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Novel Biomimetic Surface Based on a Self-Organized Metal-Polymer Hybrid Structure Daisuke Ishii,*,†,‡ Hiroshi Yabu,‡,§ and Masatsugu Shimomura†,‡ WPI-AIMR and IMRAM, Tohoku UniVersity, Aoba-ku Katahira 2-1-1, 980-8577 Sendai, Japan, and CREST, Japan Science and Technology Agency, Hon-cho 4-1-8, 332-0012 Kawaguchi, Japan ReceiVed March 11, 2009 ReVised Manuscript ReceiVed April 6, 2009
Biomimetic surfaces1 mimicking some plants and insects are pertinent to functions of the liquid-solid interface such as superhydrophobicity, water droplet moving, and wettability control. Lotus leaves possess superhydrophobic surfaces based on the bumpy microstructures.2 Butterfly wings composed of oriented microstructured surfaces present anisotropic water droplet moving.3 Gecko’s feet4 and rose petals5 show superhydrophobic surfaces with water droplet adhesion driven by large van der Waals forces of those hierarchical microstructures. With a focus on a large-size-structured surface, hydrophilic-hydrophobic submillimeter patterned surfaces of a desert beetle’s back cause water droplet collection on the hydrophilic domains.6 Wettability, especially superhydrophobicity, of biomimetic surfaces is subjcted to a relationship between the size of a surface structure and affinity to a water droplet as shown in Figure 1. Lotus leaves are composed of a microstructured bumpy surface that have a strong water repellent property (lotus effect). Superhydrophobic surfaces that mimic lotus leaves, including a fractal surface,7 a nanopin surface,8 and the other surfaces,9 have been reported. These surfaces were prepared by growing crystals of organic or inorganic materials, lithography, and self-organization. Most superhydrophobic surfaces employ microscopic structures to repel water. Recently, highly adhesive superhydrophobic structured surfaces mimicking gecko’s feet, rose petals, and others are prepared by a replica method, crystal growth, and dry * Corresponding author. E-mail:
[email protected]. † WPI-AIMR, Tohoku University. ‡ Japan Science and Technology Agency. § IMRAM, Tohoku University.
(1) (a) Koch, K.; Bhushan, B.; Barthlott, W. Soft Matter 2008, 4, 1943– 1963. (b) Xia, F.; Jiang, L. AdV. Mater. 2008, 20, 2842–2858. (2) (a) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (b) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667–677. (3) Zheng, Y.; Gao, X.; Jiang, L. Soft Matter 2007, 3, 178–182. (4) Cho, W. K.; Choi, I. S. AdV. Funct. Mater. 2008, 18, 1089–1096. (5) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Xia, F.; Jiang, L. Langmuir 2008, 24, 4114–4119. (6) Parker, A. R.; Lawrence, C. R. Nature (London) 2001, 414, 33–34. (7) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125– 2127. (8) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. J. Am. Chem. Soc. 2005, 127, 13458–13459. (9) (a) Ma, M.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193–202. (b) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621–633.
Figure 1. Relationship between affinity to a water droplet and structure sizes of biomimetic surfaces. A macroscopic hydrophilic-hydrophobic patterned surface mimicking beetle’s back has very strong affinity on wide hydrophilic domains (red), showing strong water droplet adhesion caused by its hydrophilicity. On the other hand, a novel biomimetic hybrid microscopic structured surface has conflicting affinities of adhesion on hydrophilic microdomains (red) and is repellent on hydrophobic microstructured surfaces (black), resulting in controllable water droplet adhesion on the superhydrophobic surface adjusted by the density of the hydrophilic domain.
