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Ind. Eng. Chem. Res. 2008, 47, 6354–6357
Biomimic Superhydrophobic Surface with High Adhesive Forces Jinming Xi†,‡ and Lei Jiang*,§ National Center for Nanoscience and Nanotechnology, Beijing, 100080, People’s Republic of China, Graduate School of the Chinese Academy of Sciences, Beijing, 100864, People’s Republic of China, and Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China
Superhydrophobic polydimethylsiloxane (PDMS) surfaces that contain microbinary and nanobinary structures with a water contact angle (CA) of >150° are created by duplicating the microstructures of a rose petal. The resulting surface has the same microstructures and high adhesive property as a rose petal; the water CA of the surface is as high as 154.3°. Interestingly, this surface can hold water droplets through the use of very strong adhesive forces, and the maximum adhesive force is 63.8 µN. Furthermore, the resulting surface has another important property; it shows superhydrophobic property in a wide range of pH values from 1.07 to 13.76. Therefore, it is believed that the as-prepared surface can be used in the fabrication of chemical engineering materials, microfluidic devices, and the transportation of a small amount of corrosive liquids. Introduction Biological microstructures, such as opal structures on butterfly wings,1 nanopapilla morphologies on lotus leaves,2 and needleshaped seta on water strider legs,3 are currently the subject of intense research activities, because they are expected to account for the nonwettability of those biologic surfaces.4,5 For example, water drops can easily roll off lotus leaves and carry away dust particles and debris; this mechanism often is called a selfcleaning property, and the water contact angles (CAs) on these superhydrophobic surfaces can be >150°.6 In addition, these surfaces also show a very low water roll-off angle (i.e., very low water CA hysteresis).7,8 As a result, water droplets that fall on the surfaces of lotus leaves can roll off quickly and efficiently remove a large number of dust particles that are adhering to them. Much research has revealed that lotus leaves have textured surfaces with hierarchical micrometer- and nanometer-sized structures.2,6 Furthermore, these works also show that water droplets on these textured surfaces readily sit on the apex of the nanostructures, because air bubbles fill the valleys of the structure under the droplets. Water drops on this type of surface cannot penetrate into the microstructures or nanostructures and wet the surface, resulting in extremely high CAs; thus, these naturally occurring leaves exhibit considerable superhydrophobicity. Inspired by these fancy biological microstructures, various approaches have been developed for the synthesis of artificial materials with novel special wettability. Some examples are superhydrophobic gold aggregates that form by mimicking the legs of the water strider9 and a lotus-leaf-like superhydrophobic surface that has been prepared by an electrohydrodynamics method.10 However, these limited techniques cannot directly copy the biological microstructures; furthermore, very few superhydrophobic surfaces with high adhesive forces have been achieved so far. Therefore, it is of interest to introduce a simple, * To whom correspondence should be addressed. Current contact information: Institute of Chemistry, Chinese Academy of Sciences, No. 2, First North Street, Zhongguancun, Beijing, PRC 100080. Tel.: (+86) 10-82621396. Fax: (+86) 10-82627566. E-mail address: jianglei@ iccas.ac.cn. † National Center for Nanoscience and Nanotechnology. ‡ Graduate School of the Chinese Academy of Sciences. § Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences.
easy to control, and highly reliable method to produce superhydrophobic surfaces. Soft lithography is an effective technique that is widely used in micrometer- and nanometer-scale fabrication.11–15 This method is easy to control and is highly faithful to the structures of the original template, even on the nanometer scale; thus, it is suitable for the replication of the very intricate topography of the microstructures of leaves. In this paper, we present a very simple soft lithography method to fabricate a superhydrophobic surface with high adhesive force by directly duplicating the microstructure of rose petals. The resulting replica shows superhydrophobicity with a high adhesive effect that is similar to that of rose petals. The water CA is as high as 154.3°, and the water droplet can still stick to the surface, even when the replica tilts 180°. Furthermore, the as-prepared film possesses robust double roughness and shows superhydrophobicity, even for corrosive liquids. It is expected to possess great capabilities for a wide range of applications, including the transportation of a small amount of liquids and anticorrosion of engineering materials. Materials and Methods Replica. Figure 1 shows the flowchart for creating the superhydrophobic replica surface. At the beginning of the experiment, 15 wt % of a poly(vinyl alcohol) (PVA) (molecular weight of MW ) 88000, purchased from Aldrich Chemical Co., Ltd.) water solution is cast onto a fresh rose petal surface and exposed under ambient conditions for its curving. After solidification at room temperature for 24 h, the PVA film, which has an imprinted inverse petal structure (negative template), can be peeled off. The mixture of liquid polydimethylsiloxane (PDMS) and its catalyzer (purchased from Dow Corning, Midland, MI) then are cast onto the negative template and solidified at 60 °C for 5 h, and, subsequently, the PDMS layer is peeled off, resulting in a surface structure that is very similar, topographically, to that of the original rose petal. Smooth PDMS Surface. A PDMS prepolymer is poured onto the clean glass plate and solidified at 60 °C for 10 h. Thus, a smooth PDMS film can be formed with a thickness of 3 mm and a surface area of 2 cm × 2 cm. Characterization. Scanning electron microscopy (SEM) images are obtained on a JSM-6700F SEM system (JEOL,
10.1021/ie071603n CCC: $40.75 2008 American Chemical Society Published on Web 06/10/2008
Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 6355
Figure 2. (a) and (b) SEM images of the surface of a red rose petal (low and high magnification, respectively), showing a periodic array of micropapillae on the surface, the top of which is composed of nanorumples. (c) and (d) Microstructures of the obtained replica (low and high magnification, respectively).
