Strelitzia reginae Leaf as a Natural Template for Anisotropic Wetting

Article pubs.acs.org/Langmuir

Strelitzia reginae Leaf as a Natural Template for Anisotropic Wetting and Superhydrophobicity Elisa Mele,*,† Salvatore Girardo,‡ and Dario Pisignano‡,§ †

Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia, via Barsanti, I-73010 Arnesano (LE), Italy National Nanotechnology Laboratory of Consiglio Nazionale delle Ricerche-Istituto Nanoscienze, and §Dipartimento di Ingegneria dell’Innovazione, Università del Salento, via Arnesano, I-73100 Lecce, Italy

‡

ABSTRACT: Artificial surfaces that exhibit unidirectional water spreading and superhydrophobicity are obtained by Strelitzia reginae leaves. Both green and dried leaves are used, thus exploiting the plant senescence. We demonstrate that the natural drying process of the leaves strongly affects the surface morphology and wettability. Polymeric stamps from the green leaf show an arrangement of periodic microridges/microgrooves that favor anisotropic wetting, with a water contact angle (WCA) variation of about 21% along the two principal directions. Instead, the shrinkage of the leaf tissue, as a consequence of the natural dehydration process, induces an enhancement of the superficial corrugation. This results in the establishment of a superhydrophobic state, which shows a WCA of up to 160°, and water rolling off. S. reginae leaves are therefore easily accessible stamps suitable for controlling wettability and realizing surfaces that exhibit various wetting behaviors.

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INTRODUCTION Nature provides a wide selection of surfaces characterized by hierarchical structures at micro- and nanoscales, such as lotus, rice, and taro leaves,1 rose petals,2 grasshopper3 and butterfly wings,4 gecko feet, fish scales, and others.5 Multiscale natural topographies often possess interesting wetting properties that include anisotropy3 and superhydrophobicity.6 The anisotropic wetting involves water contact angle (WCA) variations along one direction, resulting in elongated droplets. This effect finds many applications in liquid manipulation for microfluidics and lab-on-chips.7,8 Superhydrophobicity is typically defined as the condition of high WCA (above 150°) and low contact angle hysteresis (CAH; less than 10°), with the latter being expressed by the difference between the advancing (θadv) and receding (θrec) contact angles (CAs).2 Low CAH determines water roll off from the superhydrophobic surfaces, which is useful for selfcleaning paints and windows,9 nonwetting fabrics,10 and antifogging11 and anti-icing12 coatings. Artificial surfaces inspired by natural surfaces have been demonstrated by means of several micro- and nanofabrication approaches,1 such as electrochemical deposition, self-assembly, plasma etching, and chemical vapor deposition, and by replication methods based on the use of natural surfaces as templates.3 In particular, biomimetics involves the formation of functional surfaces inspired, adapted, or derived from nature13 and allows us to produce positive14 or negative15,16 copies of the micro- or nanoscale topographies existing in living organisms. In particular, anisotropic wetting phenomena are widely observed in natural surfaces.3,17,18 Therefore, these surfaces can be exploited as interesting templates to fabricate artificial systems exhibiting wettability anisotropy. For instance, © 2012 American Chemical Society

the anisotropic water repellency of Morpho nestira butterfly wings has been studied under low temperature and changeable relative humidity.4 Zhang et al. have reported the use of grasshopper wings as templates for making poly(dimethylsiloxane) (PDMS) surfaces that show anisotropic hydrophobicity.3 Wu et al. have realized sliding biosurfaces, inspired by rice leaves, by combining photolithography, PDMS imprinting, and coating.19 In addition to anisotropy, many studies have also been focused on the investigation of the superhydrophobic character of natural plant leaves, such as Nelumbo nucifera and Colocasia esculenta, whose micro- and nanoroughness and epicuticular wax crystals induce remarkable self-cleaning properties, known as the lotus effect.1,5 For instance, Sun et al. have created biosurfaces by replicating the lotus leaf by PDMS.20 Moreover, Bhushan et al. have produced superhydrophobic surfaces, inspired by rose petals and exhibiting low adhesion similar to lotus leaf, by combining molding processes and wax evaporation.2 Artificial surfaces that show anisotropic behavior have been produced by a number of other technologies, such as wrinkling,21 laser interference lithography,22 and projection lithography,23 and superhydrophobicity has been achieved by sol−gel methods,24 electrospray,25 and imprinting.26 However, direct replication methods certainly present clear advantages in terms of low cost, high reliability, and ease of processing. Here, we propose Strelitzia reginae leaves (green and dried) as novel templates to produce polymeric bioinspired surfaces Received: January 17, 2012 Revised: February 23, 2012 Published: March 8, 2012 5312

