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Langmuir 2006, 22, 11072-11076
Superhydrophobic Effects of Self-Assembled Monolayers on Micropatterned Surfaces: 3-D Arrays Mimicking the Lotus Leaf Jessica Pacifico, Kohei Endo, Stephanie Morgan, and Paul Mulvaney* Chemistry School, UniVersity of Melbourne, ParkVille VIC 3010, Australia ReceiVed April 6, 2006. In Final Form: September 14, 2006 The contact angle of water has been measured on a series of self-assembled monolayers (SAM) on thermally evaporated and sputter coated silver surfaces. It is found that micropatterning the surface using nanosphere lithography leads to large increases in the contact angle and generates superhydrophobic surfaces with contact angles >150°. The type of functional groups on the SAMs, the metal island size, and the metal island thickness all contribute to the measured contact angle. The maximum contact angle found was 161° for a fluorinated alkanethiol on 80 nm thick silver islands.
Introduction In classical colloid science, the measured contact angle of a liquid droplet on a surface is described by Young’s equation, which is a function only of the surface free energies of the various interfaces involved. The contact angle approaches zero for a liquid droplet adsorbed to a surface that it spontaneously wets, while the maximum angle can, in principle, approach 180° for a liquid on a suitably solvophobic surface. In practice, the maximum value of the contact angle for water droplets on hydrophobic surfaces is usually found to be about 110°, a value obtained on PTFE,1 while a value of about 115° is usually reported for water on octadecanethiol self-assembled monolayers on gold surfaces.2,3 An interesting problem is the relationship between the contact angle and the mechanical stability of the drops on the surface. Most hydrophobic surfaces do not actively self-clean because adsorption hysteresis still results in mechanical attachment of the liquid. As a result, attention has focused on generating even higher contact angles by fabricating artificially rough surfaces combined with hydrophobic surface chemistry.4-17 This * To whom correspondence should be addressed. E-mail: mulvaney@ unimelb.edu.au. Fax: 61-3-9347-5180. Phone: 61-3-8344-6486. (1) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oener, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (2) (a) Amirfazli, A.; Kwok, D. Y.; Gaydos, J.; Neumann, A. W. J. Colloid Interface Sci. 1998, 205, 1. (b) Iwami, Y.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Electroanal. Chem. 2004, 564, 77. (c) Li, Y.; Moon, K. S.; Wong, C. P. J. Electron. Mater. 2005, 34, 266. (d) Yan, L.; Wang, K.; Wu, J.; Ye, L. J. Phys. Chem. B 2006, 110 (23), 11241. (3) Schoenherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (4) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (5) (a) Cassie, A. B. D.; Baxter, S. Trans Faraday Soc. 1944, 40, 546. (b) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (6) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818. (7) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21 (19), 8978. (8) Burton, Z.; Bhushan, B. Nano Lett. 2005, 5 (8), 1607. (9) Li, Y.; Cai, W.; Cao, B.; Duan, G.; Sun, F.; Li, C.; Jia, L. Nanotechnology 2006, 17, 238. (10) Shiu, J. Y.; Kuo, C. W.; Chen, P.; Mou, C. Y. Chem. Mater. 2004, 16 (4), 561. (11) Zhang, J.; Huang, W.; Han, Y. Langmuir 2006, 22 (7), 2946. (12) Li, Y.; Cai, W.; Duan, G.; Cao, B.; Sun, F.; Lu, F. J. Colloid Interface Sci. 2005, 287, 634. (13) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713-4716. (14) Wei, H.; Dean, S. L.; Parkin, M. C.; Nolkrantz, K.; O’Callaghan, J. P.; Kennedy, R. T. J. Mass. Spectrosc. 2005, 40, 1338. (15) Fuerstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21 (3), 956. (16) Xia, F.; Feng, L.; Wang, S.; Sun, T.; Song, W.; Jiang, W.; Jiang, L. AdV. Mater. 2006, 18, 432.
approach mimics the behavior of the lotus leaf and other desert plants,18 which use fine filaments on their surface to minimize water adsorption. By combining both these techniques, contact angles greater than 150° can be realized, which are called superhydrophobic. Such surfaces exhibit little contact angle hysteresis (typically
