Langmuir 2006, 22, 2433-2436
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Condensation on Ultrahydrophobic Surfaces and Its Effect on Droplet Mobility: Ultrahydrophobic Surfaces Are Not Always Water Repellant Kevin A. Wier and Thomas J. McCarthy* Polymer Science and Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003 ReceiVed September 22, 2005. In Final Form: December 8, 2005 The condensation of water was studied on topography-based ultrahydrophobic surfaces containing hydrophobized silicon pillars. Optical microscopy showed that water nucleated and grew both on top of and between the pillars. As condensation progressed, water between the pillars became unstable and was forced upward to the surface. Macroscopic water droplets on top of the pillars coalesced with condensed water that remained between the pillars, pinning the droplets at their three-phase contact line. Dynamic contact angle measurements on ultrahydrophobic surfaces wet with condensation revealed a dramatic increase in hysteresis compared to that on dry surfaces, leading to a corresponding decrease in water drop mobility.
Many researchers have recently become interested in ultrahydrophobic surfaces because a variety of experimental techniques have allowed the preparation of samples with controlled roughness.1-12 Possible applications include drag reduction in microfluidic devices13-15 and so-called “self-cleaning” surfaces16-20 on which liquid droplets easily roll and take dirt and contaminants off of the surfaces. For most of the applications of these surfaces, the important issue is not the fact that the contact angle is high but how readily the liquid moves on the surface, which is a dynamic process. In the work reported here, condensation and its effect on liquid droplet mobility are studied on several ultrahydrophobic surfaces. We show that condensed water can dramatically reduce water drop mobility on ultrahydrophobic surfaces as evidenced by dynamic contact angle measurements. The motion of liquid droplets on inclined surfaces has been studied for decades.21,22 It has been shown that a force, most * Corresponding author. E-mail:
[email protected]. (1) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 21252127. (2) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 68006806. (3) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 743-744. (4) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (5) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818-5822. (6) Quere, D. Physica A 2002, 313, 32-46. (7) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457-460. (8) Shiu, J. Y.; Kuo, C. W.; Chen, P. L.; Mou, C. Y. Chem. Mater. 2004, 16, 561-564. (9) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549-5554. (10) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937-943. (11) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782-785. (12) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235-3237. (13) Torkkeli, A. Ph.D. Dissertation, Helsinki University of Technology, 2003. (14) Kim, J.; Kim, C.-J. In Proc. IEEE Int. Conf. MEMS 2002, 479-482. (15) Ou, J.; Perot, B.; Rothstein, J. P. Phys. Fluids 2004, 16, 4635-4643. (16) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (17) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857-1860. (18) Blossey, R. Nat. Mater. 2003, 2, 301-306. (19) Furstner, R.; Neinhuis, C.; Barthlott, W. Nachr. Chem. 2000, 48, 24-28. (20) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956-961. (21) Macdougall, G.; Ockrent, C. Proc. R. Soc. London, Ser. A 1942, 180, 151-173.