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with hierarchical microstructures, exploiting the plant senescence for achieving anisotropic wetting and superhydrophobicity. We investigate the morphological and wetting properties of the produced artificial surfaces, and we observe that the drying process of the leaves greatly influences the wettability. Indeed, polymeric replicas fabricated from green leaves are characterized by unidirectional water spreading, whereas replicas from dried leaves exhibit superhydrophobicity, with WCA values of up to 160°. S. reginae leaves are therefore easily accessible stamps, suitable for realizing surfaces with controlled wettability properties.

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EXPERIMENTAL SECTION

The procedure developed for processing and replicating the surfaces of S. reginae leaves is kept as simple as possible, aiming to a straightforward use of the natural templates. Elastomeric replicas of green and dried leaves are produced by replica molding with PDMS (Sylgard 184), purchased by Dow Corning (Midland, MI). The PDMS prepolymer, obtained by mixing the base and curing agent in a 10:1 (w/w) ratio, is poured onto the surface of the green and dried leaf and thermally polymerized in situ at room temperature for 24 h. We use the dried leaf as a template for obtaining multiple PDMS copies. On the contrary, the green leaf is employed for a few replication processes (most of the time, two processes), because it undergoes natural senescence over times of about 2 days, during which it shows significant morphological changes. To investigate the topography of the realized bioinspired surfaces, scanning electron microscopy (SEM) is performed by a Nova NanoSEM 450 instrument (FEI Europe) with an acceleration voltage of 10 kV and an aperture size of 30 μm. Moreover, the apparent WCA of flat PDMS substrates and PDMS replicas of the green and dried leaves is investigated by a KSVCAM200 instrument (KSV, Finland). Distilled water is used as liquid for these tests. Droplets with a typical volume of about 2 μL are dispensed onto the surfaces by a syringe, which is connected to the CA measuring system. The advancing and receding CAs are obtained by a dynamic method, carefully injecting and withdrawing water from the droplet volume through the syringe, respectively. SEM and WCA measurements are carried out on many different samples of green and dried leaves and many different regions of the same leaf. No significant differences are observed in both the superficial topography and the wetting behavior of the analyzed leaves.

Figure 1. Photographs of the (a) S. reginae plant and flower, (b) green leaf, and (c) dried leaf, with networks of microcapillaries in relief clearly visible. (d) Scheme of the molding procedure used to obtain PDMS replicas of the leaves. (e) Photograph of the resulting elastomeric stamp from the green leaf.

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RESULTS AND DISCUSSION S. reginae (also known as the “bird of paradise”) is a tropical perennial plant, native to South Africa, with exotic colorful flowers and large and lancet-shaped leaves (Figure 1a). Leaf lamina can reach 60 cm in length and 30 cm in width, and their petioles can exceed 1 m in length.27 As shown in panels b and c of Figure 1, the leaves are characterized by a network of microcapillaries in relief that correspond to the secondary veins of the vascular system of the plant and serve for liquid transport and mechanical reinforcement.28 We use green (Figure 1b) and dried (Figure 1c) leaves of S. reginae as templates for producing PDMS surfaces with multiscale topography (schematics in Figure 1d). Typical sample areas are of about 6 cm2. The replica molding method is widely employed as a simple, fast, and low-cost procedure to obtain elastomeric copies of natural surfaces.29 In fact, the chemical inertia and the elastic characteristics of the silicone elastomer make it appropriate for faithfully replicating fragile and complex structures. In particular, the PDMS is the elastomer of election for many microfluidic applications, because of its thermal stability, chemical inertia, biocompatibity, optical transparency, and low cost. These reasons, together with the possibility of exploiting the large size of S. reginae leaves, have led us to