Figure 1. Illustration of the soft lithography process and fabrication of a superhydrophobic surface as described in this paper.
Japan) at 3.0 kV. Prior to the measurements, the samples are coated with a thin gold film via vacuum sputtering, to improve electrical conductivity. The CAs are measured on a CA system (OCA20, Dataphysics, Germany) at ambient temperature. Water droplets (with a volume of 2 µL) with different pH values are carefully added dropwise onto the resulting surface. The average CA values are obtained by measuring five different positions of the same sample. The force required to take the water droplet away from the replica is measured using a high-sensitivity microelectromechanical balance system (Data-Physics DCAT 11, from Dataphysics, Germany). First of all, a 5-mg water droplet is suspended with a metal ring, and the replica film is placed on the balance table. The replica film is moved upward at a constant speed of 0.01 mm/s until the substrate (replica) contacts the water droplet. The force is increased gradually until it reaches its maximum value, and the shape of the water droplet changes from spherical to elliptical. Results and Discussion Figure 2a exhibits conical micropapillae, with a diameter of 20-30 µm, arranged compactly on the rose petal. On the top of the micropapillae, there exist nanorumples (Figure 1b); the width of the nanorumples is 170-180 nm, and their length is 5-6 µm. These typical hierarchical structures, on both micrometer and nanometer scales, provide sufficient roughness, which results in the superhydrophobicity of the rose petal. Accordingly, as shown in Figures 2c and 2d, the microstructure of the rose petal transferred to the replica with high fidelity. Practically, in our work, the difference in size of the microstructures between the template and its replica is considered to be negligible, because they have the same microstructure size
Figure 3. (a) Behavior of a water droplet on a smooth PDMS surface. Shapes of water droplets on the as-prepared replica surface with different tilt angles: (b) 0°, (c) 90°, and (d) 180°.
as that observed in Figure 2. Therefore, the replica has the same microstructure as the rose petal. PDMS is a hydrophobic material with a water CA of 104° on a smooth surface (see Figure 2a).16 However, the as-prepared replica of rose petal is superhydrophobic. Figure 3b shows the shape of a 2 µL water droplet on the as-prepared replica with a water CA of 154.3° ( 1.8°. For a better understanding of the superhydrophobicity of the resulting surfaces, theoretical considerations are necessary. We describe the CA in terms of the Wenzel equation: cos(θr) ) r cos(θ)
(1)
This equation is used to describe the CA for a liquid droplet at a rough solid surface.19 Here, r is the roughness factor, and θr ) 154.5° and θ ) 104° are the water CA values on a rough surface and a smooth surface made of the same material, respectively. With increasing surface roughness (r), based on this equation, the actual CA decreases for hydrophilic materials (θ < 90°) and increases for hydrophobic materials (θ > 90°).17,18 Obviously, the CA for water will increase on the rough surface of hydrophobic materials. Therefore, the binary microstructure, on nanometer and micrometer scales, of the surface
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CAs for the obtained replica. Therefore, it is believed that the as-synthesized surface is superhydrophobic in the pH range of 1.07-13.76. Conclusion
Figure 4. Graph showing the relationship between pH value and contact angle (CA) on the replica films.
