Directional adhesion of superhydrophobic butterfly wings{

COMMUNICATION www.rsc.org/softmatter Soft Matter Directional adhesion of superhydrophobic butterfly wings{ Yongmei Zheng, Xuefeng Gao* and Lei Jiang* Received 1st September 2006, Accepted 17th October 2006 First published as an Advance Article on the web 31st October 2006 DOI: 10.1039/b612667g We showed directional adhesion on the superhydrophobic wings of the butterfly Morpho aega. A droplet easily rolls off the surface of the wings along the radial outward (RO) direction of the central axis of the body, but is pinned tightly against the RO direction. Interestingly, these two distinct states can be tuned by controlling the posture of the wings (downward or upward) and the direction of airflow across the surface (along or against the RO direction), respectively. Research indicated that these special abilities resulted from the directiondependent arrangement of flexible nano-tips on ridging nano-stripes and micro-scales overlapped on the wings at the one-dimensional level, where two distinct contact modes of a droplet with orientation-tuneable microstructures occur and thus produce different adhesive forces. We believe that this finding will help the design of smart, fluid-controllable interfaces that may be applied in novel microfluidic devices and directional, easy-cleaning coatings. arrangement of flexible nano-tips on the top of ridging nanostripes and overlapped micro-scales on the wings. This finding will offer us an innovative insight into how to design smart fluidcontrollable interfaces. Experimental Sample preparation Specimens of the iridescent blue butterfly, Morpho aega, from Brazil were bought from a store in China. Many venations, distributed on the wings along the RO direction of the body s central-axis, could be seen easily with the naked eye. To better explore the role of the micro-nanostructure characteristics of the wings in surface wettability and adhesion, we carefully cut around Introduction The local control of anisotropic surface wettability by micropatterns, such as grooves with a simple rectangular cross section, has attracted much interest due to its potential in both fundamental research and practical applications. 1 7 It is known that a droplet tends to slide on the surface of groove microstructures more easily along the direction parallel to the grooves rather than perpendicular. 1 3 To the best of our knowledge, all studies on the anisotropy of surface wettability reported up to now were defined at the two-dimensional level. This is also frequently seen in natural plants such as rice leaves, 3 originating from the quasi-onedimensional arrangement of micro-papillae along the direction parallel to the leaf edge. Moreover, recent researches have demonstrated that superhydrophobic surfaces may present distinct adhesive properties and the rolling and pinning states of a droplet may coexist on the same surface under different conditions. 8 12 However, the direction-dependent switch of the two distinct states at the one-dimensional levelhasnotbeenreported sofar. Here, we show for the first time directional adhesive properties on the wings of a butterfly (Morpho aega), that is, a droplet easily rolls along the radial outward (RO) direction (denoted by arrows in Fig. 1a) of the central axis of the body but is tightly pinned in the opposite direction. Our experiments and analyses indicated that this unique ability is ascribed to the direction-dependent Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China. E-mail: jianglei@iccas.ac.cn; gaoxf@iccas.ac.cn { Electronic supplementary information (ESI) available: Fig. S1 showing the superhydrophobic states. See DOI: 10.1039/b612667g Fig. 1 Directional adhesion on superhydrophobic butterfly wings. (a) An iridescent blue butterfly M. aega. The black arrows denote the radialoutward (RO) direction away from the body s centre-axis. (b) The droplet easily rolls along the RO direction when the wing is tilted toward downwards by 9u. (c) The droplet is firmly pinned on the wing that is tilted upward, even when fully upright. 178 Soft Matter, 2007, 3, 178 182 This journal is ß The Royal Society of Chemistry 2007

