270° configuration. (g) Schematic of oil advancing past a 270° pinning site. (h) Length scales of the chemical patterns of the test structures and the micropillars.

reveal that the meniscus has mostly pinned between two pillars in a configuration where the liquid has not wetted the front edge of the pillars (see also Figure 3b for the corresponding pinning between two micropillars). These spots create natural re-entrant pinning sites, as the meniscus would need to expand in order to emerge from the narrowest spot between two pillars. This effect is well-known in the field of capillary microfluidics where it has been used in valving of microfluidic channels.19 The pinning

mechanism here has a lot in common with the pinning of the meniscus to overhang structures on superoleophobic surfaces. In the case of overhang, it is relatively easy to engineer features where the meniscus needs to expand into all directions when moving downward. However, in the present case, the floor and the bottom corners of the pillars create a possible way for the meniscus to bypass the valving site. Capillary flow in an isolated L-shaped corner is possible at lower contact angles,2022 and a 7317

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Langmuir meniscus leaning on the corner and hitting the next row of pillars has been observed as the principal mechanism allowing the imbibition front to move from one pillar row to the next.16,17,23 Thus, in cases with lower inherent contact angles, there exists a possible mechanism by which the oil might bypass the metastable pinning sites. Experiments with Lower Surface Tension Oil. We observed that a slightly lower surface tension oil hexadecane (≈ 28 mN/m), having a contact angle of 50° with the smooth fluoropolymer surface, was able to penetrate the barrier and also wet the fluoropolymer-coated nanopillars. The spreading of hexadecane was not isotropic, as the imbibition slowed significantly once the hexadecane reached the oleophobic area, allowing the patterns to be transiently seen replicated in the hexadecane, but the oleophobic areas did ultimately fill as well. Thus, a practical contact angle limit for this particular nanopillar geometry appears to be somewhere between 50° and 57°. This limit is also consistent with known problems with capillary geometrical valves at lower contact angles, where the very similar pinning mechanism has also been reported to be unreliable at lower contact angles.24 Even lower surface tension liquids such as isopropanol (≈ 23 mN/m) wet the nanopillar surface rapidly and seemingly isotropically, irrespective of the surface chemistry patterns. Micropillar Experiments with 9 μm Pillars. Complementary experiments were performed with micropillar topographies (see Experimental Section for details of the geometries), where the surface properties were again patterned by lithography to consist of either the fluoropolymer coating or oxidized silicon, just as with the nanopillars. Several clear differences in the olive oilconfining performance compared to the nanopillars were noticed. First, we noticed that on the 9 μm pillars with 4.5 μm spacing, the olive oil could not get pinned in a reflex 270° angle configuration (270° sector filled by oil and 90° sector unfilled), as shown in Figure 3ac. Figure 3a shows a simple circular oleophilic test structure after the filling process had reached equilibrium. We noticed that, in this case, the oil was not confined to the oleophilic circle, but instead advanced also to the fluoropolymercoated area and ultimately formed a sawtooth pattern with 90° turns. When we examined the occasional 270° corners that did successfully confine the liquid (six of these are shown in Figure 3a), we noticed that in every single one of them there was a defect in the structure that altered the geometry in a very specific way (the defects were caused by an old photomask, which was not checked for defects before use). Two of these cases are shown in Figure 3b,c, showing the pinned liquid meniscus and the micropillar structure, respectively. From this figure it appears clear that the 270° corners do not hold because, if a meniscus is pinned between a pillar and the pillar in the next row, and another meniscus is pinned between the same pillar and the pillar in the next column (the three pillars making a L shape), the two pinned menisci fuse together and can then jointly reach the next pillar. This process then generates a new 270° corner configuration, leading to a row-by-row “zipping”-like filling, that has been observed also in other studies of micropillar geometries.17,23,25 The defect sites effectively prevented the joining of the menisci on the 270° corners, and without the defects, the resulting pattern would have been a square spanned by the extent of the original circle to both dimensions of the rectangular pillar array. Micropillar Experiments with 10 μm Pillars. Olive oil patterns on 10 μm square pillars with 10 μm spacing were more successful in the sense that the oil did not penetrate great

