ARTICLE pubs.acs.org/Langmuir
Fabrication, Surface Properties, and Origin of Superoleophobicity for a Model Textured Surface Hong Zhao, Kock-Yee Law,* and Varun Sambhy Xerox Corporation, Xerox Research Center Webster, 800 Phillips Road, 147-59B Webster, New York 14580, United States
bS Supporting Information ABSTRACT: Inspired by the superhydrophobic effect displayed in nature, we set out to mimic the interplay between the chemistry and physics in the lotus leaf to see if the same design principle can be applied to control wetting and adhesion between toners and inks on various printing surfaces. Since toners and inks are organic materials, superoleophobicity has become our design target. In this work, we report the design and fabrication of a model superoleophobic surface on silicon wafer. The model surface was created by photolithography, consisting of texture made of arrays of ∼3 μm diameter pillars, ∼7 μm in height with a center-to-center spacing of 6 μm. The surface was then made oleophobic with a fluorosilane coating, FOTS, synthesized by the molecular vapor deposition technique with tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane. Contact angle measurement shows that the surface exhibits super repellency toward water and oil (hexadecane) with a water and hexadecane contact angles at 156° and 158°, respectively. Since the sliding angles for both liquids are also very small (∼10°), we conclude that the model surface is both superhydrophobic and superoleophobic. By comparing with the contact angle data of the bare silicon surfaces (both smooth and textured), we also conclude that the superoleophobicity is a result of both surface texturing and fluorination. Results from investigations of the effects of surface modification and pillar geometry indicate that both surface oleophobicity and pillar geometry are contributors to the superoleophobicity. More specifically, we found that superoleophobicity can only be attained on our model textured surface when the flat surface coating has a relatively high oleophobicity (i.e., with a hexadecane contact angle of >73°). SEM examination of the pillars with higher magnification reveals that the side wall in each pillar is not smooth; rather it consists of a ∼300 nm wavy structure (due to the Bosch etching process) from top to bottom. Comparable textured surfaces with (a) smooth straight side wall pillars and (b) straight side wall pillars with a 500 nm re-entrant structure made of SiO2 were fabricated and the surfaces were made oleophobic with FOTS analogously. Contact angle data indicate that only the textured surfaces with the re-entrant pillar structure are both superoleophobic and superhydrophobic. The result suggests that the wavy structure at the top of each pillar is the main geometrical contributor to the superoleophobic property observed in the model surface.
’ INTRODUCTION The phenomenon of superhydrophobicity displayed by nature, including many plants, insects, and waterfowls, has attracted tremendous attention in recent years in both academic and industry and is a subject of intensive investigation.1 A superhydrophobic surface generally exhibits extremely high water repellency and is characterized by a water contact angle >150° and a small sliding angle (5 independent measurements. The typical contact angle measurement error is ∼2°. For dynamic measurements, the advancing angle was measured by slowly adding a small volume of the probe liquid to the droplet on the surface, and the receding angle was measured by slowly removing the probing liquid from the drop (0.15 μL/s). The tilting angle measurement was done by tilting the base unit at a rate of 1°/s with a 10 μL droplet using tilting base unit TBU90E. All measurements were averaged from 5 to 8 measurements, using a pristine area of the substrate for each
Fabrication and Microscopy of the Model Fluorinated Textured Surface. The model textured surface studied in this
work was created by photolithographic technique on Si wafer, followed by chemical modification of the resulting textured Si surface with a fluorosilane, FOTS, synthesized from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane by the molecular vapor deposition technique on the MVD100 reactor. A schematic summarizing the surface texturing and surface modification procedure is given in Scheme 1. 5929
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir
ARTICLE
Table 1. Static and Dynamic Contact Angle Results for Various Textured Surfaces on Silicon Wafer water
hexadecane
contact angle surface
pillar structure
Si wafer
none
textured Si-wafer
dia: ∼2.7 μm
area fraction
coating
(sliding angle)
contact angle adv.
rec.
e
(sliding angle)
adv.
rec.