can enhance the superhydrophobicity and endows the surface with a high water CA. The CA hysteresis (the different between advancing and receding CAs), which is often determined by the three-phase (solid-liquid-air) contact line and can accurately assess the sliding behavior of a water droplet.20–24 In our experiment, the prepared surface show a strong CA hysteresis, and the water droplet does not slide when the replica film is tilted vertically (see Figure 3c) or even turned upside down (see Figure 3d). Furthermore, the critical weight of the water droplet on a vertical tilted replica film is as high as 10 mg and the maximum adhesive force is 63.8 µN, which is indicative of a strong adhesive effect between water and the surface. The high adhesive force of the replica can be explained according to the Wenzel model. As illustrated in Figure 3e, the water can get into the grooves of the rough solid surface that attribute to the capillary effect of the coneshaped microstructures. This capillary force is so strong that the water droplet can be pinned on the replica surface even when it turns vertically. This observation can be attributed to the difference of the microstructures and sizes between the as-prepared surface and the lotus leaf. Accordingly, the observed superhydrophobic behavior of the prepared surfaces can be understood in terms of the of three-phase contact line. For the lotus leaf, contact lines are discontinuous, which prevents water from intruding into the microstructure interspaces. Thus, the surface shows low CA hysteresis and the water droplet moves very freely. In contrast to the prepared surface, the three-phase contact line is continuous, and the water droplet can be pinned on the surface, because of the strong capillary force of the cone-shaped microstructures, which can force the water into the grooves of the surface. Therefore, the resulting replica shows superhydrophobicity with high adhesive force. The as-prepared surface shows superhydrophobic properties in the pH range of 1-14; that is, the CAs are larger than 150° not just for pure water but also for corrosive liquids, such as acidic, basic solutions. Figure 4 shows the relationship between pH values and CAs on the obtained surface. There is no obvious difference of the CA values within the error. All CA values are in the range from 151.16° to 158.28°, with little difference that is due to experimental error. These results reveal that pH values of the aqueous corrosive solution have little or no effect on
In summary, a superhydrophobic surface is fabricated via a facile soft lithography technique. Compared to the superhydrophobic surface with a small roll angle, which is prepared by conventional chemical synthesis, the resulting replica surface shows strong adhesion to water droplets. This shows that the as-prepared microstructures and nanostructures can not only enhance the hydrophobicity of the surface, but also give rise to a high adhesive force for water. Furthermore, the as-prepared surface not only shows superhydrophobicity for pure water but also for corrosive liquids, including acidic and basic solutions. From the point of practical applications, this surface will be more useful in the synthesis of chemical engineering materials and microfluidic devices.
ACKNOWLEDGMENT This work is supported by the National Research Fund for Fundamental Key Projects (No. 2007CB936403), and the National Natural Science Foundation of China (Nos. 20571077, 5073020). Literature Cited (1) Zheng, Y. M.; Gao, X. F.; Jiang, L. Directional adhesion of superhydrophobic butterfly wings. Soft Matter 2007, 3, 178–182. (2) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1–8. (3) Gao, X. F.; Jiang, L. Water-repellent legs of water striders. Nature 2004, 423, 36–36. (4) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Super-hydrophobic surfaces: from natural to artificial. AdV. Mater. 2002, 14, 1857–1860. (5) Feng, X. J.; Jiang, L. Design and creation of superwetting/antiwetting surfaces. AdV. Mater. 2006, 18, 3063–3078. (6) Blossey, R. Self-cleaning surfaces-virtual realities. Nat. Mater. 2003, 2, 301–306. (7) Extrand, C. W. Model for contact angles and hysteresis on rough and ultraphobic surfaces. Langmuir 2002, 18, 7991–7999. (8) Kijlstra, J.; Reihs, K.; Klamt, A. Roughness and topology of ultrahydrophobic surfaces. Colloid Surf. A 2002, 206, 521–529. (9) Shi, F.; Wang, Z. Q.; Zhang, X. Combining a layer-by-layer assembling technique with electrochemical deposition of gold aggregates to mimic the legs of water striders. AdV. Mater. 2005, 17, 1005–1009. (10) Jiang, L.; Zhao, Y.; Zhai, J. A lotus-leaf-like superhydrophobic surface: A porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew. Chem., Int. Ed. 2004, 43, 4338–4341. (11) Gates, B. D.; Whitesides, G. M. Replication of vertical features smaller than 2 nm by soft lithography. J. Am. Chem. Soc. 2003, 125, 14986– 14987. (12) Yan, X.; Yao, J.; Lu, G.; Li, X.; Zhang, J.; Han, K.; Yang, B. Fabrication of non-close-packed arrays of colloidal spheres by soft lithography. J. Am. Chem. Soc. 2005, 127, 7688–7689. (13) Zheng, B.; Tice, J. D.; Ismagilov, R. F. Formation of arrayed droplets by soft lithography and two-phase fluid flow, and application in protein crystallization. AdV. Mater. 2004, 16, 1365–1368. (14) Connal, L. A.; Qiao, G. G. Preparation of porous poly(dimethylsiloxane)-based honeycomb materials with hierarchal surface features and their use as soft-lithography templates. AdV. Mater. 2006, 18, 3024–3028. (15) Gargas, D. J.; Muresan, O.; Sirbuly, D. J.; Buratto, S. K. Micropatterned porous-silicon Bragg mirrors by dry-removal soft lithography. AdV. Mater. 2006, 18, 3164–3168. (16) Jin, M. H.; Feng, X. J.; Xi, J. M.; Zhai, J.; Cho, K. W.; Feng, L.; Jiang, L. Super-hydrophobic PDMS surface with ultra-low adhesive force. Macromol. Rapid Commun. 2005, 26, 1805–1809. (17) Sun, T. L.; Feng, X. F.; Gao, X. F.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38, 644–652. (18) Callies, M.; Que´re´, D. On water repellency. Soft Matter 2005, 1, 55–61.
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ReceiVed for reView November 25, 2007 ReVised manuscript receiVed January 11, 2008 Accepted January 16, 2008 IE071603N