the venations and selected an intact area with a large number of crossed-overlapping scales as our experimental sample. Measurements of contact angle and adhesion The dynamic and static behavior of water droplets on the butterfly wings was measured using an optical contact angle meter system (OCA20, Dataphysics Instruments GmbH, Germany). The static equilibrium contact angles of butterfly wings were directly obtained by depositing a Mill-Q water droplet (3 ml volume) on the as-prepared sample. Subsequently, we further explored the distinct water-shedding and adhesion behaviour of droplets on the wings along and against the RO direction by tilting the wings downward and upward, which was easily carried out using an automatically controlled tilting table accessory. Meanwhile, the samples were monitored via a high speed CCD camera which produced a video that was used for further detailed analyses of the interaction of water with the micro- and nanostructures of the wings. All the tests were at ambient conditions. In order to investigate in detail the directional adhesion behaviour of water droplets on the wings, other pushing forces were exerted by blowing N 2 airflow toward and against the RO direction, besides the gravity factor produced by tilting the wings upwards or downwards. Structure characterization The micro- and nanostructures of butterfly wings were observed under a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) at 3 kv. The three-dimensional topologies and the cross-sections were obtained by tapping mode atomic force microscopy (AFM, SPI3800N, Seiko Instruments Inc., Japan). Results and discussion Measurements of the surface wettability of the wings demonstrated that they are superhydrophobic, with an equilibrium static contact angle of 152 1.7u for droplets of 3 ml volume,whichisin accordance with previous results reported by Prof. Barthlott et al. 13 The air pocket in the rough microstructures may effectively reduce the surface contact area of water with the surface of wings, which results in a nearly spherical droplet. Unexpectedly, it was found in our dynamic experiments that a droplet deposited on the wings exhibits a distinct water-shedding behaviour along and against the RO direction carried out by the action of tilting the wings downwards or upwards, respectively. Fig. 1b shows three representative images in the process of a droplet rolling off the surface of wings as they are gradually tilted downward. As can be seen, the droplet starts to roll off the surface in the RO direction when the wing is slightly tilted downward at an angle of y9u. However, the droplet is tightly pinned on the surface in the opposite direction (Fig. 1c) when the wing is tilted gradually upward, even fully upright. In view of the knowledge of surface wettability, 11,12,14 18 two distinct superhydrophobic states, the Cassie state (with extremely low adhesion) and the Wenzel state (with high adhesion), seem to be responsible for the directiondependent rolling and pinning behavior of a droplet at the onedimensional level, which are usually related to the special surface microstructures on the wings. Scanning electron microscopic (SEM) observations reveal that the wings of the butterfly Morpho aega are covered by a large numbers of quadrate scales, with a length of y150 mmandawidth of y70 mm, which overlap each other to form a periodic hierarchy along the RO direction (Fig. 2a). Further magnified views display numerous separate ridging stripes of 184.3 9.1 nm in width and 585.5 16.3 nm in clearance on the surface of each scale (Fig. 2b). Very interestingly, these fine nano-stripes consist of multi-layers of cuticle lamellae of apparently different lengths, which are stacked stepwise along the RO direction, and these nano-tips emerge on the top of stripes which are tilted slightly upward. To attain more detailed information about the structural parameters, we used atomic force microscope (AFM) to scan the micro- and nanostructures of the wings. Fig. 2c shows a typical AFM image of the overlapping scales, which are flexibly fluctuated with a peak height of y6 mm. The nano-tips are slightly tilted with a peak height of 121.3 21.7 nm (Fig. 2d). This is a hierarchical microand nanostructure composed of nano-tips on the lamella-stacked nano-stripes and micro-scales on wings, which are both flexible andorientedtothebaseofthewings. It was reported that whether a droplet is pinned on or rolls off the superhydrophobic surface is ascribed to both the distinct contact modes 11,12,14 18 (either the liquid follows the texture or it leaves air inside the texture) and the three-phase (solid/liquid/gas) contact line (TCL). 