Water Wetting Observation on a Superhydrophobic Hairy Plant Leaf Using Environmental Scanning Electron Microscopy

Mankind has been continuously learning from nature via careful observation in attempt to improve their own lives and to further overcome the environmental issues. In the last 10 years, researchers have focused their attention on the surface functions of parts of living organisms, such as gecko’s feet, water strider’s legs, and lotus leaves for various applications (e.g., self-cleaning surfaces, nano-micro robotics, and water harvesting) (Bhushan & Her, 2010; Cheng et al., 2005; Cho & Choi, 2008; Gao & Jiang, 2004; Hansen & Autumn, 2005; Neinhuis & Barthlott, 1997). In particular, functional plant surfaces such as lotus or acacia leaves have been studied owing to their excellent characteristics of superhydrophobicity and self-cleaning, which are a result of their structures and surface materials (Cha et al., 2010; Cheng et al., 2005; Neinhuis & Barthlott, 1997). For characterizing superhydrophobicity, one can use simple water droplet measurements on target surfaces or measure the squeezing pressure at the micro-scale. Measurement of water condensation with water vapors is also a key indicator of robust superhydrophobicity at the nanoscale (Ko et al., 2012, 2015; Quéré, 2008; Shin et al., 2012; Varanasi et al., 2009). When a water droplet is placed on a superhydrophobic plant leaf possessing nanoor micro-scale roughness like hair or bump-shapes (Fig. 1A), the droplet forms a nearly perfect spherical shape, rolls off, and cleans the leaf surface as shown in Fig. 1B. This self-cleaning and water-repellent behavior is typically attributed to the roughness of the surface and lowsurface-energy coatings such as the hydrophobic epicuticular wax crystalloid coating of lotus leaves. Superhydrophobicity has been commonly characterized by higher water contact angles (i.e., higher than 160) and wetting angle hysteresis of drop rolling. However, because this method uses relatively large water droplets with millimeter-scale sizes, additional information on the quality of superhydrophobicity has been recently obtained. A robust superhydrophobic surface should be able to sustain high contact angles against condensation