often gravity, must be applied to overcome the surface tension forces holding the liquid to the surface.23-25 Equation 1,
mg sin R ) kwγlv(cos θR - cos θA)
(1)
where m is the droplet mass, g is the acceleration of gravity, k is a constant, w is the droplet contact diameter, θR and θA are the receding and advancing contact angles respectively, and γlv is the liquid vapor surface tension, can be used to predict R, the minimum tilt angle required for the droplet to move. We emphasize that this equation predicts that by decreasing contact angle hysteresis, the difference between the advancing and receding contact anglessthe inclination angle required to induce drop motionsis decreased. In fact, if there is no hysteresis, then a droplet of liquid should move spontaneously on a horizontal surface; we have reported examples of such surfaces.26 However, if the hysteresis is high enough, then a droplet will remain pinned even though the static contact angle is >160° on a vertical surface. Contact angle hysteresis on hydrophobic rough surfaces can vary dramatically depending on how the liquid wets the surface. Wenzel described a wetting regime where the liquid penetrates between the surface asperities on a rough surface.27 It was later shown that such surfaces often have high contact angle hysteresis.28-31 The three-phase contact line remains pinned because there are high-energy barriers between metastable states that the contact line can adopt. A liquid droplet can also rest on top of surface features as described by Cassie and Baxter.32,33 (22) Kawasaki, K. J. Colloid Sci. 1960, 15, 402-407. (23) Furmidge, C. G. J. Colloid Sci. 1962, 17, 309-324. (24) Dussan, E. B.; Chow, R. T. P. J. Fluid Mech. 1983, 137, 1-29. (25) ElSherbini, A. I.; Jacobi, A. M. J. Colloid Interface Sci. 2004, 273, 556565. (26) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399. (27) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988-994. (28) Johnson, R. E.; Dettre, R. H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1969; Vol. 2, pp 85-153. (29) Johnson, R. E.; Dettre, R. H. In AdVances in Chemistry Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43, pp 136-144. (30) Dettre, R. H.; Johnson, R. E. In AdVances in Chemistry Series; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43, pp 136-144. (31) Dettre, R. H.; Johnson, R. E. In Wetting; Society of Chemical Industry: London, 1967; pp 144-155. (32) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551. (33) Cassie, A. B. D.; Baxter, S. Nature 1945, 155, 21-22.
10.1021/la0525877 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/16/2006
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In this wetting regime, the hysteresis is generally lower because the droplet is sitting on a composite surface of air and solid and the energy barriers limiting the discontinuous contact line movement are small. Therefore, according to eq 1, a droplet would move more easily (at a lower tilt angle) on the lowhysteresis surface in the Cassie-Baxter wetting regime than on the higher-hysteresis Wenzel-type surface. Recently, it has been shown that a transition from the CassieBaxter to Wenzel wetting regime can be observed on some ultrahydrophobic surfaces.6,7,34-40 Starting from the CassieBaxter regime, water cannot wet between the surface features unless the hydrostatic pressure is increased above a certain critical pressure, Pc, given as
Lcp Pc ) -γlv cos θo Acp
(2)
where γlv is the liquid/vapor surface tension, θo is the advancing contact angle of the material, Lcp is the total solid perimeter in a repeat unit of the surface configuration, and Acp is the area of air space in the repeat unit.13,31 The critical pressure can be overcome in a variety of ways. Lafuma and Quere showed that a wetting transition could occur by pressing a liquid droplet between two ultrahydrophobic surfaces with enough force to overcome the critical pressure.7 He et al. discussed this wetting transition on an ultrahydrophobic poly(dimethylsiloxane) surface.34,38 They observed that a water drop placed gently on the surface sat on top of the asperities but it wet between the features when it fell onto the surface from a certain height. In both cases, a pressure-induced transition from the Cassie-Baxter to Wenzel wetting regime increases hysteresis and decreases droplet mobility. Condensation on ultrahydrophobic surfaces has also been shown to decrease drop mobility and increase hysteresis. Quere and Lafuma briefly mentioned experiments where water droplets were grown by condensation on ultrahydrophobic surfaces.7,36 Dynamic contact angle measurements showed that the hysteresis was nearly 100°, compared to about 5° on the dry surface. Narhe and Beysens noted a similar increase in hysteresis while studying the growth of condensed water droplets on ultrahydrophobic surfaces.41 It has even been shown that condensation on a lotus leaf, an inspiration and model for ultrahydrophobic surfaces, can increase contact angle hysteresis and severely limit droplet mobility.42 We have studied condensation on a series of topographybased ultrahydrophobic surfaces containing hydrophobized silicon posts. The details of the photolithographic preparation and dimethyldichlorosilane modification of the silicon post surfaces have been reported.4 The various surfaces are designated WXY,Z, where W is the post width, Y and Z are the dimensions of the unit cell, and X is the post shape (SP for square post, StP for four-arm star, IP for indented square post) (Figure 1). All of the posts are 40 µm in height. Custom-built cold stages were used to cool the samples for both contact angle measurements and optical microscopy. Contact angles were measured using a custom-built goniometer with a CCD camera allowing video capture. A 4-6 µL water drop was placed on the sample using (34) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999-5003. (35) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220-226. (36) Quere, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109-1112. (37) Patankar, N. A. Langmuir 2003, 19, 1249-1253. (38) He, B.; Lee, J.; Patankar, N. A. Colloids Surf., A 2004, 248, 101-104. (39) Patankar, N. A. Langmuir 2004, 20, 8209-8213. (40) Patankar, N. A. Langmuir 2004, 20, 7097-7102. (41) Narhe, R. D.; Beysens, D. A. Phys. ReV. Lett. 2004, 93, 76103. (42) Cheng, Y. T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 144101.