45°
34°
0°
0°
77°
0°
121°
0°
f
1.0
none
12.3°
2.6°
∼0.16a
none
super wetting
super wetting
1.0
OTSb
109° (13°)
117°
95°
OTSb
157° (8°)
161°
141°
139°g
155°
92°
i-CVD PTFE
152° (10°)
160°
133°
height ∼7 μm side wall: wavy smooth OTS
none
textured OTS
dia: ∼2.7 μm
∼0.16
a
40° (8°) super wetting
height ∼7 μm side wall: wavy smooth i-CVD PTFE
none
textured i-CVD PTFE
dia: ∼2.7 μm
i-CVD PTFEc,d
1.0 ∼0.16
a
c,d
73°g 150°116°g
height ∼7 μm side wall: wavy smooth FOTS
none
textured FOTS
dia: ∼2.7 μm
1.0
FOTS
107° (14°)
116°
95°
74°
65°
∼0.16a
FOTS
156° (10°)
161°
143°
73° (9°) 158° (10°)
161°
121°
∼0.15a
FOTS
152° (12°)
162°
135°
120°g
129°
0
∼0.2a
FOTS
151° (10°)
160°
141°
145° (17°)
158°
111°
height ∼7 μm side wall: wavy textured FOTS (straight side wall)
dia: ∼2.6 μm height ∼9 μm side wall: straight
textured FOTS
dia: ∼3.0 μm
(overhang)
height ∼5 μm side wall: with overhang
The center-to-center distance between pillars is 6 μm and the area fraction is calculated based on SEM measurement of the pillar diameter. b Selfassembled-monolayer synthesized from octadecyltrichlorosilane (ref 14). c i-CVD PTFE coating from GVD Corporation, the as-prepared surface is ∼100 nm thick and comprises 1020 nm fiber-like structure randomly distributed on the surface with rms roughness of ∼4 nm and average Rz of ∼11 nm. d The contact angles for a smooth PTFE surface are: water contact angle ∼112117° (ref 32) and hexadecane contact angle 48° (ref 34). e Drop starts to flow at ∼13°. f Drop starts to flow at ∼5°. g Drop does not slide and is pinned to the surface at 90° titled angle. a
The photomask used in our initial study was intended to create a textured pattern with 3 μm diameter pillars separated by a center-to-center distance of 6 μm on the Si wafer. This pattern size was chosen because the void space in the texture is smaller than the sizes of typical toner particles (510 μm) and toner piles (∼50 μm) and the pillar is large enough to allow for reproduction of the textured pattern consistently using photolithography. Figure 1 shows the SEM micrographs of the textured FOTS surface created according to the steps outlined in Scheme 1. The texture consists of ∼3 μm diameter pillars, ∼7 μm in height with a pitch of ∼6 μm. Careful examination of the texture reveals that the side wall in each pillar is straight but not smooth. It comprises of an ∼300 nm wavy structure from top to bottom (inset of Figure 1). We attribute it to the Bosch etching process because pillars with straight smooth side walls could be obtained with a different etching procedure. With this Bosch etching process, the height of the pillar is conveniently controlled by the number of etching cycles. Contact Angle Measurements. The surface property of the textured FOTS surface was studied by both static and dynamic contact angle measurements. Figure 2 depicts the static contact angle data for the textured FOTS surface with water and hexadecane as the test liquids. The contact angle data for all of the controlled surfaces, namely the Si wafer itself, the textured Si wafer, and the smooth FOTS surface, are
summarized in Table 1 as part of the surface modification study. The contact angles (CA) for the smooth FOTS surface with water and hexadecane are about 107°and 73°, respectively. The result is comparable to those reported by Kobrin and co-workers. 16 In contrast, the textured FOTS surface exhibits extremely high repellency to both water and oil. The water and hexadecane contact angles are at 156° and 158°, respectively. By comparing with the contact angle data from the smooth FOTS surface and from the Si wafer and textured Si wafer surfaces, we conclude that the high contact angles observed for the textured FOTS surface with water and hexadecane is the result of both surface texturing and fluorination. Dynamic contact angle measurements were also performed on the surfaces created in this work. The sliding angles for the textured FOTS surface are found to be ∼10° with both water and hexadecane. The high contact angles coupled with the low sliding angles lead us to conclude that the textured FOTS surface is both superhydrophobic and superoleophobic. Figure 3, panels a and b, shows the advancing and receding contact angle data for the textured FOTS surface with water and hexadecane, respectively. The dynamic contact angle data are also included in Table 1, indicating that the hysteresis is about 20° with water and 40° with hexadecane. The magnitude of the hysteresis is sizable, but is comparable to other superhydrophobic pillar structured surfaces reported in the 5930