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distances into the oleophobic area, but suffered from worse fidelity and lower resolution as compared to the olive oil patterns on nanopillars. As with the nanopillars, hexadecane was able to penetrate the pinning sites also on the micropillar surface. Protruding olive oil lines and diamonds as well as oil triangles in both polarities are presented in Figure 3d,e. The lower fidelity of the patterns is explained again by problems with pinning the oil in configurations where the oil forms angles over 180°. This is especially visible in the protruding lines in Figure 3d (the bases of the lines are rounded) and the intruding triangles in Figure 3e (the tips are rounded so that the sharper the tip angle on the fluoropolymer pattern, the more rounding there is in the oil droplet). An optical micrograph showing pinned 270° reflex angles on the 10 μm pillars is presented in Figure 3f (rounded tip of an intruding triangle). Other than at the very tip, the adjacent menisci are not joined and remain separated by unwetted areas of the micropillars. Meniscus Pinning in a 270° Configuration. At least the following conditions need to be fulfilled for a meniscus to successfully pin to a 270° configuration: (1) the meniscus must not advance from the pinning site across the sidewalls of the pillars by capillary imbibition; (2) the meniscus must not advance along the corners between the pillar sidewalls and the bottom; and (3) the bulging meniscus at the pinning site must not touch the next pillar. The first condition can be derived from the geometry shown in Figure 3g and combines both capillary filling of an open groove22,25 and the geometrical valve,19 in that the advancement of the meniscus a unit step forward to the direction of dx is opposed both by the increased liquidair surface area of the expanding top (open groove) as well as the expanding front edge (geometrical valve). A step of dx forward will increase the total liquidvapor surface area (Alv) and solidliquid surface area (Asv), while reducing the solidvapor area (Asv) by pffiffiffi dAlv  x dx þ 2h dx ð1Þ dAsl ¼  dAsv ¼ x dx þ 2h dx

ð2Þ

where h is the height of the pillars, and the terms that are secondorder in dx have been rounded off. The first terms (x dx) correspond to the open top and the bottom of the channels, and the second terms correspond to the increased area of the open front edge and increased contact with the pillar sidewalls, respectively. From these, the total energy change, taking into account Young’s equation, is dE=dx ¼ γlv dAlv =dx  γlv cosðθÞ dAsl =dx pffiffiffi ¼ γlv ðx þ 2hÞ  γlv cosðθÞðx þ 2hÞ giving the critical contact angle when dE/dx = 0 as pffiffiffi x þ 2h cosðθc ðxÞÞ ¼ x þ 2h

ð3Þ

ð4Þ

It is noteworthy that the critical contact angle gets lower as the meniscus advances further, so in theory it would be possible to get equilibrium states where the meniscus has advanced only partially along the sides of the pillars. The meniscus will touch the next pillar when it has advanced along the sidewalls the distance of the pillar spacing a, and, since the initial position of the fused menisci is at x = a, θ > θc(2a) is one necessary condition for meniscus pinning into the 270° configuration. For both micropillar cases, θc(a) ≈ 35° and θc(2a) ≈ 31°, so this condition 7318

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Langmuir should be satisfied for both geometries. Also the second condition should be satisfied, since for 90° sidewall angles, the upper contact angle limit for open corner flow is 45°.20 A few tentative explanations are offered for the observed difference: first, the meniscus shape is in reality not a straight line like that depicted in Figure 3g, but is instead bulging out from the pinning site (this bulging was simulated for the straight line case by Forsberg et al.18), and it is plausible that the bulging of the fused meniscus is able to bridge the shorter 4.5 μm gap. Second, the pinning sites shown in Figure 3f actually do not appear to be at x = a, but instead the meniscus has advanced somewhat further, possibly driven by additional forces from the interaction between the imbibing meniscus and the droplet edge, and this mechanism again could lead to bridging the shorter 4.5 μm gap. Resolution Comparison. The resolution of the olive oil patterns on micropillars was limited by the length scale of the micropillars themselves, since the pinning effect depends on a combination of both the surface properties and the geometry. Figure 3h shows a fluoropolymer pattern on top of micropillars, demonstrating that it is not possible for this structure to confine the oil to the same resolution as the fluoropolymer pattern, as the first proper pinning sites can be many micrometers into the actual fluoropolymer area. This effect is seen in the overall shape of all the oil patterns in Figure 3d,e and also in Figure 3f, where the shapes of the menisci observed at the pinning sites are affected by where exactly the surface chemistry changes in relation to the geometrical pinning sites. Because of the dependency of the pinning on the specific sites between pillars, the regular lattice and the size scale of the micropillars also lead to a directional dependency on the accuracy of the oil patterns, so that, for example, lines to the direction of the two principal axes of the lattice are more accurate than lines that make awkward angles with the principal axes. This effect can be seen, for example, in the diamond shapes in Figure 3e, where the bottom one making a 45° angle with the axes has a reasonably nice shape (as the oil can get pinned in a regular pattern alternating between pinning one step to the x direction and then one step to the y direction), while the other diamonds have more poorly defined shapes as the meniscus has to travel between the x and y directions in a nonregular fashion to keep up with the overall shape of the oleophilic area. In comparison, the same patterns on the nanopillar surface (Figure 1d) were all accurate, and had no directional dependency.