choose PDMS for the production of bioinspired surfaces. Because replica molding allows us to replicate surfaces over large areas, the size of the bioinspired molds can reach 25−30 cm2, depending upon the dimensions of the plant leaf. A photograph of the elastomeric replica of the green leaf is presented in Figure 1e. The SEM planar views of the polymeric replicas, obtained from the green (stampgreen; Figure 2a) and dried (stampdried; Figure 2d) leaves, provide evidence of the oriented periodic ridges and grooves. Specifically, in the stampgreen, the ridges are intersected by a network of interconnected and unstructured grooves. The distribution of the feature periodicity (Λ) is welldescribed by a Gaussian curve peaked at 282 μm, with a dispersion [full width at half maximum (fwhm)] of about 37 μm (Figure 2b). Moreover, about 80% of the ridges have a width (w) in the range of 140−200 μm, resulting in a broad Gaussian curve, with a peak at 177 μm and fwhm of 65 μm (Figure 2c). Instead, the stampdried exhibits defined and continuous ridges alternated with microstructured grooves (Figure 2d). The corresponding Λ distribution is well-fitted by a Gaussian curve peaked at 263 μm, with a fwhm of 19 μm. A 5313

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Figure 2. Planar views of the (a) stampgreen and (d) stampdried, imaged by SEM and corresponding distributions of the period (b and e) and width (c and f) of the resulting microstructure. The superimposed lines are the best fits by Gaussian curves. The shown data are from different areas of the analyzed leaves.

by micropapillas that have a size of 10−20 μm. During the drying process, the natural dehydration rate is low and ordered arrays of fine wrinkle waves are formed.28 In fact, in the stampdried (panels b and d of Figure 3), the surface corrugation is accentuated and microstructures, parallel to the main axis of the ridges, are visible both on the ridges and in the grooves. Moreover, the height of the ridges increases up to 100 μm. The roughness of the produced bioinspired surfaces affects their wettability. The CA of a water droplet on an ideal, smooth solid surface is known to be described by Young’s equation, which considers the balance of the interfacial tension at liquid− air, solid−air, and liquid−solid interfaces.30 When a topographic roughness is present on the surface, two wetting regimes are possible, namely, a noncomposite state that is governed by Wenzel’s equation and a composite state that can be described by the Cassie−Baxter law.31 In the Wenzel model, the roughness enhances both hydrophobicity and hydrophilicity, depending upon the chemical nature of the surface. The CA of the rough surface (θW) is related to the CA of the smooth surface (θ) by cos θW = RW cos θ, where RW > 1 is the roughness factor, defined as the ratio of the actual area wet by the liquid to its projected area.31 The Cassie−Baxter regime is valid for composite solid−liquid−air interfaces, resulting from air trapped within the surface asperities. The CA of the rough surface (θCB) increases when more air pockets are formed, as described by the equation cos θCB = f CB(cos θ + 1) − 1, where f CB < 1 is the fraction of the solid surface wet by the liquid.31 Furthermore, if the geometry of the roughness is isotropic, the drop deposited on the surface takes a spherical shape, with the CA almost uniform along the contact line, in any direction.32,33 Instead, the presence of a superficial asymmetric topography, typically a one-dimensional arrangement of parallel micro- and nanostructures, induces a CA different for the principal directions of the rough surface (parallel and perpendicular to the structures). This results in the consequent

total of 40 and 27% of the measured period values are in the range of 250−260 and 260−270 μm, respectively. The distribution of w shows a peak at 146 μm, with a dispersion of 30 μm. Overall, the morphological analysis of the polymeric replicas of the S. reginae leaves highlights a reduction of the period and the width of the microfeatures of about 7 and 18%, respectively, after the drying process. Moreover, the dispersion of Λ and w is narrower for the stampdried than for the stampgreen, indicating more defined and periodic microstructures. The topographic variations, observed in the leaves upon drying, are associated with the shrinkage of the leaf tissues, as a consequence of the senescence process of the plant.28 This natural wrinkling phenomenon is most evident in Figure 3, where we show SEM images of the cross-section of the replicas at different magnifications. In the stampgreen (panels a and c of Figure 3), the ridges are ∼50 μm in height, and they are separated by smooth microchannels, which correspond to the secondary veins of the green leaf. The surface is characterized