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir literature.17 It is important to point out that, the contact angle hysteresis for hexadecane on the textured-FOTS surface is ∼4 times larger than that of the smooth FOTS surface. We attribute it to the sagging of the hexadecane droplet into the void space between the pillars. This interpretation is supported by the SEM microscopy data presented in the next section (Figure 4). A more detailed discussion of the origin of the contact angle hysteresis for this kind of pillar structures will be given in a forthcoming report.18 States of Liquid Droplets on Textured Surfaces. Scheme 3 summarizes the two states commonly used to describe the composite liquidsolid interface between liquid droplets and rough surfaces.1921
ARTICLE
The static contact angles for the droplet at the CassieBaxter state (θCB) and the Wenzel state (θW) are given by eqs 1 and 2, respectively. cos θCB ¼ rf f cos θY þ f 1
ð1Þ
cos θW ¼ r cos θY
ð2Þ
where f is area fraction of projected wet area, rff is solid area fraction, r is the roughness ratio, and θY is the contact angle for the flat surface. In the CassieBaxter state, the liquid droplet “sits” primarily on air with a very large contact angle (θCB). Since the textured FOTS surface in Figure 1 is superoleophobic, we decide to seek direct physical evidence for the existence of the CassieBaxter state. Figure 4a shows the droplet of a molten polyethylene wax ink on the model superoleophobic surface at an elevated temperature of 105 °C. The contact angle and the sliding angle were measured to be about 155° and 33°, respectively, consistent with the hexadecane contact angle data. More importantly, when the wax drop was cooled, we were able to carefully detach the waxy drop from the textured surface and examine the geometry of the composite interface by microscopy. Figure 4b shows the SEM micrograph of the composite interface. The result indicates that wax surface is “flat” with holes corresponding to the location of the pillars. From the thickness of the “rim”, one can estimate the penetration depth of the molten wax drop into the void space between the pillars, which is less than a micrometer. Since the height of the pillar is ∼7 μm, our result positively reveals that the molten ink droplet is indeed sitting on air at the interface on the superoleophobic surface. However, the composite interface is not perfectly flat; the molten ink does penetrate into the void space between the pillars. The result also seems to suggest that there may be more than one pinning location for the penetrating liquid drop. In any event, we have evidence to suggest that this sagging is contributing to the large contact angle hysteresis and details will be discussed in a forthcoming report.18 Scheme 3. States of Liquid Droplets on Rough Surfaces
Figure 3. Advancing and receding contact angle measurements for the textured FOTS surface with (a) water and (b) hexadecane.
Figure 4. (a) Static contact angle for a molten droplet of polyethylene wax on the textured FOTS surface and (b) SEM micrograph of the composite interface of the droplet after solidification. 5931
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir
Figure 5. (a) SEM micrograph of the textured FOTS surface with straight smooth side wall pillars on Si wafer and (b) static contact angles and sliding angles for water and hexadecane on the textured surface in panel a.