Figure 1. (a) Diagram showing the dimensions used to designate the various silicon posts surfaces: W is the width of the post, and Y and Z are the dimensions of the unit cell. (b) Illustrations of the top of an indented square post, IP, and a four-armed star post, StP. (c) Representative SEM of lithographically prepared silicon posts. Table 1. Dynamic Contact Angle Measurements on Various Hydrophobic Silicon Post Surfaces Before (Dry) and After (Wet) Condensationa surface 8 µm
32,32 µm
dry wet 8 µm SP32,80 µm dry wet 2 µm dry SP4,8 µm wet 8 µm SP16,32 µm dry wet 8 µm 16,32 µm dry IP wet 8 µm StP16,32 µm dry wet 32 µm square dry hollow wells wet SP
θA (deg) θR (deg) θA - θR 172 120 170 113 168 97 172 137 171 128 171 138 152 140
135 24 145 51 130 0 123 72 128 0 140 0 127 39
37 96 25 62 38 97 49 65 43 128 31 138 25 101
f1
Λ (µm-1)
0.13
0.063
0.05
0.025
0.25
0.50
0.25
0.13
0.19
0.15
0.13
0.14
0.13
0.12
a f1 is the fractional contact area between a liquid droplet and the post tops; Λ is the contact line density44 (i.e., the post top perimeter per unit area).
a syringe, and the advancing and receding contact angles were recorded by adding or withdrawing water from the drop, respectively. Multiple measurements were made per sample during the course of condensation, and the same spot was never used twice. The relative humidity was maintained at 100% for contact angle measurements. A representative scanning electron microscopy image of the lithographically prepared dimethyldichlorosilane hydrophobized silicon posts is shown in Figure 1c. Water droplets placed on such surfaces rest on top of the posts on a solid/air composite surface. We have previously shown that the surfaces are ultrahydrophobic and have reasonably low contact angle hysteresis.4 Water droplets move easily when the surfaces are slightly tilted. The growth of condensed water was observed on a variety (Table 1) of these surfaces by optical microscopy. A custombuilt cold stage was used to cool the samples below the dew point. Figure 2 shows a succession of optical micrographs of condensation on sample 2 µmSP4,8 µm. The water droplets initially nucleate on the sides of the posts and grow to span the gap between neighboring features. The droplets grow larger as more water condenses, but they are laterally confined by the hydrophobic posts. Surface tension forces keep the drop
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Figure 2. (a-d) Sequential optical micrographs of condensed water growth on hydrophobized silicon posts: 40 µm tall, 8 µm wide, 32 µm × 80 µm unit cell. White arrows indicate drops that protrude above the tops of the posts.