Figure 3. Cross-section of PDMS replicas of green (a and c) and dried (b and d) leaves at different magnifications, collected by SEM at a tilting angle of 20°. 5314

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drop shape at the equilibrium derives from the minimization of the reduced free energy of the droplet, which is f = G/γlv, where G is the free energy of the system and γlv is the surface tension between the liquid and vapor.38−42 For the two directions, f can be expressed by41

elongation of the water droplet in a particular direction and in the occurrence of anisotropic wettability.32−34 Indeed, both experiments and simulations demonstrate that the CA perpendicular to the features is higher than the parallel CA, because of pinning effects.21,33 In fact, according to Gibbs’ criterion, the wetting contact line remains pinned at the edges of the features when the advancing CA along the perpendicular direction has a value between θ and θ + α, where α is the maximum inclination of the side walls with respect to the top of the structures.35−37 In particular, here, we obtain anisotropy using the green S. reginae leaf as the template. Smooth PDMS surfaces have an hydrophobic character with a WCA of 101° ± 3°. The pristine PDMS hydrophobicity is therefore enhanced by the micropattern. In addition, we notice that, when the water droplet contacts the surface of the stampgreen, it spreads in all directions until it reaches the vertical walls of the ridges that represent energy barriers for liquid spreading. Indeed, the droplet shape is strongly affected by the presence of the oriented microfeatures, and the contact line deviates from an ideal circle and becomes elongated along one direction.23,33 The superficial morphology therefore induces a preferential liquid imbibition along the microgrooves/ridges (∥) and liquid pinning in the perpendicular direction (⊥), as shown in Figure 4a. Along the

f⊥ =

D⊥θ⊥ − D⊥ cos θ sin θ⊥

f =

Dθ sin θ

− D cos θ

where D⊥, D∥, θ⊥, and θ∥ are the droplet base length and the WCA measured for the perpendicular and parallel directions, respectively. We estimate the droplet D parameters along the two directions from Figure 4a, obtaining D⊥ ≅ 1.9 mm and D∥ ≅ 2.6 mm. The difference in the resulting reduced free energy is of about 20%. This effect is demonstrated to originate a different wetting behavior along the two directions and possibly to favor complex oscillation patterns of the CA.38,42 Overall, the arrangement of microgrooves in the stampgreen (corresponding to the leaf veins) continues to favor liquid control and transport, such as in the green leaf of S. reginae. Instead, the increase in the hydrophobicity along the perpendicular direction with respect to smooth PDMS surfaces is related to the enhancement of the roughness, according to the Wenzel model, where a roughness factor of about 1.6 has been estimated by our SEM characterization. From the equation describing the Wenzel state and considering the WCA values of the flat PDMS and the stampgreen, we calculate RW ≅ 2, which is in agreement with the value measured by SEM. When the contact line is parallel to the ridges, roughness enhances water spreading in the grooves, with a consequent reduction of the WCA, because no energy barriers are responsible for liquid pinning.21 Furthermore, one observes that, for this noncomposite wetting state, where the water completely penetrates into the grooves, the height of the microstructures strongly affects the degree of anisotropic wetting. In fact, the larger depth of the ridges is typically associated with an increment of the energy barrier and, consequently, higher values of Δθ, as demonstrated by experiments and simulations.21,22,43 In particular, Wu et al. have studied the effect of the height of grooves on the anisotropic behavior of surfaces patterned by laser interface lithography, obtaining an increment of Δθ from 9° to 48° by varying the depth of the grooves from 100 nm to 1.3 μm.22 Zhao and co-workers have structured azobenzene surfaces by laser interference, observing a Δθ change of about 7° and 10° when the increment of the depth of the features is of about 60 and 80 nm, respectively.43 These studies also provide evidence that the effect of the groove depth is more significant along the perpendicular direction with respect to that parallel to the patterned features.44 Different from the stampgreen, the water droplet deposited on the surface of the stampdried keeps its spherical shape, without preferential spreading directions (Figure 4c). After the drying process of the leaf, the overall surface corrugation is accentuated, with the formation of hierarchical fine wrinkled structures. We believe that the appearance of this second length scale, more than the simple increment of the height of the ridges, is responsible for the ultimately superhydrophobic state of the elastomeric replicas. Indeed, this is a well-known effect, leading to a favored formation of air pockets between the water droplet and the top protruding features, and this would result in high WCA values, as described by a composite wetting model.2 As known, this state is not affected by the anisotropy of the surface topography.44 Indeed, same WCA values are obtained