Although we have not imaged the composite interface for water on the textured FOTS surface, we believe that the water droplet is also in the CassieBaxter state based on the contact angle and sliding angle data. The contact angle hysteresis with water is a factor of 2 less than that with hexadecane, indicative of less sagging. Again, this point will be discussed in a forthcoming report with additional data.18 Origin of the Superoleophobicity. Due to simplicity of the procedure, model textured surfaces with pillars fabricated on Silicon wafer are very common. What differentiates this work from earlier investigations is the observation of superoleophobicity. For instance, in 2000 Oner and McCarthy17 reported the fabrication of a large number of pillar structures on Silicon wafers and the study of the effects of topography length scales on wettability. In that study, all the pillars had a smooth side wall and the textured surfaces were chemically modified with both hydrocarbon and fluorinated silanes. Very large advancing and receding water contact angles, up to ∼170° and ∼140°, respectively, were reported. The oleophobicity of the surfaces was not tested. Later, Furstner et al.22 reported the creation of micrometer size pillars 24 μm in height by X-ray lithography on silicon wafer followed by surface treatment with Au 1-hexadecanethiol. A water contact angle up to 160° and sliding angle as low as 3° were reported. Regular patterned nano pillars around 110 nm in diameter and 100800 nm tall were fabricated by Martines and co-worker via e-beam lithography on silicon wafers.23 The surface of the nanopatterns was modified by a self-assembledmonolayer synthesized from octadecyltrichlorosilane. The best surface in that study has a water contact angle of ∼164° with very little hysteresis. In all of these studies, the pillars are all straight and smooth, and only superhydrophobicity was reported, not superoleophobicity. In any event, based on the results in this work, superoleophobicity would not have been observed even if it was measured. Very recently, Tuteja and co-workers24 reported the fabrication of electrospun mats that exhibited superoleophobicity. The
ARTICLE
Figure 6. (a) SEM micrograph of the textured FOTS surface with an overhang pillar structure on Si-wafer (inset: higher magnification micrograph showing details of the pillar structure) and (b) static contact angles for water and hexadecane on the textured surface in panel a.
mat was made from F-POSS and PMMA, and the flat surface of the same material was fairly oleophobic (hexadecane contact angle close to 80°). To elucidate the mechanism for superoleophobicity, these authors created the so-called “micro hoodoo” structure on silicon wafer using photolithography and surface fluorination. These authors concluded that the geometry of the re-entrant structure in the pillar is critical to achieving superoleophobicity and the electrospun mat has a similar geometry at the liquidsolid interface. The conclusion is now further substantiated by additional experimental and modeling studies.13,25,26 Careful examination of the highly magnified SEM micrograph (inset of Figure 1) reveals that the side wall in each pillar actually consisting of a wavy structure made up of repeating “loops” ∼300 nm in dimension. Since Tuteja et al. have shown that the re-entrant structure is crucial in achieving superoleophobicity in their study of the electrospun fiber mat and the micro hoodoo structure,13,2426 we suggest that the superoleophobicity observed in this work may be due to (1) the re-entrant structure(s) at the top of the wavy side wall, (2) the entire wavy structure, or (3) some combination of the above possibilities. To differentiate the above possibilities, we fabricated a similar fluorinated, textured surface with the same photomask with the exception that all the pillars have a straight and smooth side wall. The fabrication procedure is outlined in the left-hand-side of Scheme 2. Figure 5 shows the SEM micrograph of the FOTS textured surface with the straight side wall pillars. Included in Figure 5 are the static contact angle data with water and hexadecane, which are also summarized in Table 1. The FOTS textured surface with the straight side wall pillars is superhydrophobic with water contact angle and sliding angle at 152° and 12°, respectively. The result is consistent with those reported by Oner and McCarthy.17 The result from hexadecane is more interesting. The hexadecane contact angle is about 120° and the droplet does not slide even at 90° tilting angle (Figure 5b) which 5932
