Superhydrophobicity of Lotus Leaves versus Birds Wings: Different

Article pubs.acs.org/Langmuir

Superhydrophobicity of Lotus Leaves versus Birds Wings: Different Physical Mechanisms Leading to Similar Phenomena Edward Bormashenko,*,†,‡ Oleg Gendelman,§ and Gene Whyman† †

Physics Department, and ‡Department of Chemistry and Biotechnology Engineering, Ariel University Center of Samaria, POB 3, Ariel 40700, Israel § Faculty of Mechanical Engineering, Technion−Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: Remarkable water repellency of birds’ feathers and lotus leaves is discussed. It is demonstrated that physical mechanisms of superhydrophobicity of birds’ feathers and lotus leaves are very different. The topography of lotus leaves is a truly hierarchical one, whereas birds’ feathers manifest pseudohierarchical relief, where various scales do not interact. The pronounced stability of the Cassie state observed on birds’ feathers is due to the high value of critical pressure necessary for their total wetting, which is on the order of magnitude of 100 kPa. This high value allows feathers to withstand large dynamical pressure of rain droplets and remain dry under the rain. The energy barrier separating the Cassie state from the complete wetting situation calculated for a feather is also very high, allowing the increased stability of superhydrophobicity.

1. INTRODUCTION Wetting of rough surfaces attracted significant attention from investigators in the last few decades.1−20 The interest in the problem was stimulated by revealing the so-called “lotus effect”, giving rise to surfaces upon which pronounced water repellency was demonstrated.1−4 The phenomenon of superhydrophobicity was revealed in 1997 when Barthlott and Neinhuis studied the wetting properties of a number of plants and stated that the “interdependence between surface roughness, reduced particle adhesion and water repellency is the keystone in the self-cleaning mechanism of many biological surfaces”.1 They revealed the extreme water repellency and unusual self-cleaning properties of the “sacred lotus” (Nelumbo nucifera) and coined the notion of the lotus effect, which is now one of the most studied phenomena in surface science.1−20 Subsequently, the group led by Barthlott studied a diverse range of plants and revealed the deep correlation between surface roughness, surface composition of plants, and their wetting properties varying from superhydrophobicity to superhydrophilicity.2−4 The amazing diversity of the surface reliefs of plants observed in nature was reviewed in reference 2. Barthlott et al. noted that plants are coated with a protective outer membrane coverage called a cuticle. This cuticle is a composite material built up by a network of polymer cutin and waxes.2 One of the most important properties of this cuticle is hydrophobicity, which prevents desiccation of the inner cells of plants.2,3 It is noteworthy that the cuticle demonstrates only a moderate inherent hydrophobicity (or even hydrophilicity for certain plants, such as the famous lotus5), whereas the rough surface of the plant may be extremely water repellent. Since Barthlott et al. reported the extreme water repellency of lotus, similar phenomena have been described for a diverse © 2012 American Chemical Society

range of biological objects: water strider legs, birds’ feathers (Figures 1 and 2), and butterfly wings.6−9 Barthlott et al. have

Figure 1. Ten microliter droplet deposited on a pigeon feather.

also drawn the attention of investigators to hierarchical reliefs, such as those depicted in Figure 3, inherent to plants characterized by superhydrophobicity. Our paper is devoted to the interrelation between the hierarchical topography of surfaces and their water repellency. We will also demonstrate that physical mechanisms responsible for hydrophobicity may be very different for various biological objects. Received: August 17, 2012 Revised: September 19, 2012 Published: September 19, 2012 14992

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increasing. Herminghaus also stressed that θ0 corresponding to the Young angle θY ≡ θ0 on a flat surface must only be nonzero but needs not exceed π/2 to obtain high resulting apparent contact angles on hierarchical surfaces.16 This makes the existence of natural and artificial superhydrophobic surfaces based on inherently hydrophilic materials (including lotus leaves and birds’ wings) possible.1−5,9,16,19,20 On the other hand, sticky hierarchical biosurfaces, which demonstrate high apparent contact angles, accompanied with a high contact angle hysteresis were reported recently.40−42 A relation between hierarchical topographies of the surfaces and their wetting properties most certainly calls for clarification. Consider two hierarchical surfaces: cutin- and waxes-built surfaces of the lotus leaf and keratin-built birds’ feathers, depicted schematically in Figure 4, panels A and B. Both

Figure 2. SEM image of pigeon feather. Scale bar is 200 μm.