1,3,19 In the first case (the Wenzel state), 14 the amplification of the contact angle of the hydrophobic textured surface is carried out by the increase of the surface area due to the rough microstructures (Fig. S1a in the ESI{). Considering that water completely fills the valleys of the textured surface (also frequently referred to as a wet contact) the droplet will be firmly pinned on the textured surface, accompanied with the formation of a continuous and stable TCL. Thus, the Wenzel state exhibits an extremely high adhesive property. 11 In contrast, Cassie and Baxter 15 subsequently proposed another distinct mode of composite contact: the liquid only contacts the top of asperities with air trapped in the submicrometer-scale hollows of rough solid surfaces (Fig. S1b in the ESI{). Thus, the dramatic decrease of the surface contact area of a droplet with the microstructure can greatly enhance the hydrophobicity of such surfaces, which results in a nearly spherical droplet, showing superhydrophobicity with an apparent contact angle above 150u. In this case, the droplet will easily roll off a surface with extremely low adhesion if a discontinuous TCL can effectively form, depending on the special construction of surface microstructures. 1 3,11 However, in most cases, water may partially wet superhydrophobic textured surfaces with air trapped in the valleys (Fig. S1c in the ESI{). Such a solid/liquid contact is part way between the Wenzel and the Cassie, and is, thus, called the intermediate or metastable state. 12,16 This is the origin of the coexistence of two superhydrophobic states on the same microstructured surface depending on external disturbance, 10 12 for example, a pressing force. 12 Otherwise, the superhydrophobic state may be artificially tuned to the Wenzel with the high adhesive force or the Cassie with the extremely low adhesive force, through the design of robust microstructures with variable geometric parameters to control certain solid/liquid contact modes. 20,21 Thus, it is possible for the same surface to possess two distinctly directiondependent states at the one-dimensional level as the flexible surface structures may be designed with a tuneable orientation. This journal is ß The Royal Society of Chemistry 2007 Soft Matter, 2007, 3, 178 182 179

Fig. 2 Hierarchical micro- and nanostructures on the surface of the wings. (a, b) SEM images of the periodic arrangement of overlapping micro-scales on the wings and fine lamella-stacking nano-stripes on the scales. Scale bars: (a) 100 mm; (b) 100 nm. (c, d) AFM images of the overlapping micro-scale and nano-stripe structures. The black lines present the respective cross-section profiles at the bottom. As for the butterfly s wings, the case seems more complex. Further experiments showed the pinning state of droplets when the wings are tilted upward does not simply result from the Wenzel state as discussed above. By changing the tilt direction of the wing, the droplet pinned against the RO direction can be made to roll along the RO direction again. Meanwhile, the lengthening front contact can be receded and the pinned rear contact can be released. Moreover, we found that the pinning and rolling state of droplets on the wing may be switched reversibly by blowing a directiondependent N 2 gas flow (Fig. 3). When the airflow is against the RO direction, the droplet is pinned (Fig. 3a) as the micro-scales are apparently tilted to stick to the droplet in the rear and confine its movement a strong adhesive force. However, the droplet will roll off as the airflow blows along the RO direction (Fig. 3b). Provided the pinning state on the wing is the Wenzel state, the above physical measures would be unable to change the Wenzel state into the Cassie state due to the extremely strong adhesive force caused by the complete penetration of liquid into the nanogrooves. 11 Thus, we may conclude that the pinning state of droplets on wings that are tilted upward is an intermediate state, not the Wenzel state. It is noted that several microlitres of droplets can hardly penetrate into such nano-grooves containing an air pocket; this is also verified by the high contact angle value of above 150u, regardless of whether the wings are tilted upward or downward. To better clarify the essential discrepancy of such marvellous biological phenomena, we emphasise how two Fig. 3 The directional adhesion of butterfly wings. (a) The water droplet was pinned on the wings with the scale tilted and adhered to the rear of the drop when N 2 airflow was blew against the RO direction. (b) The water droplet easily rolled off the surface as N 2 airflow was blew against the RO direction. Two distinct states can be reversibly switched by turning the direction of the airflow towards either side of droplet. 180 Soft Matter, 2007, 3, 178 182 This journal is ß The Royal Society of Chemistry 2007