’ CONCLUSIONS Patterned oil droplets were demonstrated based on surface chemistry patterns on nanopillar and micropillar topographies. On the oleophobic areas, the oil meniscus gets pinned to a metastable state between the pillars and is unable to advance, even though global free energy considerations (Wenzel model) would favor enhanced wetting. The oil patterns on nanopillar topographies accurately replicated the shape of the lithographic surface chemistry pattern, while the accuracy of the oil droplets on micropillar topographies was compromised. Comparison of the nano- and micropillar cases suggests several rules for designing surfaces for patterning oils. The length scale of the topography should be smaller than the resolution of the chemical pattern, as otherwise the shape of the oil pattern is not solely determined by the chemical pattern, as desired, but instead depends on both the chemical pattern and the pillar and lattice geometry. Also, a random array has an advantage over a regular

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lattice, as a regular lattice tends to lead to a directional dependency on the frequency of pinning sites, while a sufficiently small length scale random array should present workable pinning sites equally to all directions. Finally, when working with regular geometries, care has to be taken so that some specific meniscus configurations, such as the 270° angles discussed in this work, are not able to bypass the pinning sites. Controlling the spreading and pinning of oil droplets could lead to some interesting applications such as surface capillary fluidics and accurately defined depositions from oil droplets. In addition, for oil repellent superoleophobic surfaces based on reentrant geometries, controlling the lateral pinning is a useful backup mechanism, so that a single failure of the metastable Cassie state does not result in complete wetting of the surface by lateral imbibition. A key design issue for the future is how to tie the geometric parameters of both random and regular surfaces to the critical (lowest) contact angle that still allows confinement of the oil droplet through the use of metastable nonfilling states.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].fi (V.J.); sami.franssila@tkk.fi (S.F.).

’ ACKNOWLEDGMENT V.J. received financial support from the National Graduate School in Nanoscience (NGS-NANO), and L.S. received financial support from the Academy of Finland (#138674) and the Aalto University Committee for Research Affairs and Doctoral Education. ’ REFERENCES (1) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (b) Parker, A. R.; Lawrence, C. R. Nature 2001, 414, 33–34. (c) Gao, X.; Jiang, L. Nature 2004, 432, 36. (d) Kennedy, R. J. Nature 1970, 227, 736–737. (2) (a) Bhushan, B.; Jung, Y. C.; Koch, K. Langmuir 2009, 25, 3240–3248. (b) Choi, C.; Kim, C. Phys. Rev. Lett. 2006, 96, 066001. (c) Abdelgawad, M.; Wheeler, A. R. Adv. Mater. 2009, 21, 920–925. (d) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Analyst 2004, 129, 284–287. (e) Lapierre, F.; Brunet, P.; Coffinier, Y.; Thomy, V.; Blossey, R.; Boukherroub, R. Faraday Discuss. 2010, 146, 125–139. (f) Verplanck, N.; Coffinier, Y.; Thomy, V.; Boukherroub, R. Nanoscale Res. Lett. 2007, 2, 577–596. (g) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H.A. Adv. Mater. 2011, 23, 673–678. (3) Cassie, A.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (4) Wenzel, R. Ind. Eng. Chem. 1936, 28, 988–994. (5) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457–460. (6) (a) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsuji, K. J. Colloid Interface Sci. 1998, 208, 287–294. (b) Darmanin, T.; Guittard, F. J. Am. Chem. Soc. 2009, 131, 7928–7933. (c) Nguyen, T. P. N.; Brunet, P.; Coffinier, Y.; Boukherroub, R. Langmuir 2010, 26, 18369–18373. (d) Ahuja, A.; Taylor, J. A.; Lifton, V.; Sidorenko, A. A.; Salamon, T. R.; Lobaton, E. J.; Kolodner, P.; Krupenkin, T. N. Langmuir 2008, 24, 9–14. (e) Dufour, R.; Harnois, M.; Coffinier, Y.; Thomy, V.; Boukherroub, R.; Senez, V. Langmuir 2010, 26, 17242–17247. (f) Karlsson, M.; Forsberg, P.; Nikolajeff, F. Langmuir 2010, 26, 889–893. € (7) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, J.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (8) (a) Jokinen, V.; Sainiemi, L.; Franssila, S. Adv. Mater. 2008, 20, 3453–3456. (b) Jokinen, V.; Franssila, S.; Baumann, M. Microfluid. Nanofluid. 201110.1007/s10404-011-0781-x. 7319

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dx.doi.org/10.1021/la200511q |Langmuir 2011, 27, 7314–7320