Biomimic from the superhydrophobic plant leaves in nature: Binary structure and unitary structure
Introduction
Wettability of the solid surface is a characteristic property of materials and largely depends on both the surface energy and the surface structure [1], [2], [3], [4], [5]. A surface on which water contact angle is greater than 150°, and but sliding angle less than 10°, is commonly called a superhydrophobic surface. Many plants in nature, including the lotus leaf, exhibit the unusual wetting characteristics of superhydrophobicity and self-cleansing property. This property of self-cleansing has been investigated thoroughly and the main reason is ascribed to the weak interaction between the surface and the water, resulting in superhydrophobicity [6], [7], [8], [9], [10], [11], [12].
The discovery of the lotus effect is of great technological interest [13], [14], [15]. Many research results demonstrate that nature accomplishes this fascinating mainly effect by virtue of the especial surface topography in regardless of surface energy that presents two different length scales to the outside environment (only for lotus leaves) [16], [17], [18], [19], [20]. By transferring this fascinating effect to artificial surfaces, and yielding surfaces that can be cleaned by a simple rainfall, numeral technical applications are possible [21], [22], [23], [24], [25]. The biomimic artificial superhydrophobic surfaces are applied not only for resisting water [26] and fog condensation [27], but also preventing contamination [28]. Up to date, with the development of nanotechnology and their potential applications have extended to some new fields, such as biocompatibility [29], lubricity [30] and durability of materials [31], for their low free energy surface.
Although the especial surface structures of lotus leaves were still being researched in past decades, a few questions remain unanswered. Is the micro-and nanostructure the necessary condition for fabricating an artificial superhydrophobic surface? Is a surface microstructure in many other superhydrophobic plant leaves similar to that of lotus leaf? What is the hydrophobic wax-like material covering the lotus surface? To look for some potential clues about these, our research group did some work in past five years continuously. This paper focuses on the superhydrophocity of some natural plant leaves and their corresponding microstructures to try to explain the above questions, and on several reported superhydrophobic surfaces by means of biomimicing the structures of some natural superhydrophobic plant leaves.
Section snippets
Sample preparation
The localities of plant leaves used in this paper and their corresponding habitats are shown in Table 1. The as-prepared samples of the plant leaves were cut to about 4 mm × 4 mm size, and prefixed beyond 24 h at about 4 °C with 2.5 wt % glutaraldehyde, and then treated with 1.5 wt% osmium acid at 2 h. The treated samples were spurted a film of gold, about 5 nm thick, to enhance their conductivity before characterization by means of SEM.
Surface characterization
The samples were examined by FTIR spectrometry (on a bio-Rad
Binary structures
In several Asian countries and regions, especially in China, the lotus flower is revered as the purity symbol due to its self-cleansing property. The lotus’ leaves unfold are not untouched by the pollution even when emerging from muddy waters (Fig. 1a). We often see, in a pond after a rainfall, spherical water droplets on floating lotus leaves, carrying effortlessly the contaminations attached on the leaves when the surface is slightly tilted, showing self-cleansing function and low hysteresis (
Conclusion
Studying from nature can give us inspirations to engineer special wettability on functional surfaces through constructing appropriate surface morphologies and chemical modifications. In this paper, we have studied the eight plant leaves with superhydrophobicity by measuring the water contact angle measurement and by using SEM. We make a conclusion that there exist two main surface morphologies, one is the hierarchical micro-and nanostructure, and the other is unitary under nanoscale or
Acknowledgments
The authors acknowledge the National Natural Science Foundation of China for continuing financial support (50421502 and 50432020) and National 973 Project (No. 2007CB607601), and thank Yan Xin, Bo Wang, and Jiazheng Zhao for fruitful discussons.
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