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir demonstrates the high adhesion between the hexadecane droplet and the textured FOTS surface with the straight side wall pillars. The results imply that either the wavy side wall or the re-entrant structure(s) at the top of the pillar is the key contributor for superoleophobicity. To test this hypothesis, we devised another etching procedure to prepare pillars with an overhang re-entrant structure. The schematic for this procedure is given in the right-hand-side of Scheme 2. Figure 6 shows the SEM micrograph of the textured FOTS surface with the overhang pillar structure. A higher magnified micrograph is given in the inset, clearly showing that the pillar has a straight side wall with a 500 nm thick SiO2 layer on the top to create the re-entrant structure. The static contact angles with both water and hexadecane are included in Figure 6 as well as in Table 1. The sliding angles for water and hexadecane are 10° and 17°, respectively. The data clearly shows that both liquids are in the CassieBaxter nonwetting state on the textured FOTS surface. The overall results lead us to conclude that the reentrant structure on the top of the pillar is a dominating driver for achieving superoleophobicity in the textured FOTS surface. Basically the contact line is pinned around the overhang or the re-entrant structure, enabling a “stable” nonwetting CassieBaxter state. As for the model FOTS textured surface in Figure 1, the SEM result of the composite interface (Figure 4) indicates that there may exist more than one pinning locations among the pillars within the textured surface. Since the liquid drop is shown to penetrate by less than a micrometer into the void space between the pillars, the data suggest that only the first couple of “waves” at the top of the wavy side wall is contributing to the pinning. The conclusion reached in this work not only is consistent with those reported by Tuteja and co-workers13,2426 it is also supported by recent theoretical and experimental results in this general area. For instance, by analyzing the free energy of the composite interface, both Nosonovsky27 and Marmur28 showed independently that a multiscale roughness structure with the convex geometry will likely achieve the nonwetting CassieBaxter state. Ahuja et al.29 reported the fabrication of the so-called “nanonails” and found that these “nanonail” surfaces were able to form the nonwetting composite interfaces with a variety of liquids with surface tensions ranging from 21.8 to 72.0 mN/m, indicating the importance of contact line pinning in the reentrant or overhang structure. Similarly, Kumar and coworkers30 also demonstrated the achievement of high contact angle with benzyl alcohol with an overhanging nanostructure. According to Tuteja and co-workers,25 the robustness of the composite interface for the FOTS textured surface can be estimated by the two breakthrough pressure design parameters, robustness height H* and robustness angle T*. H* is the dimensionless measure of the pressure required to force the sagging height for the liquidvapor interface to reach the maximum pillar height and T* is the dimensionless pressure when the composite interface transitions and wets the next wave of the side wall. Assuming that the hexadecane droplet is pinned at the “first” wave of the pillar structure in Figure 1, the H* and T* values are calculated to be 3744 and 296, respectively. A description of the assumptions and equations used in the calculation are provided as Supporting Information. The breakthrough pressure for the FOTS textured surface can then be estimated from the T* value to be ∼7779 Pa as T* is smaller than H*. Qualitatively, the breakthrough pressure exhibited by the FOTS textured surface is higher than those of the microhoodoo structures and
ARTICLE
is attributable to the smaller surface texture, e.g., 3 μm pillars versus the 520 μm microhoodoo structures. Effect of Chemistry on Surface Hydrophobicity and Oleophobicity. Flat Surface. In addition to the fluorosilane surface FOTS, we also examine the effect of different surface chemistry on the surface properties. The two conformal coatings studied are a self-assembled-monolayer synthesized from octadecyltrichlorosilane (OTS) using the solution coating technique14 and a 100 nm i-CVD PTFE layer made by polymerization of tetrafluoroethylene with the initiated chemical vapor deposition technique.15 On Si-wafer, these two coatings are highly hydrophobic with water contact angles at 109° and 139°, respectively. While the water contact angle for the smooth OTS surface is in agreement with the literature value,31 the water contact angle for the i-CVD PTFE coating on Si-wafer is significantly higher than that of a smooth PTFE film, which is typically ∼112 - 117°.32 The high water contact angle can be attributable to the surface roughness effect.33 Unlike the smooth OTS surface, which is as flat as the Si-wafer upon SEM examination, the