Figure 4. Water droplets on the surface of the (a) stampgreen and (c) stampdried, imaged by optical microscopy. The arrows indicate the direction parallel and perpendicular to the grooves/ridges. (Right panels) WCA values for five different samples measured along the perpendicular (●) and parallel (○) direction for the (b) stampgreen and (d) stampdried. Lines are guides for the eyes. (Insets) Optical micrographs of water droplets and corresponding WCA values for the perpendicular (left insets) and parallel (right insets) directions.

perpendicular direction (●), θgreen⊥ is 115° ± 3°, whereas along the parallel direction (○), θgreen∥ is 91° ± 8°, i.e. decreasing by about 20%. The measured degree of wetting anisotropy, defined as the difference of the CA values for the two directions Δθ = θgreen⊥ − θgreen∥, is of about 24°. Because α is of about 30° for the stampgreen (Figure 3a), the measured WCA in the perpendicular direction is consistent with Gibbs’ prediction, and, with θgreen⊥ being higher than θgreen∥, the drop spreads preferentially along the direction parallel to the grooves, achieving the observed elongation. This determines anisotropic wetting, as displayed in Figure 4b. Moreover, the 5315

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along the perpendicular direction (● in Figure 4d; θdried⊥ = 164° ± 3°) and along the parallel direction (○ in Figure 4d; θdried∥ = 162° ± 3°). Moreover, the advancing and receding WCAs of the stampdried are measured to be about 168° and 160°, respectively. The water droplet tends to roll off from the surface of the stampdried with a measured CAH (=θadv − θrec) of 8°. Because of the low CAH, the water droplet easily moves across the superhydrophobic surface, and the critical line force needed to initiate the movement can be expressed by F = πRγlv(cos θrec − cos θadv),45,46 where R is the radius of the contact line. For the stampdried, F is about 10 μN, which is consistent with other findings on superhydrophobic PDMS surfaces with low adhesion.47 The roughness topography can indeed determine the formation of air pockets between the microstructured surface and the water droplet, thus leading to a composite interface underneath the liquid.48 Consequently, the solid−liquid contact area is reduced, resulting in a high CA and low CAH. The corrugation responsible for the superhydrophobic behavior is characterized by a solid fraction, f CB, of about 0.08, as obtained by taking into account the microscale top protruding features imaged by SEM. This is consistent with the value (≅0.05) calculated from the equation of the Cassie− Baxter state, by considering the wetting behavior of the analyzed surface.