Fig. 4 The models proposed for elucidating the potential mechanism of distinct adhesion dependent on the direction along and against the RO direction. (a) As the wing is tilted down, the oriented nano-tips on the nano-stripes and micro-scales separate from each other so that the water droplet deposited on the wing not only presents a composite contact fashion with the top of the nano-stripes and a dry contact with the air pocket trapped in the nano-grooves, but forms an extremely discontinuous TCL, which both make it easy for it to roll off the surface. (b) As the wing is tilted upward, the flexible nano-tips and micro-scales take a close arrangement so that the droplet presents the wet contact to the nanostripes and forms a quasi-continuous TCL, which pins it on the surface while the intact area of the dry contact with the air pocket in the nanogrooves remains to ensure the high CA and superhydrophobicity of wings. distinct direction-dependent contact modes are performed only at the one-dimensional level on the lamella-stacked nano-stripes, through tuning of flexible nano-tips tilted slightly upward along the RO direction (Fig. 2b). Based on the above experiments, and a theoretical knowledge of the superhydrophobic field as contributed to by previous scientists, 12,16 21 we propose two reasonable hypothetical modes as shown in Fig. 4 to clarify the distinct adhesive properties of butterfly wings. It should be especially noted that, to clearly elucidate such novel and complex biological behaviours, we introduce the new concept of a dry contact in the following discussion to describe the local state of the water droplet contacting with the air pocket in the nanogrooves, while other two important concepts widely used in the Cassie and Wenzel models: 11,12 the composite contact and the wet contact will be used to describe the local state of droplets contacting the composite interface of air and the top of nano-tips and wetting the top of nano-stripes, respectively. When the wing is tilted downward, the micro-scales with ridged nano-stripes are spatially separated from each other and the oriented nano-tips tend to be unwound with flexible micro-scales (Fig. 4a, top). In this case, air can be efficiently trapped in these nanoscale voids among the nano-tips extended by lamellae and the ridged nano-stripes and thus the droplet only touches the top of nano-tips, with a minimal contact area. 15 This ensures the superhydrophobicity of the wings, with a high contact angle above 150u, which has been verified by Prof. Barthlott and his coworkers. 13 Moreover, it is known that the ordered arrangement of the microstructures may influence the contour, length and continuity of TCL and thus control the way a droplet tends to move. 1 3,19 Accordingly, the ordered arrangement of the micro-scales and nano-stripes on the wings along the RO direction, as shown in Fig. 2, and the formation of the extremely discontinuous TCL as illustrated in the bottom of Fig. 4a, make the droplet easily roll off the wings along the RO direction. So, the rolling state of the droplet on the wings tilted slightly downward may be considered the Cassie state with a composite contact on the tops of ridged nano-stripes and a dry contact on the air pockets trapped in the nanogrooves. However, when the wing was tilted upward, the flexible nanotips and micro-scales take on a close arrangement as shown in Fig. 4b. The nano-tips on the top of the nano-stripes are raised with the flexible micro-scales to closely contact the droplet (Fig. 4b, top). Although the fraction of air trapped in the nanogrooves almost remains constant, the complete wet contact of water with the nano-tips and lamellae would increase the fraction of solid/ liquid surface area. As a result, a quasi-continuous TCL is formed, as shown in the bottom of Fig. 4b when the droplet attempts to move under the gravitational potential against the RO direction. In this case, the pinning at numerous corners (nano-tips) of the steps between the neighbouring lamellae on top of the ridged nano-stripesmayproduceaveryhighenergybarrier,whichmakes the droplet pin tightly on the wing as it is tilted upward, even when it is fully upright. Thus, the pinning