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Superior Water Repellency of Water Strider Legs with Hierarchical Structures: Experiments and Analysis Xi-Qiao Feng,† Xuefeng Gao,‡ Ziniu Wu,† Lei Jiang,*,‡ and Quan-Shui Zheng*,† Department of Engineering Mechanics, Tsinghua UniVersity, Beijing 100084, PR China, and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China ReceiVed October 16, 2006. In Final Form: January 31, 2007 Water striders are a type of insect with the remarkable ability to stand effortlessly and walk quickly on water. This article reports the water repellency mechanism of water strider legs. Scanning electron microscope (SEM) observations reveal the uniquely hierarchical structure on the legs, consisting of numerous oriented needle-shaped microsetae with elaborate nanogrooves. The maximal supporting force of a single leg against water surprisingly reaches up to 152 dynes, about 15 times the total body weight of this insect. We theoretically demonstrate that the cooperation of nanogroove structures on the oriented microsetae, in conjunction with the wax on the leg, renders such water repellency. This finding might be helpful in the design of innovative miniature aquatic devices and nonwetting materials.
1. Introduction The hydrophobicity of solid surfaces plays a significant role in various biological processes and industrial applications.1-7 Lotus leaves are a typical example of superhydrophobic natural materials that possess perfect self-cleaning and nonwetting properties with a water contact angle (CA) larger than 150° and a sliding angle below 5°. Their surfaces consist of numerous micropapillae with branchlike nanostructures, which is the very origin of the so-called lotus effect or self-cleaning effect.1-3 In view of the extremely low adhesive property of water beads on such surfaces, superhydrophobization of solid surfaces5-7 is expected to be able to bring about some innovative applications (e.g., easy-cleaning windows and traffic indicators for raindrops, antisticking antennas for snow and stain-resistant textiles) and, therefore, has attracted extensive attention. A large variety of methods have been developed to synthesize superhydrophobic surfaces by constructing rough microstructures associated with the use of low-surface-energy matter.8-12 Moreover, various bioinspired drag-reducing ideas5,13-16 are currently being investigated to improve the fast propulsion of air or aquatic vehicles. It has been reported that engineering the surface into superhy* Corresponding authors. E-mail:
[email protected]; jianglei@ iccas.ac.cn. † Tsinghua University. ‡ Chinese Academy of Sciences. (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (2) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667-677. (3) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 14, 1857-1860. (4) Gao X.; Jiang L. Nature 2004, 432, 36. (5) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31-41. (6) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644-652. (7) Blossey, R. Nat. Mater. 2003, 2, 301-306. (8) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 21252127. (9) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662-6665. (10) Xie, D.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Han, C. C. AdV. Mater. 2004, 16, 302-305. (11) Shi, F.; Wang, Z.; Zhang, X. AdV. Mater. 2005, 17, 1005-1009. (12) Wu, X.; Shi, G. J. Phys. Chem. B 2006, 110, 11247-11252. (13) Ball, P. Nature 1999, 400, 507-508. (14) Bechert, D.; Bruse, M.; Hage, W.; Meyer, R. Naturwissenschaften 2000, 87, 157-171. (15) Kim, J.; Kim, C. J. IEEE Conf. MEMS, Las Vegas, NV, Jan 2002; pp 479-482. (16) Cottin-Bizonne, C.; Barrat, J. L.; Bocquet, L.; Charlaix, E. Nat. Mater. 2003, 2, 237-240.