from a natural stamp: Grasshopper wings. Soft Matter 2011, 7, 7973− 7975. (4) Mei, H.; Luo, D.; Guo, P.; Song, C.; Liu, C.; Zheng, Y.; Jiang, L. Multi-level micro-/nanostructures of butterfly wings adapt at low temperature to water repellency. Soft Matter 2011, 7, 10569−10573. (5) Liu, K.; Jiang, L. Bio-inspired design of multiscale structures for function integration. Nano Today 2011, 6, 155−175. (6) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (7) Extrand, C. W. Retention forces of a liquid slug in a rough capillary tube with symmetric or asymmetric features. Langmuir 2007, 23, 1867−1871. (8) Xia, D.; Brueck, S. R. J. Strongly anisotropic wetting on onedimensional nanopatterned surfaces. Nano Lett. 2008, 8, 2819. (9) Parkin, I. P.; Palgrave, R. G. Self-cleaning coatings. J. Mater. Chem. 2005, 15, 1689−1695. (10) Choi, W.; Tutej, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H. Fabrics with tunable oleophobicity. Adv. Mater. 2009, 21, 2190−2195. (11) Gao, X.; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography. Adv. Mater. 2007, 19, 2213−2217. (12) Cao, L.; Jones, A.; Sikka, V.; Wu, J. Anti-icing superhydrophobic coatings. Langmuir 2009, 25, 12444−12448. (13) Bhushan, B. BiomimeticsLessons from nature: An overview. Philos. Trans. R. Soc., A 2009, 367, 1445−1486. (14) Losic, D.; Mitchell, J. G.; Lal, R.; Voelcker, N. H. Rapid fabrication of micro- and nanoscale patterns by replica molding from diatom biosilica. Adv. Funct. Mater. 2007, 17, 2439−2446. (15) Lee, S.-M.; Lee, H. S.; Kim, D. S.; Kwon, T. H. Fabrication of hydrophobic films replicated from plant leaves in nature. Surf. Coat. Technol. 2006, 201, 553−559. (16) Sharma, C. S.; Abhishek, K.; Katepalli, H.; Sharma, A. Biomimicked superhydrophobic polymeric and carbon surfaces. Ind. Eng. Chem. Res. 2011, 50, 13012−13020. (17) Zheng, Y.; Gao, X.; Jiang, L. Directional adhesion of superhydrophobic butterfly wings. Soft Matter 2007, 3, 178−182. (18) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, 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. (19) Wu, D.; Wang, J.-N.; Wu, S.-Z.; Chen, Q.-D.; Zhao, S.; Zhang, H.; Sun, H.-B.; Jiang, L. Three-level biomimetic rice-leaf surfaces with controllable anisotropic sliding. Adv. Funct. Mater. 2011, 21, 2927− 2932. (20) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Artificial lotus leaf by nanocasting. Langmuir 2005, 21, 8978−8981. (21) Chung, J. Y.; Youngblood, J. P.; Stafford, C. M. Anisotropic wetting on tunable micro-wrinkled surfaces. Soft Matter 2007, 3, 1163−1169. (22) Wu, D.; Chen, Q.-D.; Yao, J.; Guan, Y.-C.; Wang, J.-N.; Niu, L.G.; Fang, H.-H.; Sun, H.-B. A simple strategy to realize biomimetic surfaces with controlled anisotropic wetting. Appl. Phys. Lett. 2010, 96, 053704. (23) Chu, K.-H.; Xiao, R.; Wang, E. N. Uni-directional liquid spreading on asymmetric nanostructured surfaces. Nat. Mater. 2010, 9, 413−417. (24) Wang, J. X.; Wen, Y. Q.; Hu, J. P.; Song, Y. L.; Jiang, L. Fine control of the wettability transition temperature of colloidal-crystal films: From superhydrophilic to superhydrophobic. Adv. Funct. Mater. 2007, 17, 219−225. (25) Mizukoshi, T.; Matsumoto, H.; Minagawa, M.; Tanioka, A. Control over wettability of textured surfaces by electrospray deposition. J. Appl. Polym. Sci. 2007, 103, 3811−3817. (26) Lee, Y.; Park, S. H.; Kim, K. B.; Lee, J. K. Fabrication of hierarchical structures on a polymer surface to mimic natural superhydrophobic surfaces. Adv. Mater. 2007, 19, 2330−2335.

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CONCLUSION In summary, we demonstrate that green and dried leaves of S. reginae are promising templates for producing artificial surfaces with different wetting properties. The imposed multiscale microstructures endow the bioinspired surfaces with anisotropic wettability and superhydrophobicity. In particular, PDMS stamps of the green leaf exhibit an arrangement of periodic microridges, separated by interconnected and smooth grooves, appertaining to the vascular system of the plant. This morphology favors water spreading in the direction parallel to the microfeatures and liquid trapping in the perpendicular direction, determining anisotropic wetting with a WCA variation of about 21% along the two directions. This anisotropic wettability has important potential applications in microfluidic control and directional water transport. During the natural dehydration, the shrinkage of the leaf tissue induces the formation of ordered arrays of fine wrinkle structures and the reduction of the width of the microfeatures by about 18%. In the polymeric replicas, this enhancement of superficial corrugation leads to an isotropic superhydrophobic state that shows WCA of about 164° and water rolling off, ideal for selfcleaning and nonwetting surfaces.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Guo, Z.; Liu, W.; Su, B.-L. Superhydrophobic surfaces: From natural to biomimetic to functional. J. Colloid Interface Sci. 2011, 353, 335−355. (2) Bhushan, B.; Her, E. K. Fabrication of superhydrophobic surfaces with high and low adhesion inspired from rose petal. Langmuir 2010, 26, 8207−8217. (3) Zhang, T.; Li, M.; Su, B.; Ye, C.; Li, K.; Shen, W.; Chen, L.; Xue, Z.; Wang, S.; Jiang, L. Bio-inspired anisotropic micro/nano-surface 5316