i-CVD PTFE coating on Si-wafer does exhibit surface texture in the nano dimension. Figures 7a shows that SEM micrograph of the textured i-CVD PTFE surface on Si-wafer. The inset gives a higher magnified view of the surface on the top of the pillar. The overall results indicate that the i-CVD PTFE coating consists of fibrous structures of 1020 nm in dimensions randomly coated on the Si-wafer surface. In the textured surface, the coating is uniform and conformal covering the entire pillar structure from top to bottom. The oleophobicity of the smooth OTS surface and the i-CVD PTFE coating was examined by the hexadecane contact angle and are about 40° and 73°, respectively. The results are included in Table 1. Although the hexadecane contact angles for the i-CVD PTFE coating and the smooth FOTS surface are about the same, we believe that the i-CVD PTFE coating is less oleophobic intrinsically because the contact angle value tends to be inflated on the rough surface. In fact, the hexadecane contact angle for a smooth PTFE film was measured to be ∼48° and is comparable to the literature value.34 The data in Table 1 therefore indicate that the oleophobicity of the “smooth” surfaces studied decreases in this order: FOTS > i-CVD PTFE > OTS > bare silicon. Textured Surfaces. The results in the middle of Table 1 show that all the modified textured surfaces are superhydrophobic with water CAs range from 152° to 157° and sliding angles range from 8° to 12°. In contrast to the controlled bare textured Silicon surface, which is superhydrophilic, all textured surfaces are superhydrophobic. According to the CassieBaxter equation, liquid droplets will be in the CassieBaxter state if the liquid and the surface have a high degree of phobicity (e.g., θY g90°). Since all the surface coatings studied in this work are highly hydrophobic with water contact angle ranges from ∼105° to 130°, the result is easily attributable to the high hydrophobicity of the surface coatings. From the advancing and receding angles, the hysteresis of these textured surfaces are found to range from 20° to 30° and are comparable to those observed by Oner and McCarthy earlier.17 Results from the hexadecane contact angle data are a lot more interesting. Similar to the controlled bare textured Silicon surface, the textured OTS surface is superoleophilic also. Hexadecane wets the surface immediately upon contact. Since the smooth OTS surface is fairly oleophilic, the result merely indicates that the texture just turns the oleophilic OTS surface to superoleophilic. This is consistent with the surface texturing effect for surface with low contact angle.19 5933
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir
ARTICLE
Figure 7. (a) SEM micrograph of the textured i-CVD PTFE surface (inset: higher magnified view at the top of the pillar) and (b) contact angle data on the textured i-CVD PTFE surface for (i) water, (ii) hexadecane, and (iii) tilting at 90° for a 10 μL hexadecane droplet.
The hexadecane droplet is metastable on the textured i-CVD PTFE surface and the contact angle data is summarized in Figure 7b. The sessile drop has an initial contact angle of ∼150°. Upon slight vibration on the bench or after several minute sitting, the hexadecane drop transitions to a stable sessile drop with a hexadecane contact of ∼116° on the goniometer. Although the oleophobicity of this texture surface is still very high, the hexadecane drop shows extremely high adhesion toward the textured i-CVD PTFE surface. It does not slide when the surface is tilted. It actually sticks on the surface at 90° tilting angle. The receding angle is literally zero, indicating that the contact line is pinned at the wetted area when the bulk of the liquid is removed from the surface. Only the textured FOTS surface is superoleophobic with a hexadecane contact angle of 158° and a sliding angle of 10°. Among the surface modifications investigated in this work, surface oleophobicity increases from bare silicon to OTS to i-CVD PTFE to FOTS. The result clearly indicates that one of the driving factors to achieve superoleophobicity is to have a highly oleophobic surface modification. Although the contact angle of the smooth FOTS surface is only at θY = 73°, the textured FOTS surface is still able to achieve the CassieBaxter state at the liquidsolid interface, owing to contact line pinning at the re-entrant structure. However, as the oleophobicity of the surface coating decreases, the textured surface actually can no longer maintain a stable CassieBaxter state. For example, the hexadecane droplet actually transitions from the CassieBaxter state to the Wenzel state for the textured i-CVD PTFE surface. As
for the OTS coating, due to its oleophilicity, the textured surface simply becomes superoleophilic.