drophobic structures can dramatically reduce flow resistance15 and that the reduction of the length scale of surface structures to the slip flow regime (∼100 nm) leads to a decrease in viscous drag and a measurable slip velocity near the surface.16 Nowadays, biomimetics is becoming an increasingly vital and effective approach to developing advanced materials and to solving engineering problems.5,6,13,17 In nature, water striders are a type of insect with remarkable abilities18-29 to stand effortlessly and slide and jump promptly on water surfaces using their water-resistant legs. That is to say that they possess superior aquatic weight-bearing and fast-propulsion abilities. Bush et al.24-26 studied through skillfully designed experiments the physical mechanisms of such water-walking creature as water striders and spiders in propelling themselves and climbing menisci and, very recently, gave an excellent review of the hydrodynamics of water walkers. However, another crucial issue of why their legs possess striking water repellency without piercing the water surface during the jerky propelling processes has long been neglected. Previous research indicated that it is the rough microstructure, in conjunction with the wax cover, that seems to be dominantly responsive for the superior water repellency of water strider legs by the formation of a stable air cushion on the leg/water interface.4,18-20 Here, we further investigated the quantitative relationship among the maximal supporting force, the dimple depth, the contact angle of the legs, and the hierarchical structure of microseta and nanogrooves in detail. A model representing the observed hierarchical structures consisting of setae with nanosized grooves is also introduced to elucidate why (17) Dickinson, M. H. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14208-14209. (18) Andersen, N. M. Vidensk. Meddr dansk Naturh. Foren. 1976, 139, 337396. (19) Andersen, N. M. Vidensk. Meddr dansk Naturh. Foren. 1977, 140, 7-37. (20) Andersen, N. M. The Semiaquatic Bugs (Hemiptera, Gerromorpha): Phylogeny, Adaptations, Biogeography, and Classification; Entomonograph Series; Scandinavian Science Press: Klampenborg, Denmark, 1982; Vol. 3, pp 1-455. (21) Caponigro, M. A.; Erilsen, C. H. Am. Midland Nat. 1976, 95, 268-278. (22) Suter, R. B. J. Exp. Biol. 1997, 200, 2523-2538. (23) Vogel, S. Life in MoVing Fluids: The Physical Biology of Flow, 2nd ed.; Princeton University Press: Princeton, NJ, 1994. (24) Hu, D. L.; Chan, B.; Bush, J. W. M. Nature 2003, 424, 663-666. (25) Hu, D. L.; Bush, J. W. M. Nature 2005, 437, 733-736. (26) Bush, J. W. M.; Hu, D. L. Annu. ReV. Fluid Mech. 2006, 38, 339-369. (27) Denny, M. W. J. Exp. Biol. 2004, 207, 1601-1606. (28) Cheng, L. Nature 1973, 242, 132-133. (29) Dickinson, M. Nature 2003, 424, 621-622.
10.1021/la063039b CCC: $37.00 © 2007 American Chemical Society Published on Web 03/27/2007
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the legs possess such exceptional hydrophobicity. These findings might be very helpful in designing novel miniature aquatic robots. 2. Experimental Section The supporting force of a single leg against the water surface was measured by fixing it to a high-sensitivity balance system (Dataphysics DCAT 11, Germany) and moving a water vessel toward the leg. To record the deformation of the leg and the water surface, an optical microscope lens and a CCD camera system are used to take photographs at 1 frame/s. The water vessel was moved upward at a constant speed of 0.01 mm/s. Once the leg was in contact with the water surface, the equipment automatically started to record the force-distance curves and make photographs showing the whole water-treading process. SEM images were taken on a field-emission scanning electron microscope (JEOL JSM-6700F, Japan) at 3.0 kV. Samples were sputtered with a layer of gold (∼10 nm thick) prior to imaging.