Figure 3. Wetting hierarchical surfaces according to Herminghaus.16

Figure 4. (A) Typical hierarchical relief inherent to lotuslike surfaces. (B) Pseudohierarchical structure of a bird feather.

2. RESULTS AND DISCUSSION 2.1. What is the True Hierarchical Relief? It is agreed that the extreme water repellency of biological objects is due to the so-called Cassie−Baxter wetting regime when a large amount of air is trapped by a droplet located on a superhydrophobic surface.1−4,21,22 Various external factors, such as pressure, vibration, or bouncing, may promote the Cassie−Wenzel wetting transition leading to penetration of liquid into the grooves which constitutes the relief.23−39 The Wenzel wetting is characterized by the high contact angle hysteresis; when the droplet is “sticky”, the Cassie−Wenzel transition destroys the water repellency of the surface.23−39 Thus, true superhydrophobicity requires not only a high apparent contact angle but also a low contact angle hysteresis and high stability of a Cassie (called also “fakir” in the case of pillar reliefs) state. As was shown experimentally, some biological objects (e.g., birds wings) demonstrate remarkable stability to wetting transitions.9,37 It is commonly assumed that the low contact angle hysteresis and high stability of the Cassie state observed for biological subjects are due to the hierarchical topography of their surfaces.1−4,9,11,16,20 For hierarchically indented substrates, such as those depicted in Figure 3, Herminghaus deduced the following recursion relation: cos θn + 1 = (1 − fLn )cos θn − fLn

surfaces are hierarchical; indeed micro and nanometric bumps are recognized at the surface of the lotus leaf shown schematically in Figure 4, panel A. Several scales are also distinguished at the surface of the bird feather sketched in Figure 4, panel B and built from the network of barbs (larger tubes constituting the feather) and barbules (smaller tubes), shown in Figure 2. These scales are the distance between barbs, which is on the order of magnitude of 0.5 mm, and the distance between the barbules’ centers, 2d = 7.5 μm [the diameter of barbs a equals ∼20 μm, and the diameter of barbules 2r equals ∼5 μm (see Figures 2 and 4)].9 However, the surface of a lotus leaf could be called “the true hierarchical surface”, whereas the surface of birds’ feathers could be called “the pseudo-hierarchical surface”. We mean that the hierarchy of elements constituting the Lotus leaf strengthens the air trapping and increases the apparent contact angle, which should be given by the modified Cassie eq 1, considering this hierarchy.16,20,42,43 At the same time, the hierarchy of the scales of barbs and barbules does not strengthen the water repellency of the feather; eq 1 is useless for feathers, and the apparent contact angle in this case is given by the well-known traditional Cassie−Baxter equation:

(1)

cos θ = − 1 + fS (cos θY + 1)

where θn is the apparent contact angle, f Ln is the fraction of free liquid surfaces suspended over the indentations of the relief where n specifies a number of generations of the indentation hierarchy.16 Larger n values correspond to a longer length scale. According to eq 1, cos θn+1 − cos θn = −f Ln(1 + cos θn) < 0, so that the sequence of apparent angles in eq 1 is monotonically

(2)

where f S is the relative fraction of wetted solid. Thus, the mathematical criterion distinguishing between the true and pseudohierarchical reliefs is desirable. Such criterion should be based on observable topography of the relief, rather than on fractions of free surfaces which are hardly measurable. 14993

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Consider a crude scheme of (A) true and (B) pseudo hierarchical surfaces presented in Figure 5, which are exposed

Figure 6. Scheme of water penetration between barbules of the feather. The cross section of two neighboring cylinders modeling the system of feather barbules with the curved water−air interface between them.

Figure 5. Schemes of (A) true and (B) pseudo hierarchical surfaces.

to the Cassie wetting (i.e., side walls of pillars are nonwetted). Both reliefs are characterized by two characteristic scales, l1 and l2 (Figure 5); however, on the pseudohierarchical reliefs depicted in Figure 5B, these scales do not interact and for the relative fraction of wetted solid we have fS = fS1 + fS2

From the contact angle definition and geometrical consideration (Figure 6), it follows that α = π − φ − [π/2 − (π − θa)] = 3π/2 − φ − θa and

(3)

R=

where f S1 and f S2 are the relative wetted fractions due to the scales l1 and l2, respectively. Obviously, for the relief featured by n scales, eq 3 could be generalized as