’ SUMMARY AND REMARKS This work describes the fabrication of a model superoleophobic surface on silicon wafer by a simple photolithography and surface fluorination procedure. While similar processes have been used to prepare textured surfaces on silicon wafer, most of the reported surfaces in the literature were superhydrophobic. Relatively little attention has been received on the oleophobic aspects of the surfaces. Surface oleophobicity is a more desirable property for practical applications due to its high oil repellency. Surfaces with superoleophobicity are more valuable, but achieving superoleophobicity is more difficult because the surface tension of hexadecane and oil are lower than that of water (1540 mN/m versus 72.7 mN/m). Careful examination of the structure of our model surface reveals that the pillars created in this work have a wavy side wall due to the Bosch etching process. Each repeating wavy unit is ∼300 nm in dimension. Systematic investigation indicates that both surface chemistry (oleophobicity) and pillar geometry are major contributors to the achieved superoleophobicity. Specifically, the curvature in the re-entrant structure at the top of the pillar presumably is able to pin the contact line of the hexadecane drop, leading to a large static contact angle and a small sliding angle. In this work, using molten polyethylene wax as liquid, we are able to acquire direct evidence that liquid droplets are indeed sitting on air at the 5934
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
Langmuir composite interface of our model superoleophobic surface. The liquidsolid composite interface however is not flat; the liquid drop does sag and penetrate into the void space by about less than a micrometer.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed description of the assumptions and equations used to calculate the two breakthrough pressure design parameters (H* and T*) of the model textured surface. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: 585-422-5229. Fax: 585422-3833.
’ ACKNOWLEDGMENT The authors thank the technical staff at the Cornell NanoScale Science & Technology Facility (CNF) for their support throughout this work.
ARTICLE
(20) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (21) Marmur, A. Langmuir 2003, 19, 8343. (22) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956. (23) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (24) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618. (25) Tuteja, A.; Choi, W.; Mabry, J. M.; G., C.; McKinley, G. H.; Cohen, R. E. Proc. Natl. Acad. Sci. 2008, 105, 18200. (26) Choi, W.; Tuteja, A.; Shreerang, C.; Mabry, G. C.; Cohen, R. E.; McKinley, G. H. Adv. Mater. 2009, 21, 2195. (27) Nosonovsky, M. Langmuir 2007, 23, 3157. (28) Marmur, A. Langmuir 2008, 24, 7573. (29) 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. (30) Kumar, R. T. R.; Mogensen, K. B.; Boggild, P. J. Phys. Chem. C 2010, 114, 2936. (31) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (32) Murase, H.; Fujibayashi, T. Prog. Org. Coat. 1997, 31, 97. (33) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872. (34) Lee, S.; Park, J. S.; Lee, T. R. Langmuir 2008, 24, 4817.
’ REFERENCES (1) For recent reviews, see:Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. andZhang, X.; Feng, S.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. andNeinhuis, C.; Barthlott, W. Annals. Botany 1997, 79, 667. (3) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978. (4) Cheng, Y. T.; Rodak, D. E.; Wong, C. A.; Hayden, C. A. Nanotechnology 2006, 17, 1359. (5) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (6) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (7) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (8) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (9) Artus, G. R. J.; Jung, S.; Zimmermann, J.; Gautschi, H. P.; Marquardt, K.; Seeger, S. Adv. Mater. 2006, 18, 2758. (10) Puukilainen, E.; Rasilainen, T.; Suvanto, M.; Pakkanen, T. A. Langmuir 2007, 23, 7263. (11) Wang, S.; Song, Y.; Jiang, L. Nanotechnology 2007, 18, 1. (12) Lai, Y.; Lin, C.; Huang, J.; Zhuang, H.; Sun, Z.; Nguyen, T. Langmuir 2008, 24, 3867. (13) Tuteja, A.; Choi, W.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F. MRS Bull. 2008, 33, 752. (14) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (15) Tenhaeff, W. E.; Gleason, K. K. Adv. Funct. Mater. 2008, 18, 979. andMartin, T. P.; Lau, K. K. S.; Chan, K.; Mao, Y.; Gupta, M.; O’Shaughnessy, W. S.; Gleason, K. K. Surf. Coat. Technol. 2007, 201, 9400. (16) Kobrin, B.; Chinn, J.; Ashurst, R. W. NSTI Nanotech. 2005, 2, 347. (17) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (18) Zhao, H.; Law, K. Y. manuscript in preparation. (19) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988; and J. Phys. Colloid. Chem. 1949, 53, 1455. 5935
dx.doi.org/10.1021/la104872q |Langmuir 2011, 27, 5927–5935