3. Results and Discussion Figure 1A shows a photograph of a water strider (Gerris remigis) at rest on the water surface. They have a flattened body that is ∼15 mm long and weighs ∼10 dynes. Scanning electronic microscope (SEM) observations clearly revealed that the leg is covered with large numbers of tiny chitinous setae (Figure 1B), which are oriented at an angle of inclination of ∼20° with respect to the leg surface. Most of these spindly setae are about 50 µm in length and less than 3 µm in diameter (Figure 1C). A highresolution SEM image shows that the surfaces of microsetae are marked with elaborate nanosized grooves (Figure 1D). Thus, the surfaces of the legs are rough and have hierarchical micro- and nanostructures. Such an interesting structure was first observed by Cheng28 and Andersen,18-20 but its quantitative influence on the superhydrophobic property of the leg has not been addressed. Figure 2A shows several typical side views that represent the process of the leg gradually depressing the free surface of the water. For several representative initial stepping angles θ between the leg and the water surface, the force-distance curves were automatically recorded in Figure 2B, where a negative force means that the leg is water-repellent and is subjected to an upward force. When θ is larger than 28°, the tips of legs pierced the water surface on contact. However, the supporting force increases as the stepping angle gradually decreases. The largest force appears in the range of θ between 0 and 10°, which corresponds to the actual posture of a leg rowing on water surface. Because of the high flexibility of legs, the force-distance relationships are almost identical for different angles in this range, as shown by the solid red curve in Figure 2B. Surprisingly, the maximal dimple under the leg is as deep as 4.38 ( 0.02 mm, yielding a maximal supporting force of 152 ( 3 dynes. This means that a single leg can hold a force of about 15 times the weight of the insect’s body without piercing the water surface. An overheadview photograph of the maximal dimple just before the leg penetrates the water surface is shown in Figure 2C. The volume of water displaced is roughly 300 times that of the leg itself, indicating that such legs possess this striking hydrophobicity. For such legs partially submerged in water, the supporting force or weight bearing actually equals the weight of water displaced by the dimple.30,31 The topography of the maximal dimple (Figure 3A) is simulated by using the Young-Laplace equation25 Fgw ) σ∇‚n and the measured depth curve (Figure 2A) of the leg as the boundary condition, where w denotes the displacement of the dimple surface with respect to the remote (30) Mansfield, E. H.; Sepangi, H. R.; Eastwood, E. A. Philos. Trans. R. Soc. London, Ser. A 1997, 355, 869-919. (31) Keller, J. B. Phys. Fluids 1998, 10, 3009-3010.
Figure 1. (A) Water strider resting on water. (B) SEM images of a water strider leg covered by numerous oriented needle-shaped microsetae. (C) SEM image of grooved nanostructure on the seta surface.
water horizontal plane, F is the water density, g is the gravity constant, σ () 0.07275 J m-2) is the water-air interfacial energy, n is the unit outward normal of the dimple surface, and 3‚n the divergence of n that can be expressed in the form
∇‚n )
(1 + wx2)wyy - 2wxwywxy + (1 + wy2)wxx (1 + wx2 + wy2)3/2
which is referenced to a Cartersian coordinate system {x, y} on the remote water horizontal plane. The calculated weight of the water displaced by the dimple is 147.8 mg, which is quite close to the value of the measured maximal supporting force. Two typical simulated cross-sections of the free water surface interacting with a cylindrical leg are also plotted in Figure 3B. To understand why the dimple can reach a maximum depth of 4.38 ( 0.02 mm, we model the leg by an infinitely long solid column on the x axis of diameter D that is horizontally and
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Figure 2. Force-distance curves and dimples. (A) Several representative side views of the leg gradually walking across the water surface as θ ) 10°. (B) Supporting force on a single leg versus the ascending distance of a water vessel under different stepping angles θ. (C) Top view of the maximal dimple (red inset) just before the leg penetrates the water surface.
differential equation, we can establish the dependence of hmax on D and the contact angle φl as
φleg )
Figure 3. Numerical results of the dimple caused by a hind leg. (A) Three-dimensional topography of the dimple treaded by a hind leg. (B) Transects of the dimple crossing the two joints.
[
]
σ(hmax ) π + arctan σ′(hmax ) + arcsin 2 2 D
where 2σ(w) is the horizontal distance separating the two free water surfaces beside the leg. For the measured hmax ) 4.38 ( 0.02 mm, the curves of contact angles φl with respect to the diameter are plotted in Figure 5. Using the measured diameter (140-180 µm) of the leg corresponding to the maximum depth segment, we deduced that to press down the water surface to be able to form such a deep dimple the water contact angle of the leg must be at least 168°, which agrees well with our previously reported experimental results.4 The water contact angle φw of wax secreted on the legs is ∼105°,32 which is not high enough to account for its striking hydrophobicity. Usually, the wettability of a solid surface is governed by both its chemical composition and geometrical morphology. Microstructures on the surface of low-energy materials may greatly enhance the hydrophobicity.33-36 As the case stands, the hierarchical micro- and nanostructures on the legs should play the most significant roles. Figure 6A is an illustrative transect of a leg, where the small circles represent transects of setae. On the basis of the Cassie-Baxter law for surface wettability,33 we build the model for the oriented microseta structures and derive the following contact angle equation
cos φl ) (π - φs)f cos φs - (1 - f sin φs) gradually sunk under quasi-static loading. The model dimple is thus characterized by its cross-section as illustrated in Figure 4, which is defined by the Young-Lapace equation with
κ)
wyy (1 + wy2)3/2
Whenever the two free water surfaces beside the leg (i.e., the column) come into contact, the leg reaches a maximum depth of hmax and pierces the water surface. By solving the ordinary
where φs is the contact angle of setae and d and s are the mean diameter and spacing of setae, respectively. Recently, we reported a quantitative study on the wetting-mode transformation.37 According to our measurement of microstructures, the parameter (32) Holdgate, M. W. J. Exp. Biol. 1955, 32, 591-617. (33) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546-551. (34) Bico, J.; Marzolin, C.; Que´re´, D. Europhys. Lett. 1999, 47, 220-226. (35) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777-7782. (36) Herminghaus, S. Europhys. Lett. 2000, 52, 165-170. (37) Zheng, Q.-S.; Yu, Y.; Zhao, Z.-H. Langmuir 2005, 21, 12207.