(4)

provided that φ + θa > π

n

fS =

r sin φ − d sin(φ + θa)

(5)

since d > r and the case R > 0 is considered (positive pressure). From eq 5, it is seen that filling with water starts from a nonzero initial value given by

∑ fSi i=1

In the situation of the true hierarchical topography presented in Figure 5A, the characteristic scales are not independent, and generally we can assume that

φin = π − θa

(6)

when the surface is a plane, R = ∞. The value of R decreases with further water penetration and reaches its minimum at φ = φ0:

n

fS = Φ(∏ fSi ) i=1

cos(φ0 + θa) = −ssin θa ;

where Φ is a function depending on the specific relief. In this case inherent to lotus leaves (see Figure 4, panel A, and reference 2), the hierarchical roughness decreases the wetted fraction of solid and consequently increases the apparent contact angle. It should be stressed that the relief of birds’ feathers, shown in Figures 2 and 4, panel B, is pseudohierarchical. Thus, the reasonable question is what is the physical reasoning of the remarkable water repellency of birds' feathers? 2.2. The Physical Reasoning of the Remarkable Stability of the Cassie State on Birds’ Feathers. In Figure 6, a scheme is presented modeling the system of feather barbules by an array of parallel cylinders of radius, r, with the distance between their centers being 2d. The curvature radius, R, of the water−air interface is supposed to be constant at this interface and depends on the water penetration depth characterized by the angle of filling, φ. It is natural to put the contact angle between the solid and liquid surfaces equal to the so-called advancing angle, θa, which is the highest possible one under the action of external factors like the adding of liquid, pressure, oscillations, or bouncing. The angle θa is a characteristic of a given liquid−solid pair and does not change in the course of water penetration.

φ0 = ψ − θa + π

(7)

where s=

r ; d

ψ = arccos(s sin θa)

(8)

The corresponding minimal radius value is R 0 = d(sin ψ + s cos θa)

(9)

The capillary pressure Pcap = γ/R rises in turn (where γ = 72 mN/m is the water surface tension). For some φ values, it compensates an external pressure, Pext, exerted on the feather (e.g., the dynamic pressure of falling rain droplets) Pcap =

γ sin(φ + θa) = Pext r sin φ − d

(10)

and water penetration is terminated. The graph of the pressure dependence (eq 10) on φ is presented in Figure 7. Its maximum is given by eqs 7−9. Thus, the maximum possible pressure that can be sustained by the system equals γ P0 = d(sin ψ + s cos θa) (11) 14994

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Figure 7. Dependence of pressure on the filling angle φ. For geometrical parameters of the feather see the text.

The geometrical parameters describing the feather structure in Figure 2 and presented in a simplified form in Figures 4, panel B, and 6 were measured in references 9 and 44: d = 7.5 μm, r = 2.5 μm, and θa = 94°. The parameters corresponding to these values and calculated according to eqs 6−9, and 11 are s = 0.33, φin = 86°, φ0 = 157°, R0 = 6.9 μm, and P0 = 10.4 kPa. It should be emphasized that the presented value of the maximum pressure, P0, which the pigeon feather can withstand, is 2 orders of magnitude higher than that of artificial superhydrophobic reliefs.24 It is of the same order as the dynamic pressure of a falling rain droplet.24 Note that unlike the mentioned artificial reliefs made of hydrophobic materials, the feather tissue is inherently hydrophilic; the Young contact angle established on a flat feather tissue (keratin) was θY = 78° (see reference 44, where the advancing contact angle for keratin was established as θa = 94°). The water repellence of the feather is due to the very special relief structure modeled in Figure 6 (also see the following text). The analyses of eqs 8 and 11 show that the limit for capillary pressure, P0, is the increasing function of both the advancing contact angle, θa, and the linear density of barbules, s. The first conclusion is quite natural: the higher the θa value, the stronger the resistance of an interface to wetting. The increase of P0 with increasing s looks less obvious. The bird feather resistance to wetting is also characterized by the energetic barrier: W = Etrans − Ein, separating the initial wetting state and the transition state prior to the complete wetting regime.29 The initial wetting state corresponds to the start of water penetration into the space between the cylinders and is characterized by a plane water−air interface with the filling angle, φin. The transition state, separating the onset of the penetration and the complete wetting, was introduced in ref 29. The transition state in our case is prior to the total wetting, when water−air interfaces touch one another and coalesce, as depicted in Figure 8, immediately preceding the total wetting of the lower feather surface. This occurs when the curvature radius of the water−air interface becomes equal to the half-distance between neighboring cylinder centers (i.e., R = d). The corresponding filling angle is determined by a counterpart of eq 4 d=