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(27) Szot, P.; Pogroszewska, E.; Rybczyñski, R. Effect of growth regulators on mechanical properties of strelitzia leaves. Int. Agrophys. 2009, 23, 87−96. (28) Xiao, H.; Chen, X. Modeling and simulation of curled dry leaves. Soft Matter 2011, 7, 10794−10802. (29) Gao, J.; Liu, Y.; Xu, H.; Wang, Z.; Zhang, X. Biostructure-like surfaces with thermally responsive wettability prepared by temperature-induced phase separation micromolding. Langmuir 2010, 26, 9673−9676. (30) Young, T. An essay on the cohesion of fluids. Philos. Trans. R. Soc. London 1805, 95, 65−87. (31) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem. Soc. Rev. 2007, 36, 1350−1368. (32) Chen, Y.; He, B.; Lee, J.; Patankar, N. A. Anisotropy in the wetting of rough surfaces. J. Colloid Interface Sci. 2005, 281, 458−464. (33) Semprebon, C.; Mistura, G.; Orlandini, E.; Bissacco, G.; Segato, A.; Yeomans, J. M. Anisotropy of water droplets on single rectangular posts. Langmuir 2009, 25, 5619−5625. (34) Liu, M.; Zheng, Y.; Zhai, J.; Jiang, L. Bioinspired superantiwetting interfaces with special liquid−solid adhesion. Acc. Chem. Res. 2010, 43, 368−377. (35) Blow, M. L.; Yeomans, J. M. Anisotropic imbibition on surfaces patterned with polygonal posts. Philos. Trans. R. Soc., A 2011, 369, 2519. (36) Kusumaatmaja, H.; Vrancken, R. J.; Bastiaansen, C. W. M.; Yeomans, J. M. Anisotropic drop morphologies on corrugated surfaces. Langmuir 2008, 24, 7299−7308. (37) Kusumaatmaja, H.; Pooley, C. M.; Girardo, S.; Pisignano, D.; Yeomans, J. M. Capillary filling in patterned channels. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2008, 77, 067301. (38) Kusumaatmaja, H.; Yeomans, J. M. Modeling contact angle hysteresis on chemically patterned and superhydrophobic surfaces. Langmuir 2007, 23, 6019−6032. (39) Oliver, J. F; Huh, C.; Mason, S. G. Resistance to spreading of liquids by sharp edges. J. Colloid Interface Sci. 1977, 59, 568−581. (40) Extrand, C. W. Modeling of ultralyophobicity: Suspension of liquid drops by a single asperity. Langmuir 2005, 21, 10370−10374. (41) Zhang, D.; Chen, F.; Yang, Q.; Siac, J.; Hou, X. Mutual wetting transition between isotropic and anisotropic on directional structures fabricated by femotosecond laser. Soft Matter 2011, 7, 8337−8342. (42) Zhang, X.; Mi, Y. Dynamics of a stick-jump contact line of water drops on a strip surface. Langmuir 2009, 25, 3212−3218. (43) Zhao, Y.; Lu, Q.; Li, M.; Li, X. Anisotropic wetting characteristics on submicrometer-scale periodic grooved surface. Langmuir 2007, 23, 6212−6217. (44) Li, W.; Fang, G.; Li, Y.; Qiao, G. Anisotropic wetting behavior arising from superhydrophobic surfaces: Parallel grooved structure. J. Phys. Chem. B 2008, 112, 7234−7243. (45) Kawasaki, K. Study of wettability of polymers by sliding of water drop. J. Colloid Sci. 1960, 15, 402−407. (46) Rothstein, J. P. Slip on superhydrophobic surfaces. Annu. Rev. Fluid Mech. 2010, 42, 89−109. (47) Jin, M.; Feng, X.; Xi, J.; Zhai, J.; Cho, K.; Feng, L.; Jiang, L. Super-hydrophobic PDMS surface with ultra-low adhesive force. Macromol. Rapid Commun. 2005, 26, 1805−1809. (48) Dorrer, C; Ruhe, J. Some thoughts on superhydrophobic wetting. Soft Matter 2009, 5, 51−61.

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