state of the droplet on the wing when it is tilted upward is an intermediate state with the wet contact on the top of ridging nano-stripes and the dry contact on the air pockets trapped in nanogrooves. Finally, we emphasize that the case of tilting the wings upward and downward was not quite the same as that of blowing N 2 airflow along and against the RO direction, although they both induced the surface structure arrangement and consequent direction-dependent adhesion. The decisive factor inducing the reversible switch between the rolling state and the pinning state is the external driving force exerted in the latter case by blowing airflow and in the former case by gravity. Besides, our findings are quite different from previous reports on the irreversible forceinduced transition from the rolling state (the Cassie state) to the pinning state (the Wenzel state). 12 This is because the reversible transition between the distinct rolling and pinning state on the wings might result from the reversible transition between the composite contact and the wet contact, which only occurred on the local nano-stripes, and may recover via structural arrangement, while the dry contact upon air pockets trapped in numerous nanogrooves remained unchanged despite the conditions, which may greatly reduce the energy requirement for the reversible transition. Conclusions In summary, we revealed for the first time directional adhesive properties on the wings of a butterfly, M. aega, and the reversible switching of the distinct rolling and pinning state at onedimensional level by tuning the posture of wings, tilted downward or upward, and controlling the direction of airflow across the surface, along or against the RO direction, respectively. The combined experiments and theoretical analyses have indicated that these marvellous properties should be attributed to the directiondependent arrangement of flexible nano-tips on the lamellastacked nano-stripes and micro-scales overlapped on the wings. This journal is ß The Royal Society of Chemistry 2007 Soft Matter, 2007, 3, 178 182 181

Actually, these marvellous properties are of crucial biological significance. They can endow a butterfly s wings with the ability of directional easy-cleaning in a watery environment, and thus ensure its stability of flight by avoiding the accumulation of dirt particles on the end of its wings near the central axis of its body in the process of the removal of such particles deposited on the wings. Moreover, this finding helps to offer an insight into how to design smart fluid-controllable interfaces that may be applied in novel microfluidic devices 7,22 and directional easy-cleaning coatings. 23 Notes and references 1 Z. Yoshimitsu, A. Nakajima, T. Watanabe and K. Hashimoto, Langmuir, 2002, 18, 5818 5822. 2 Y.Chen,B.He,J.LeeandN.L.Patankar,J. Colloid Interface Sci., 2005, 281, 458 464. 3 L.Feng,S.Li,Y.Li,H.Li,L.Zhang,J.Zhai,Y.Song,B.Liu,L.Jiang and D. Zhu, Adv. Mater., 2002, 14, 1857 1860. 4 T. Sun, L. Feng, X. Gao and L. Jiang, Acc. Chem. Res., 2005, 38, 644 652. 5 M. Gleiche, L. Chi, E. GedigandH. Fuchs, ChemPhysChem, 2001, 3, 187 191. 6 J. Bico, C. Marzolix and D. Quéré, Europhys. Lett., 1999, 47, 220 226. 7 R. Seemann, M. Brinkmann, E. J. Kramer, F. F. Lange and R. Lipowsky, Proc.Natl.Acad.Sci.U.S.A., 2005, 102, 1848 1852. 8 T. Onda, S. Shibuichi, N. Satoh and K. Tsujii, Langmuir, 1996, 12, 2125 2127. 9 M.Jin,X.Feng,L.Feng,T.Sun,J.Zhai,T.LiandL.Jiang,Adv. Mater., 2005, 12, 1977 1981. 10 Y. T. Cheng and D. E. Rodark, Appl. Phys. Lett., 2005, 86, 144101 144103. 11 D. Quéré, A. Lafuma and J. Bico, Nanotechnology, 2003, 14, 1109 1112. 12 A. Lafuma and D. Quéré, Nat. Mater., 2003, 2, 457 460. 13 T. Wagner, C. Neinhuis and W. Barthlott, Acta Zool., 1996, 77, 213 225. 14 R. N. Wenzel, Ind. Eng. Chem., 1936, 28, 988 994. 15 A. B. C. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546 551. 16 A. Marmur, Langmuir, 2004, 20, 3517 3519. 17 N. A. Patankar, Langmuir, 2003, 19, 1249 1253. 18 N. A. Patankar, Langmuir, 2004, 20, 7097 7102. 19 W. Chen, A. Y. Fadeev, M. C. Hsieh, D. Öner, J. Youngblood and J. McCarthy, Langmuir, 1999, 15, 3395 3399. 20 B. He, N. A. PatankarandJ. Lee, Langmuir, 2003, 19, 4999 5003. 21 C. W. Extrand, Langmuir, 2004, 20, 5013 5018. 22 G. M. Whitesides, Nat. Biotechnol., 2003, 21, 1161 1165. 23 I. P. Parkin and R. G. Palgrave, J. Mater. Chem., 2005, 15, 1689 1695. 182 Soft Matter, 2007, 3, 178 182 This journal is ß The Royal Society of Chemistry 2007