Introduction

The study of biological microstructures has been an active area of research because these microstructures bring about many unique properties.^1-6 For example, many plant leaves,² and insect wings⁵ and legs⁶ exhibit unusual self-cleaning character, i.e., the so-called lotus effect. Water droplets do not stay stably on these surfaces, where they can spontaneously roll off with a slight tremble. During this process, dust particles on the surface are removed. This self-cleaning property is not only useful in biology, but also very instructive for the design of new materials, where a natural force might be used to clean a surface.

The self-cleaning phenomenon is usually explained as the cooperation of rough surface with special micro- and nanostructures and low surface energy materials, which lead to superhydrophobic property with both a high contact angle (greater than 150) and a low sliding angle (less than 5).² Up to now, a variety of such surfaces have been theoretically studied and also artificially prepared,⁷ including films of carbon,⁸ polymers,^9-12 nanostructured inorganic oxide,^13,14 and some others.^15-17 Another superhydrophobic adhesive surface, showing both a large contact angle and a high contact angle hysteresis, represents its importance in wetting behavior. We have reported the preparation of superhydrophobic polystyrene nanotubes layer with high adhesive force mimicking gecko's foot.¹⁸ This material has been successfully applied to no lost reversible transport of microliter-sized superparamagnetic liquid droplets by alternating magnetic fields.¹⁹

Generally, there are two superhydrophobic states on a rough surface: Wenzel's state and Cassie's State. The former represents a wet-contact mode of water and rough surface, where water droplets pin the surface to form a high contact angle hysteresis. The latter represents a nonwet-contact mode and water droplets can roll off easily owing to the low contact angle hysteresis. We recently clarified the definition of superhydrophobic surface as five states, in which lotus and gecko attribute to the special case of Cassie's state.²⁰ Compared with the lotus effect widely observed in the nature, less examples are known showing the adhesive property with an important Cassie impregnating wetting state. Therefore, the study of the sixth superhydrophobic state in the natural system is of significance not only for academic reasons but also for their importance in practical applications.

Here, we disclose for the first time that there is a close array of micropapillae on the surfaces of the petal of red rose (rosea Rehd). We also show that many nanofolds exist on each papillae top. These hierarchical micro- and nanostructures provide sufficient roughness for superhydrophobicity but have high adhesive force with water. A water droplet on the surface of these petala is sphere in shape, which cannot roll off even when the petal is turned upside down. We define this phenomenon as the "petal effect" as compared with the familiar "lotus effect". Artificial fabrication of biomimic polymer films, with well-defined nanoembosses by duplicating the petal's microstructures, indicates that the superhydrophobic and adhesive petal is in Cassie impregnating wetting state. Note that much research has previously been performed on the lotus leaf that tends to be in Cassie's state, whereas little has been studied on the Cassie impregnating wetting state in the nature. Therefore, the finding of petal effect should be of great biological and technological importance.

Experimental Section

Synthesis. All the chemicals are commercially available analytical grade reagents and were used without further purification. The duplicated processing steps involved in the preparation of superhydrophobic adhesive surfaces are illustrated in the Supporting Information (Figure S1). Poly(vinyl alcohol) (PVA, M_w = 22 000 g mol^-1, ca. 10 wt %) water solution was poured onto the surface of a red rose petal and exposed to air under ambient conditions. When water was evaporated completely at room temperature, the PVA film was peeled off, which imprinted the inverse petal structures. Polystyrene (PS, M_w = 100 000 g mol^-1) films were then obtained by pouring 15 wt % PS chloroform solution onto the prepared PVA film, which were subsequently dried and peeled off.

Characterization. The morphological characterization of the samples was examined by using scanning electronic microscope (SEM). SEM measurements of the fresh petals were conducted on a Hitachi S-3000N scanning electronic microscope in the low vacuum mode, while SEM measurements of the polymer films were obtained on a JEOL 6700F scanning electronic microscope in the high vacuum mode. Contact angles were measured on a dataphysics OCA20 contact angle system at ambient temperature. Water droplets (2.0 L) were dropped carefully onto the surface of samples. The average contact angle was obtained by measuring at five different positions of the same sample.