Figure 8. Transition to wetted regime in the system modeling barbules of the feather.

over the lower surface of a feather, is unessential since the nonwetted state stabilization is provided in the present case by the energy barrier between the initial and transition states, not by the energy difference between nonwetted and wetted regimes themselves, as occurs in the situation of wetting artificial pillar-built surfaces29 or as takes place under the wetting of lotus leaves. This kind of Cassie-state stabilization was discussed recently.45,46 The energy barrier, W, separating the transition and initial states (per unit length of cylinder axis) is given by W /2 = (γSL − γSA )(φtrans − φin)r + γdβtrans − γ(d − r sin φin)

In eq 13, the first term presents the energy change at the solid−liquid and solid−air interfaces with γSL and γSA being interface tensions on them, respectively. The rest of eq 13 is due to the increase in the liquid−air interface (recall that in the initial state this interface is a plane) where βtrans = φtrans + θa − π is the angle at the center of the later interface in the transition state (Figure 8). Taking into account s = r/d, φin = π − θa, and the Young formula, γ cos θY = γSA − γSL, one finally gets W = 2γd[(φtrans + θa − π )(1 − s cos θY ) + s sin θa − 1] (14)

For the geometrical parameters of the feather used above, W = 922 nJ/m; the corresponding barrier calculated per 1 mm2 of the barbules’ network equals 61.5 nJ/mm2. It is worthwhile to compare the last value with the energetic barrier of artificial superhydrophobic reliefs such as those produced by Barbieri, Wagner, and Hoffman.29 The calculations reported in reference 29 supplied the highest values of the energetic barriers ranging from 1 nJ to 10 nJ for perfluorinated microscaled reliefs for droplets of a volume of 3 μL (that corresponds to the 2.5 mm2 contact area). Again, as in the case of comparison of pressures, the energy barrier for the feather is 1−2 orders of magnitude higher. Recall that the feather tissue (keratin) is hydrophilic in nature, unlike the previously mentioned artificial materials.

r sin φtrans − d sin(φtrans + θa)

(13)

(12)

It should be stressed, that when we consider birds’ feathers, there is no Wenzel wetting in the ordinary sense. In fact, the energy of the final (wetted) state, when water spreads freely 14995