etching.4,5,10 Adhesion forces of these microstructured surfaces were attributed to a large van der Waals force. On the other hand, a desert beetle’s back is part of its unique water collection system. Submillimeter-sized hydrophilic domains are distributed on waxy hydrophobic bumps. Water droplets repelled by the waxy hydrophobic bumps are collected onto the wide hydrophilic domains because of very strong affinity to a water droplet. This approach to water collection has been imitated by top-down hydrophilic-hydrophobic patterning of a glass surface.11 This patterned surface provides siteselective water droplet adhesion on the macroscopic hydrophilic domains, which are approximately 500 µm in diameter. As summarized in Figure 1, biomimetic surfaces such as the microscopic structured surface with strong water repellency, the hierarchical microscopic structured surface with water droplet adhesion, and the hydrophilic-hydrophobic macroscopic patterned surface with very strong affinity on wide hydrophilic domains have been reported. Our aim is to make a novel biomimetic microscopic structured surface with conflicting affinities of adhesion on hydrophilic microdomains that are repellent on a superhydrophobic microstructured surface. It is expected that water droplet adhesion on the biomimetic superhydrophobic surface is controlled by density of the hydrophilic microdomain and affinity to water droplets. In this letter, we fabricate a novel biomimetic hybrid surface based on superhydrophobic polymer microscopic structured surfaces with metal microdomains as hydrophilic components. Adhesion force of the novel hybrid metal-polymer surface is driven by affinity between water (10) (a) Lai, Y.; Lin, C.; Huang, J.; Zhuang, H.; Sun, L.; Nguyen, T. Langmuir 2008, 24, 3867–3873. (b) Li, Y.; Zheng, M.; Ma, L.; Zhong, M.; Shen, W. Inorg. Chem. 2008, 47, 3130–3143. (c) Liao, K.-S.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2008, 24, 4245– 4253. (d) Winkleman, A.; Gotesman, G.; Yoffe, A.; Naaman, R. Nano Lett. 2008, 8, 1241–1245. (e) Jin, M.; Feng, X.; Feng, L.; Sun, T.; Zhai, J.; Li, T.; Jiang, L. AdV. Mater. 2005, 17, 1977–1981. (11) Zhai, L.; Berg, M. C.; Cebeci, F. C¸.; Kim, Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 1213–1217.
10.1021/cm9006926 CCC: $40.75 2009 American Chemical Society Published on Web 04/15/2009
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Figure 2. SEM images showing top and side views of the HP film, the plated HP film, and the hybrid dome-spike film, respectively (Scale bar: 10 µm).
and the hydrophilic metal domains. It is expected that water droplet adhesion is regulated by varying its affinity which is controlled by density of the hydrophilic metal component. The hybrid biomimetic surface consists of two different microstructures: hydrophilic metal domes and hydrophobic polymer spikes. The hybrid surface was prepared by nickel electroless plating12 of self-organized honeycomb-patterned polystyrene-based films (HP film) formed by breath figuring,13 and successive simple peeling14 of the plated HP film (see Figure S1, Supporting Information). Figure 2 shows SEM images of the HP film, the nickel-plated HP film, and the hybrid dome-spike surface after peeling. The HP film consists of hexagonally ordered array of microcavities having a center-to-center interval of about 10 µm and many narrow pillars that hold the top and bottom porous layers together (Figure 2a). After plating, some pores were covered by the nickel layer, but other pores were fully plated into their cavities, resulting in the formation of metal micropots (Figure 2b). The hybrid dome-spike surface was then formed by peeling; the surface consists of a hexagonally ordered array of hydrophobic polystyrene spikes and hydrophilic nickel microdomes (Figure 2c). Each metal dome was the base of a metal micropot in the fully plated cavity. The wettability of the HP film in the catalytic solution was a key factor in forming the metal pot. The contact angles of the catalytic solution of 5.0 mg on the HP film at 25 and 60 °C were 118 ( 5° and 96 ( 3°, respectively. Wettability at 60 °C is better than that at 25 °C, because an interfacial energy between liquid and substrate becomes large with increasing liquid temperature. The density of the metal pots was controlled by the wettability of the catalytic solution, and the density of the metal domes could be regulated by the catalyzation conditions. At the low temperature of 25 °C, no metal domes were formed in the polymer spike array. As Figure 3 shows, the dome density in the hybrid film increases linearly from 0% to 30% with increasing catalytic solution temperature. These results indicate that the dome (12) (a) Ishii, D.; Nagashima, T.; Udatsu, M.; Sun, R.-D.; Ishikawa, Y.; Kawasaki, S.; Yamada, M.; Iyoda, T.; Nakagawa, M. Chem. Mater. 2006, 18, 2152–2158. (b) Yabu, H.; Hirai, Y.; Shimomura, M. Langmuir 2006, 22, 9760–9764. (c) Ishii, D.; Yabu, H.; Shimomura, M. Colloids Surf., A 2008, 313-314, 590–594. (13) (a) Maruyama, N.; Koito, T.; Nishida, J.; Sawadaishi, T.; Cieren, X.; Ijiro, K.; Karthaus, O.; Shimomura, M. Thin Solid Films 1998, 327329, 854–856. (b) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071– 6076. (14) Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235–3237.
Figure 3. Graph of the metal dome density against the catalytic solution temperature. Insets show SEM images of the hybrid surfaces with metal dome densities of 0, 12, and 25%.