Results and Discussion

Surface Morphology and Surface Wettability. Figure 1a illustrates the typical scanning electronic micrograph of a usual red rose petal observed at low vacuum, showing a periodic array of micropapillae with an average diameter of 16 m and height of 7 m. The magnified SEM image in Figure 1b clearly reveals that these micropapillae exhibit cuticular folds in the nanometer scale, about 730 nm in width on each top. It is known that the hydrophobicity of a surface can be enhanced by being textured with different scale structures. In nature, the surface of the lotus leaf is famous for its self-cleaning property, which is induced by the roughness at two length scales amplifying the intrinsic hydrophobicity. Similar to this effect, the petal's surface also exhibits superhydrophobicity with a contact angle of about 152.4 (Figure 1c) owing to its surface micro- and nanostructures. However, the diverse design in the surface microstructures and the different sizes of the lotus leaf and the red petal result in different dynamic wetting. That is, water droplets with the same volume can effortlessly roll off the surface of a lotus leaf, while they stay pinned to the surface of a red rose petal. Water droplets on the petal's surface maintain the sphere shape when the surface is facing up or even when it is turned upside down (Figure 1d), showing a high contact angle hysteresis. The crucial parameter for this effect is the volume of the droplet. For a small droplet, the weight is small compared to the surface tension force, and thus it is expected that a droplet will stick to the surface. When the volume of the water droplet is about 10 L, a balance of the weight and the surface tension is reached, above which the droplet will fall (for more details, see Supporting Information Figure S2). This character imparts flowers special properties in that small water droplets can stay stably on the petals maintaining their fresh looking, while the bigger ones such as raindrops can roll off.

Figure 1 (a, b) SEM images of the surface of a red rose petal, showing a periodic array of micropapillae and nanofolds on each papillae top. (c) Shape of a water droplet on the petal's surface, indicating its superhydrophobicity with a contact angle of 152.4. (d) Shape of water on the petal's surface when it is turned upside down.

Duplication of Petal's Surface. The surface microsructure and surface property of natural petals provide us inspiration to fabricate biomimic polymer films by using the petal as a duplicated template and the solvent-evaporation-driven nanoimprint pattern transfer process at room temperature. In a typical experiment, 10 wt % PVA solution in water was first poured onto the surface of a fresh red rose petal and exposed to air under ambient conditions. When water was evaporated completely at room temperature, the PVA film could be peeled off, and it imprinted the inverse petal's surface microstructures. Subsequently, PS film with the exact petal structure can be obtained by pouring 15 wt % PS chloroform solution onto the prepared PVA film, allowing it to dry, and then peeling off. The SEM images of the PVA and PS films are given in Figure 2. From panels a and b of Figure 2, we can see that PVA film is characterized as the inverse petal's structures with a close-packed array of approximately hemispherical concaves and ditches in the middle of the concave. Panels c and d of Figure 2 show the representative SEM images of the PS film, which was duplicated from the textured PVA film. It is worth to note that the surface of the PS film with a periodic array of embossment shows remarkable microstructures and sizes similar to that of the original red rose petal. This PS film with rough structure shows adhesive superhydrophobicity with a contact angle of 154.6, although the flat PS film exhibits only hydrophobicity with a contact angle of about 95.¹⁸ Importantly, the duplicated PS film shows a high contact angle hysteresis, i.e., a water droplet placed on the film stably stays on the surface and cannot roll off even when the film is tilted until turned upside down (insert in Figure 2d).

Figure 2 (a, b) SEM images of the duplicated PVA film with inverse petal structures. (c, d) SEM images of the duplicated PS film with the similar petal's surface structures. (Inset in panel d is the shape of a water drop on the PS film when it is turned upside down, indicating its superhydrophobic adhesive property.)

Theoretical Analysis. In general, surfaces with a static contact angle higher than 150 are defined as superhydrophobic surfaces. As previously reported, there are two possible origins for superhydrophobicity: (1) the liquid attaches to the solid surface (Wenzel's state), and (2) it leaves air inside the texture (Cassie's state).²¹ The Wenzel model describes homogeneous wetting by the equation²²

The Cassie model describes heterogeneous wetting by the equation²³

As for the details of contact angle hysteresis, Wenzel's state can induce a high contact angle hysteresis and Cassie's state a low contact angle hysteresis. In the past, much research has been performed on these two states, especially on lotus leaf that tends to exhibit Cassie's state. However, considerably little has been studied on another important superhydrophobic Cassie impregnating wetting state. In Cassie impregnating wetting state, grooves of the solid are wetted with liquid and solid plateaus are dry.²⁴ The Cassie impregnating wetting regime is described with equations which are different from the Wenzel and Cassie ones. In this regime, the liquid film impregnates the texture; however, there will always remain islands that emerge above the "absorbed" liquid film. On the basis of the hierarchical micro- and nanostructures on the surface of petal and the duplicated polymer, it could be suggested that the wetting regime of rose petals when both contact angles and adhesion are large is in the Cassie impregnating wetting state (Figure 3).

Figure 3 Schematic illustrations of a drop of water in contact with the petal of a red rose (the Cassie impregnating wetting state) and a lotus leaf (the Cassie's state).