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(6) Feng, X.-Q.; Gao, X.; Wu, Z.; Jiang, L.; Zheng, Q.-S. Superior water repellency of water strider legs with hierarchical structures: Experiments and analysis. Langmuir 2007, 23, 4892−4896. (7) Sun, T; Feng., L.; Gao, X.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38, 644−652. (8) Zheng, Y.; Gao, X.; Jiang, L. Directional adhesion of superhydrophobic butterfly wings. Soft Matter 2007, 3, 178−182. (9) Bormashenko, E.; Bormashenko, Y.; Stein, T.; Whyman, G.; Bormashenko, E. J. Why do pigeon feathers repel water? Hydrophobicity of pennae, Cassie−Baxter wetting hypothesis and Cassie− Wenzel capillarity-induced wetting transition. J. Colloid Interface Sci. 2007, 311, 212−216. (10) de Gennes, P. G.; Brochard-Wyart, F.; Quéré, D. Capillarity and Wetting Phenomena; Springer: Berlin, 2003. (11) Shibuichi, A.; Onda, T.; Satoh, N.; Tsujii, K. Super waterrepellent surfaces resulting from fractal structure. J. Phys. Chem. 1996, 100, 19512−19517. (12) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Effects of surface structure on the hydrophobicity and sliding behavior of water droplets. Langmuir 2002, 18, 5818−5822. (13) Blossey, R. Self-cleaning surfaces: Virtual realities. Nat. Mater. 2003, 2, 301−306. (14) Patankar, N. A. Mimicking the lotus effect: Influence of double roughness structures and slender pillars. Langmuir 2004, 20, 8209− 8213. (15) Quéré, D. Wetting and roughness. Annu. Rev. Mater. Res. 2008, 38, 71−99. (16) Herminghaus, S. Roughness-induced non wetting. Europhys. Lett. 2000, 52, 165−170. (17) Nosonovsky, M.; Bhushan, B. Roughness-induced superhydrophobicity: A way to design non-adhesive surfaces. J. Phys.: Condens. Matter 2008, 20, 225009. (18) Nosonovsky, M.; Bhushan, B. Biologically Inspired Surfaces: Broadening the Scope of Roughness. Adv. Funct. Mater. 2008, 18, 843−855. (19) Kietzig, A.-M.; Hatzikiriakos, S. G.; Englezos, P. Patterned superhydrophobic metallic surfaces. Langmuir 2009, 25, 4821−4827. (20) Bormashenko, E.; Stein, T.; Whyman, G.; Bormashenko, Y.; Pogreb, R. Wetting properties of the multiscaled nanostructured polymer and metallic superhydrophobic surfaces. Langmuir 2006, 22, 9982−9985. (21) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (22) Cassie, A. B. D. Contact angles. Discuss. Faraday Soc. 1948, 3, 11−16. (23) Boreyko, J. B.; Chen, C.-H. Restoring superhydrophobicity of lotus leaves with vibration-induced dewetting. Phys. Rev. Lett. 2009, 103, 174502. (24) Zheng, Q.-S.; Yu, Y.; Zhao, Z.-H. Effects of hydraulic pressure on the stability and transition of wetting modes of superhydrophobic surfaces. Langmuir 2005, 21, 12207−12212. (25) Ishino, C.; Okumura, K.; Quéré, D. Wetting transitions on rough surfaces. Europhys. Lett. 2004, 68, 419−425. (26) Shirtcliffe, N. J.; McHale, G.; Newton, G. M. I.; Perry, C. C. Wetting and wetting transitions on copper-based super-hydrophobic surfaces. Langmuir 2005, 21, 937−943. (27) Sbragaglia, M.; Peters, A. M.; Pirat, C.; Borkent, B. M.; Lammertink, R. G. H.; Wessling, M.; Lohse, D. Spontaneous breakdown of superhydrophobicity. Phys. Rev. Lett. 2007, 99, 156001. (28) Bahadur, V.; Garimella, S. V. Preventing the Cassie-Wenzel transition using surfaces with noncommunicating roughness elements. Langmuir 2009, 25, 4815−4820. (29) Barbieri, L.; Wagner, E.; Hoffmann, P. Water wetting transition parameters of perfluorinated substrates with periodically distributed flat-top microscale obstacles. Langmuir 2007, 23, 1723−1734. (30) Bartolo, D.; Bouamrirene, F.; Verneuil, E.; Buguin, A.; Silberzan, B.; Moulinet, S. Bouncing of sticky droplets: Impalement transitions on superhydrophobic micropatterned surfaces. Europhys. Lett. 2006, 74, 299−305.

As in the case of the limiting capillary pressure, the energetic barrier to the wetted regime is the increasing function of both the advancing contact angle, θa, and the linear density of barbules, s. The surface energy increase in the course of water penetration between barbules is due to the sharp growth of the high-energetic liquid−air interface. Two structural factors favor this increase as far as this interface proceeds: the raise of the distance between barbules and the growth of its angle at the center, β = φ + θa − π. Both are determined by the physical properties and topography of the barbules’ network. It should be emphasized that the pronounced water repellency of lotus leaves is due to the true hierarchical topography of their relief, as shown in reference 45. As it was demonstrated in reference 45, the hierarchical topography increases the potential barrier separating the Cassie and the Wenzel wetting states. It was also shown in reference 24 that the small-scale roughness inherent to lotus leaves increases the critical pressure, allowing the Cassie wetting regime.

3. CONCLUSIONS We conclude that the fascinating water repellency of birds’ feathers and lotus leaves is due to very different physical mechanisms. The superhydrophobicity of lotus leaves is stipulated by the true hierarchical topography of their surfaces, whereas, the topography of birds’ feathers is pseudohierarchical. Hence, it does not promote formation of the high apparent contact angle. The high stability of the Cassie wetting regime observed on birds’ feathers is due to two factors: high critical pressures preventing water penetration into feathers’ network and high values of potential barriers separating the Cassie and the transition wetting states (there is no traditional Wenzel wetting for feathers). The high values of these factors arise from the topography of barbules-built network constituting birds’ feathers. Thus pronounced superhydrophobicity may be achieved by different physical and chemical means, not necessarily manifested by a true hierarchical topography of the relief.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Yelena Bormashenko for her kind help in preparing this manuscript. REFERENCES