Figure 4. Contact angles and sliding angles of water droplet on the hybrid dome-spike surfaces. Droplets on hybrid surfaces with dome densities over 15% did not fall off when the surfaces were inverted. The inset shows a photograph of the water droplet on an inverted surface with a dome density of 25%.
density in the superhydrophobic hybrid dome-spike surface can be easily controlled by merely varying the catalytic solution temperature before electroless plating. Figure 4 shows plots of a contact angle (CA) and a sliding angle (SA) of 5.0-mg water droplet against the dome density of the hybrid surfaces. The CA and SA of the hybrid surface without domes were 160 ( 2° and less than 5°, respectively; these angles are almost the same as those of a superhydrophobic pin-cushion-structured HP film without metal plating. Even though the dome density increased, the CA decreased slightly up to ca. 145°. This is conformance with Cassie’s model of hydrophobicity, in which air cavities in the HP film repel water and the wettability decreases with decreasing the air cavity because of formation of the metal domes. By contrast, the SA increased dramatically with increasing metal dome density with a high surface free energy. Water droplets on hybrid surfaces with dome densities of over 15% were strongly pinned and did not fall off even when the hybrid surface was inverted (inset of Figure 4). These results clearly demonstrate that the metal domes increase the adhesion of water droplets to the superhydrophobic surface and that the
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Figure 5. Plots of the apparent contact areas and sliding angles of the hybrid dome-spike surface (dome density: 12%) against the mass of water droplet (a) before and (b) after thermal treatment.
adhesion properties of hybrid dome-spike surfaces can be controlled by varying the metal dome density. The dependence of the adhesion force on the dome density of the hybrid dome-spike surfaces is shown in Table S1 (Supporting Information). The apparent adhesion force per unit area was calculated from the 5.0-mg water droplet and the apparent contact area as observed by an optical microscope. The apparent adhesion force of the hybrid films increased with the dome density. The apparent adhesion force of a single metal dome was calculated by dividing the apparent adhesion force by the number of metal domes per unit area. The apparent adhesion force of a single metal dome was about 10 nN and independent of the metal dome density of the hybrid surfaces. This result indicates that, in the case of the same mass of water droplets on the same hybrid surface, the adhesion force is strong with increasing apparent contact area. Control of the water droplet adhesion was achieved by thermal treatment. Figure 5 shows the effect of droplet size before and after thermal treatment on the apparent contact area and the SA. The apparent contact area at 25 °C increases with increasing the mass of water droplets on the hybrid film (dome density: 12%) before thermal treatment, whereas the SA decreases because water droplets’ own weight becomes
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large (Figure 5a). On the other hand, different curves of the apparent contact area and the SA were depicted when the water droplet on the hybrid surface was reduced by thermal treatment at 60 °C. A 7.0-mg water droplet was placed on the hybrid surface, and the surface was then heated at 60 °C for a few minutes. The mass of the droplet decreased gradually with heating time due to evaporation. The SA and apparent contact area at 25 °C after thermal treatment are plotted against the mass of the water droplet (Figure 5b). The apparent contact area remained the same even though the droplet mass was reduced from 7.0 to 5.5 mg by evaporation. The thermal-treated water droplet of 5.5 mg remained strongly attached to the hybrid film even when the surface was inverted, whereas the same mass of a water droplet before thermal treatment rolled off the hybrid surface with a sliding angle of 75°. The adhesion force of the water droplet is only dependent on the apparent contact area. As a result of the strong affinity between water and the metal domes, once water contacts the metal domes, its apparent contact area is not so much changed by thermal treatment. This result means that the water droplet adhesion is only driven by the balance between the apparent contact area and the mass of its water droplet. The apparent contact area decreased gradually on further evaporation of the droplet. After complete drying of the surface, a new 5.5 mg water droplet at 25 °C without heating was not pinned when the film was inverted. These results mean that the surface structure of the hybrid film under the water droplet was not deformed after heating at 60 °C. Pinning of the water droplet could be demonstrated by thermally changing the apparent contact area of the hybrid surface. In conclusion, a biomimetic hybrid surface composed of metal-dome and polymer-spike superhydrophobic surfaces were prepared by electroless plating of self-organized honeycomb-patterned polymer films and simple peeling. The hybrid dome-spike surface is characterized by “affinitydriven adhesion” between water and hydrophilic metal domes, in contrast with the “van der Waals force-driven adhesion” of other highly adhesive superhydrophobic surfaces.4,5,12 The water droplet adhesion on the hybrid surface could be controlled by not only varying the dome density but also by the application of thermal treatment. These results demonstrate that the surfaces can be used as novel fluidic devices for manipulating microdroplets by controlled wettability. Further investigation of the wettability change of these hybrid metal-polymer surfaces is currently being conducted. In particular, the possibility of controlling the properties of the hydrophilic metal domains by applying external stimuli, such as electric and magnetic fields, is being investigated. Supporting Information Available: Preparation method and adhesion force measurement of the metal-polymer hybrid surface (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM9006926
