Investigation on hydrophobicity of lotus leaf: Experiment and theory

A detail experiment investigation on hydrophobicity of lotus leaf had been given, including contact angle, entrapped gas, micrometer and nanometer surface topographies. Four kinds of lotus leaf surfaces were involved: fresh front (FF), dried front (DF), fresh back (FB) and dried back (DB) surfaces. The basic mechanisms of the liquid spreading on a rough surface and the entrapped gas under a drop had been discussed. Based on the mechanisms and experiment, hydrophobicity of the four lotus leaf surfaces was analyzed. The investigation results showed that the hydrophobicity of the front surfaces was dominated by the entrapped gas under the drop and the absolute stable position of the contact line while the back surfaces by metastable positions of the contact line, resulting in the ultrahydrophobicity on the front surfaces and large contact angle hysteresis on the back surfaces. The effects of dual-size surface structures on the hydrophobicity were discussed in detail. Because the nano-pillars were firmly close together, there was no entrapped gas in the nano-structure on the front surfaces. The structure of micro-pillars was a main cause for the ultrahydrophobicity of the front surfaces. However, on the FB surface, the structure of nano-pillars was a main cause to induce entrapped gas.