This observation can be attributed to the difference of the microstructures and chemical composition between the petal and lotus leaf. For the low contact angle hysteresis, such as in the case of lotus leaf with hydrophobic waxes covered, triple contact lines on a randomly rough surface are expected to be contorted and extremely unstable, preventing water from intruding into the microstructure spaces.^25-28 Thus, the droplet is constantly advancing and receding at different contact line points, and it moves very easily. As to the petal, the sizes of hierarchical micro- and nanostructures are both larger than those of the lotus leaf. Water droplets are expected to enter into the larger scale grooves of the petal but not into the smaller ones, thus, to form the Cassie impregnating wetting regime.²⁹ It can be readily understood that water sealed in micropapillae would be clinged to the petal's surface, showing a high contact angle hysteresis in the range of volume, when the surface is tilted to any angle or even turned upside down. The adhesive property of petal provides us an effective way to duplicate the surface microstructure. For comparison, the same process was performed using a lotus leaf as the template. Unfortunately, the prepared PVA film did not exhibit the exact inverse microstructures of lotus leaf, that is, only the top of each papilla was duplicated owing to the nonwetting between lotus leaf and PVA water solution (see Supporting Information Figure S3). Sun et al.³⁰ have previously reported on how to create a superhydrophobic surface using a lotus leaf as the template and to duplicate it by nanoscale casting. In their study, thermoplastic materials, heating, and solidification were necessary. The simple duplication of petal surface provides us not only a theoretical explanation of the phenomenon but also an inspiration for the preparation of biomimic polymer films, which should be of great biological and technological importance.

Other Petals. The microstructures and the special superhydrophobicity with a high contact angle hysteresis can also be found on other flower petals due to their periodic array of microsturctures. As a typical example, panels a and b of Figure 4 show representative SEM images of the petal of Chinese Kafir lily, which is characterized as close-packed hexagons with an average side length of 75 m and a strip width of 780 nm in each hexagon. The SEM image of the PVA film duplicated from the petal of Chinese Kafir lily is shown in Figure 4c, while that of the corresponding PS film from the prepared PVA film is shown in Figure 4d, whose surface exhibits adhesive superhydrophobicity with a large contact angle and a high contact angle hysteresis (inset in Figure 4d). In another example, the sunflower petal shows a periodic array of parallel lines with an average diameter of 15 m and a helix width of 2.5 m on each line, (Figure 4e,f). Panels g and h of Figure 4 show the SEM images of the corresponding PVA and PS films, duplicated from the sunflower petal and the prepared PVA film. The PS film has the microstructures and wetting behavior similar to that of the original sunflower petal (inset in Figure 4h is the shape of a water droplet on the upside down PS film). We propose that these wetting behaviors mentioned above can be mainly attributed to the special microstructures and the Wenzel's state they possess. On the basis of the duplicated technique used herein, polymer films with different periodic microstructures can be easily obtained by using different types of flower petals as the templates. Moreover, such a duplication process can be applied to different polymer precursors, such as polyacrylonitrile, polyethylene, polypropylene, polyvinyl chloride, polydimethylsiloxane, polymethyl methacrylate, polyesters, and polyamides.

Figure 4 (a, b) SEM images of the surface of a Chinese Kafir lily petal, showing a periodic array composed of close-packed hexagons and strips in two scales. (c, d) SEM images of the duplicated PVA and PS films from Chinese Kafir lily petal (inset image is the shape of a water droplet on the upside down PS film). (e, f) SEM images of the surface of a sunflower petal, showing a periodic array composed of parallel lines and helices in two scales. (g, h) SEM images of the duplicated PVA and PS films from the sunflower petal (inset image is the shape of a water droplet on the upside down PS film).

Conclusion

In conclusion, the understanding of the petal effect provides us with an example of the nature of a superhydrophobic surface with a high adhesive force to water, which shows a unusual Cassie impregnating wetting state. The observation of the petal effect also prompts us to develop a simple method for fabricating biomimic polymer films that possesses both the superhydrophobicity and the adhesive property. Large-scale fabrication can be achieved by using the petal as a mold in the duplicating process, which is possible for industrial production with high throughputs. This study not only improves our understanding of the self-cleaning properties of natural species but also provides important insights into the design of new materials for applications in coatings, functional fibers, and decoration. Material fabrication using the natural petals, an environment friendly material, as templates has the obvious merit over many other conventional techniques, which are not accessible for this purpose.

Acknowledgment

The authors thank the project funded by the National Nature Science Foundation of China (50703020), Tsinghua Basic Research Foundation (JCpy2005059), and Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (200526) for continuing financial support. Thanks to Professor Xi Zhang and Professor Lei Liu (Tsinghua University) for the helpful discussions.