(1) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (2) Koch, K.; Bhushan, B.; Barthlott, W. Multifunctional surface structures of plants: An inspiration for biomimetics. Progress in Materials Sci. 2009, 54, 137−178. (3) Yan, Y. Y.; Gao, N.; Barthlott, W. Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces. Adv. Colloid Interface Sci. 2011, 169, 80−105. (4) Koch, K.; Barthlott, W. Superhydrophobic and superhydrophilic plant surfaces: An inspiration for biomimetic materials. Philos. Trans. R. Soc. London, Ser. A 2009, 367, 1487−1509. (5) Cheng, Y.-T.; Rodak, D. E. Is the Lotus leaf superhydrophobic? Appl. Phys. Lett. 2005, 86, 144101. 14996

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(31) Moulinet, S.; Bartolo, D. Life and death of a fakir droplet: Impalement transitions on superhydrophobic surfaces. Eur. Phys. J. E: Soft Matter and Biological Physics 2007, 24, 251−260. (32) Jung, Y. C.; Bhushan, B. Wetting transition of water droplets on superhydrophobic patterned surfaces. Scr. Mater. 2007, 57, 1057− 1060. (33) Jung, Y. C.; Bhushan, B. Dynamic effects induced transition of droplets on biomimetic superhydrophobic surfaces. Langmuir 2009, 25, 9208−9218. (34) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Vibration-induced Cassie−Wenzel wetting transition on rough surfaces. Appl. Phys. Lett. 2007, 90, 201917. (35) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Cassie− Wenzel wetting transition in vibrating drops deposited on rough surfaces: Is dynamic Cassie−Wenzel wetting transition a 2D or 1D affair? Langmuir 2007, 23, 6501−6503. (36) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Resonance Cassie−Wenzel wetting transition for horizontally vibrated drops deposited on a rough surface. Langmuir 2007, 23, 12217− 12221. (37) Bormashenko, E.; Pogreb, R; Stein, T.; Whyman, G.; Erlich, M.; Musin, A.; Machavariani, V.; Aurbach, D. Characterization of rough surfaces with vibrated drops. Phys. Chem. Chem. Phys. 2008, 27, 4056− 406. (38) Reyssat, M.; Yeomans, J. M.; Quéré, D. Impalement of fakir drops. Europhys. Lett. 2008, 81, 26006. (39) Liu, B.; Lange, F. Pressure induced transition between superhydrophobic states: Configuration diagrams and effects of surface feature size. J. Colloid Interface Sci. 2006, 298, 899−909. (40) Feng, L.; Zhang, Y.; Xi, J.; Zhu, Y.; Wang, N.; Fan Xia, F.; Jiang, L. Petal effect: A superhydrophobic state with high adhesive force. Langmuir 2008, 24, 4114−4119. (41) Bormashenko, E.; Stein, T.; Pogreb, R.; Aurbach, D. “Petal effect” on surfaces based on lycopodium: High stick surfaces demonstrating high apparent contact angles. J. Phys. Chem. C 2009, 113, 5568−5572. (42) Bhushan, B.; Nosonovsky, M. The rose petal effect and the modes of superhydrophobicity. Philos. Trans. R. Soc. London, Ser. A 2010, 368, 4713−4728. (43) Bormashenko, E.; Stein, T.; Whyman, G.; Pogreb, R.; Sutovsky, S.; Danoch, Y.; Shoham, Y.; Bormashenko, Y.; Sorokov, B.; Aurbach, D. Superhydrophobic metallic surfaces and their wetting properties. J. Adhes. Sci. Technol. 2008, 22, 379−385. (44) Bormashenko, E.; Grynyov, R. Plasma treatment induced wetting transitions on biological tissue (pigeon feathers). Colloids Surf., B 2012, 92, 367−371. (45) Whyman, G.; Bormashenko, Ed. How to make the Cassie wetting state stable? Langmuir 2011, 27, 8171−8176. (46) Savoy, E. S.; Escobedo, F. A. Molecular simulations of wetting of a rough surface by an oily fluid: Effect of topology, chemistry, and droplet size on wetting transition rates. Langmuir 2012, 28, 3412− 3419.

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