Figure 3. Large condensed water droplets on a surface of hydrophobized silicon posts: 40 µm tall, 2 µm wide, 4 µm × 8 µm unit cell. Black arrows indicate distortions of the contact line. Darker areas, both underneath and between the drops, indicate where water penetrates between the posts.
reasonably spherical, so instead of growing around the posts, the droplets grow upward and protrude above the surface features. Figure 3 shows the same surface after condensed water droplets, with diameters of hundreds of micrometers, have formed. The droplets function as lenses and reveal the area beneath them that consists of a mosaic of areas where either water or air is present between the posts. These droplets are in both the Wenzel and Cassie-Baxter wetting regimes, sitting on both a solid/air and solid/liquid composite surface. The contact lines of the large droplets are contorted and not circular as pointed out by the arrows. Water between the posts has coalesced with the droplets and has pinned the contact line. As shown by contact angle measurements (Figure 4), the mobility of the liquid droplets on the surface is decreased dramatically. Contact angle measurements were taken on the dry surface and at intervals during condensation. The advancing and receding contact angles both decreased as more water condensed. Figure 4 shows contact angles on the same surface exhibited in Figures 2 and 3. On the dry surface, shown in Figure 4a, the advancing and receding contact angles are 168 and 130° respectively.
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Figure 4. Advancing and receding contact angles on a surface with hydrophobized silicon posts: 40 µm tall, 2 µm wide, 4 × 8 µm unit cell: (a) a dry surface and (b) a surface with condensed water.
Although there is hysteresis, water droplets slide easily on this surface when it is tilted. Cooling this sample below the dew point induces dramatic changes in wettability; as shown in Figure 4b, the dynamic contact angles decrease substantially after water has condensed. The advancing angle still shows a hydrophobic value of 97°, but the receding angle decreases to 0°, resulting in an enormous increase in hysteresis. This increase in hysteresis upon condensation is similar to that observed by others.7,36 Looking back at eq 1, an increase in hysteresis leads to higher surface tension forces opposing drop motion and a decrease in liquid mobility. The optical microscopy in Figures 2 and 3 and contact angle behavior in Figure 4 suggest very strongly that water condensed between surface features pins the contact lines of the macroscopic water droplets. The large water droplets placed on the surface no longer rest only on top of the surface features but also intrude between the surface features. We believe that the increase in the contact line length is more important than the increase in the solid/liquid contact area underneath the drop in regard to an increase in hysteresis. The dynamic contact angles for a number of other surfaces based on hydrophobized silicon pillars before and after condensation are shown in Table 1.43 The dry surfaces are ultrahydrophobic with relatively low hysteresis, varying from 25 to 49°. The advancing and receding contact angles decrease in all cases after water condensation with some surfaces having contact angle hysteresis values over 100°. Although the advancing contact angles all remained hydrophobic (>90°), all of the receding angles dropped below 90°. The values of the receding contact angles varied from complete wetting, 0°, to 72°. Surfaces with low hysteresis but minimal roughness (i.e., not ultrahydrophobic) would be more appropriate choices for applications where liquid droplet mobility needs to be maintained during condensation. We hydrophobized a polished silicon wafer with dimethyldichlorosilane via the same vapor-phase reaction used to modify the post surfaces. Dynamic contact angle measurement showed that the dry surface, θA ) 101°, had very little hysteresis, 2°, which was maintained even after water (43) Optical micrographs of condensation on each surface can be found in the Supporting Information. (44) Extrand, C. W. Langmuir 2004, 20, 5013-5018.
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condensed on it (Supporting Information). Water droplets moved easily on both the dry and wet surfaces when they were slightly tilted. We have examined condensation on a variety of ultrahydrophobic surfaces and found that it causes an increase in contact angle hysteresis and a resultant decrease in macroscopic water drop mobility. The water condenses both between and on top of the surface features. A macroscopic water drop coalesces with the condensed water and undergoes a wetting transition from sitting on top of the posts to wetting between them. In applications where liquid mobility is important and condensation is likely to
Letters
occur, an alternative to ultrahydrophobic surfaces would be a smooth surface with little or no hysteresis. Acknowledgment. This work was supported by the National Science Foundation-sponsored Materials Research Science and Engineering Center and Research Site for Educators in Chemistry. We thank A. Crosby for the use of his contact angle goniometer. Supporting Information Available: Optical microscopy of condensation on other ultrahydrophobic samples. This material is available free of charge via the Internet at http://pubs.acs.org. LA0525877