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Figure 4. Transects of the water surface for the leg contacting the water to different depths until the maximum depth hmax is reached before piercing the water surface.
on the setae by a wreath with circular arcs (Figure 6C) for their up surfaces and adopt the corresponding contact angle equation
cos φs ) (π - φw)f ′ cos φw - (1 - f ′ sin φw) in the Cassie-Boxter wetting mode, where f ′ ) 2d′/s′ with d′/2 and s′ being the radius of curvature and spacing of the grooves, respectively. Substitution of the measured value f ′ ≈ 2/3 and φw ) 105° yields a φs value that is approximately equal to 125°, which agrees well with the results predicted above. Therefore, it becomes clear that the excellent superhydrophobicity of the legs is a cooperative effect of multiscale microsetae and nanogroove structures based on the hydrophobic wax cover.
4. Conclusions Figure 5. Contact angle of the leg and dimple depth. Dependence of the contact angle φleg on the diameter D of the leg in order to form the maximal dimple with depth hmax ) 4.38 ( 0.02 mm.
f ) d/s is in the range of 0.05-0.1. To achieve superhydrophobicity with φl being ∼168°, the seta must have a contact angle φs of not less than 125°, which, however, is impossible for a smooth seta because φw ) 105°. In other words, the oriented microsetae themselves are not enough to induce the superhydrophobicity of legs. Thus, the grooved nanostructures on the seta surface (Figure 1E) are necessary to yield a φs that is larger than φw. Through detailed microscope studies, we may obtain accurate parameters of the nanogrooves. The average width and depth of the nanogrooves are ∼410 and ∼100 nm, respectively. Accordingly, we also model the transect (Figure 6B) of nanogrooves
Water strider legs possess superior water repellency. It is the nanosized grooves on the microseta surfaces that play a crucial role in inducing the superhydrophobicity of legs with a higher contact angle, which ensures that the legs can tread tremendous dimples that are as deep as possible without piercing the water surface. As a result, the maximal supporting force exerted by a water strider on the water surface reaches at least 750 dynes, which is more than 60 times the weight of its body, exhibiting a striking flotation ability. Such structural design makes water striders unsinkable, even in violent rainstorms or rushing currents. Note that the superhydrophobic nanopatterned surfaces may dramatically reduce flow resistance15 and fluidic drag.16 For water striders, the special micro- and nanostructures also endow the legs with higher hydrophobicity and lower drag, which ensures that they may walk rapidly on the water surface. Thus, we believe that these findings will aid in the design in the near future of
Figure 6. Transect models of oriented microsetae and nanogrooves on the strider leg used to predict its apparent contact angle. (A) Illustrative transect of a leg in contact with water, where small circles represent transects of setae. (B) Model of the oriented arrangement of microsetae, with d and s being the mean diameter and spacing of setae, respectively. (C) Model of oriented nanogrooves, with d′/2 and s′ denoting the radius of curvature and spacing of grooves, respectively.
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novel superfloating and drag-reducing skins for aquatic miniature robots.11,12 Acknowledgment. We thank the National Nature Science Foundation of China, the Innovation Foundation of the Chinese
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Academy of Sciences, and the Education Ministry of China. Mr. Z. H. Zhao and Mr. Z. Chen helped with the numerical simulations. LA063039B
