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The “Lotus Effect” Explained: Two Reasons Why Two Length Scales of Topography Are Important Lichao Gao and Thomas J. McCarthy* Polymer Science and Engineering Department, UniVersity of Massachusetts, Amherst, Massachusetts 01003 ReceiVed NoVember 28, 2005. In Final Form: February 15, 2006 Surfaces containing 4 × 8 × 40 µm staggered rhombus posts were hydrophobized using two methods. One, using a dimethyldichlorosilane reaction in the vapor phase, introduces a smooth modified layer, and the other, a solution reaction using methyltrichlorosilane, imparts a second (nanoscopic) length scale of topography. The smooth modified surface exhibits contact angles of θA/θR ) 176°/156°. Arguments are made that the pinning of the receding contact line by the post tops (with θA/θR ) 104°/103°) is responsible for the hysteresis. The second level of topography raises the contact angles of the post tops and the macroscopic sample to θA/θR ) >176°/>176° and eliminates hysteresis. The increase in Laplace pressure due to the increase in the advancing contact angle of the post tops is a second reason that two length scales of topography are important.
The ability of the lotus leaf and other natural materials to promote water repellency and self-cleaning by inducing water droplets to roll on them has inspired numerous research groups to prepare synthetic analogues.1-8 Roughness at two or more length scales has been implicated as the cause of this effect,9-12 but no speculation as to why multiple length scales are important has been offered. We show here that there are two reasons: one involving the kinetics of droplet movement, and one involving the thermodynamics of wetting. A smooth silicon surface and one containing staggered rhombus posts13 (Figure 1a) were hydrophobized using a vapor-phase reaction14 with dimethyldichlorosilane that induces no topographical changes. These reaction conditions promote “vertical polymerization”14 and produce a conformal liquid layer of oligodimethylsiloxane that is ∼25 Å thick, as measured by ellipsometry on smooth silicon. The smooth silicon wafer exhibits advancing and receding contact angles of θA/θR ) 104°/103°, and the rhombus-patterned surface exhibits θA/θR ) 176°/156°.15 The high contact angles on the post-containing surface are expected and can be rationalized by the Cassie theory (which does not address hysteresis). Water droplets come to rest on this * Corresponding author. E-mail:
[email protected]. (1) See Hikita, M.; Tanaka, K.; Nakamura T.; Kajiyama, T.; Takahara, A. Langmuir 2005, 21, 7299-7302 and references therein. (2) See Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458-13459 and references therein. (3) See Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644652 and references therein. (4) See Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978-8981 and references therein. (5) See Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662-6665 and references therein. (6) See Qian, B.; Shen, Z. Langmuir 2005, 21, 9007-9009 and references therein. (7) See Zhang, G.; Wang, D.; Gu, Z.-Z.; Mohwald, H. Langmuir 2005, 21, 9143-9148 and references therein. (8) See Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O ¨ ner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395-3399 for a discussion of ultrahydrophobicity and references to prior interest on this topic. (9) Jisr, R. M.; Rmaile, H. H.; Shlenoff J. B. Angew. Chem., Int. Ed. 2005, 44, 782-785. (10) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349-1353. (11) Zhao, N.; Xu, J.; Xie, Q.; Weng, L.; Guo, X.; Zhang, X.; Shi, L. Macromol. Rapid Commun. 2005, 26, 1075-1080. (12) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 21252127. (13) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (14) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274.
surface of hydrophobic posts, but roll when it is tilted slightly and do so with advancing and receding contact angles. When a droplet moves on most surfaces, the contact line advances (on the downhill side) and recedes (on the uphill side), moving over barriers (roughness, chemical heterogeneity, defects) from metastable state to metastable state. These kinetic barriers give rise to contact line pinning and hysteresis. When a droplet rolls on the tops of posts, however, the barriers are very different. Figure 1c shows a two-dimensional (2D) schematic representation of the advancing events that occur during droplet movement (rolling). We can envision no kinetic barrier to advancing. The discontinuous contact line does not move, but, instead, sections of the liquid-vapor interface descend onto the next posts to be wet. The droplet is at θA ) 176°, and the tops of the posts exhibit θA ) 104°, so water should spontaneously advance over the post tops. Receding events are very different (Figure 1d). The droplet is at θR ) 156°, and the post tops exhibit θR ) 103°, so the discontinuous contact line cannot recede across the post tops and must disjoin from entire post tops in concerted events in order to move. This receding contact line pinning, due to the disjoining pressure, gives rise to the 20° hysteresis observed. The obvious strategy for reducing or eliminating this hysteresis is to increase the receding contact angle of the post tops. We accomplished this by introducing nanoscopic topography on this surface (Figure 1b) in the form of a cross-linked hydrophobic polymer network. This network formation chemistry involves the reaction of silicon surfaces with methyltrichlorosilane in toluene solution with controlled amounts of water present as controlled-humidity air. A cross-linked, toluene-swollen methylsiloxane network is formed that phase-separates upon rinsing with ethanol to give the topography shown in Figure 1b. The preparation of this surface is included as Supporting Information and will be discussed in detail in a future publication. Water droplets do not come to rest, and roll effortlessly on this surface (15) We report advancing and receding water contact angle data for three surfaces here that were measured with a Rame´-Hart telescopic goniometer, a Gilmont syringe, and a 25 gauge flat-tipped needle.14 The smooth dimethyldichlorosilane-derived surface was prepared over 100 times in our lab, and contact angles are within 2° of the reported values for all samples. Some samples exhibit less than 1° hysteresis. Values for the surface shown in Figure 1a were determined recently, and measurements were made on four different areas of four samples with all measurements being within 1° of the reported values. We previously reported identical values for this surface.13 We could not accurately measure the contact angles for the surface shown in Figure 1b using this technique, but all values measured were greater than 176°.
10.1021/la0532149 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006
Letters
Langmuir, Vol. 22, No. 7, 2006 2967
Figure 1. (a) Scanning electron microscopy (SEM) image of the surface containing staggered 4 × 8 × 40 µm rhombus posts. (b) SEM image of the surface shown in panel a after being coated with a cross-linked hydrophobic polymer network. (c) Contact line events upon advancing. (d) Contact line events upon receding.
containing these two length scales of topography. Contact angles are θA/θR ) >176°/>176° with no apparent hysteresis. The dual length-scale topography affects the kinetics of contact line recession by lowering the transition state energy between metastable states. There is a thermodynamic effect in addition to this kinetic one that involves very different physics: the Laplace pressure at which water intrudes between the posts.16 For a given geometry (area and contact perimeter of a unit cell), the Laplace pressure is only a function of the advancing contact angle (-cos θA). Increasing the contact angle of the post tops from 104° to >176° increases the Laplace pressure by a factor greater than 4. This does not impact the surfaces described here at ambient pressure, but would allow spacing the posts at greater distances and taking greater advantage of the “apparent slip” on the air between posts that yields drag reduction. (16) Dettre, R. H.; Johnson, R. E., Jr. S. C. I. Monograph No. 25; Society of Chemical Industry: London, 1967; pp 144-163.
We wish to emphasize that the process of receding has a greater activation energy than the process of advancing for the surface shown in Figure 1a. This underscores the points that we made in the past13 that a high advancing contact angle does not indicate hydrophobicity and that hydrophobicity should be quantitated in terms of contact angle hysteresis. Acknowledgment. We thank Toyota and the National Science Foundation-sponsored Materials Research Science and Engineering Center and Research Site for Educators in Chemistry for financial support. Supporting Information Available: Experimental details of the preparation of the cross-linked hydrophobic polymer network. This material is available free of charge via the Internet at http://pubs.acs.org. LA0532149
