Micro-structures and coupling mechanism of hydrophobicity on locust wing surface

Using a stereo microscope, a scanning electron microscope, a Fourier transform infrared spectrometer, and a video-based contact angle meter, the micro-structure, chemical composition, and hydrophobicity of wing surface of five butterfly species were investigated. The hydrophobicity model based on micro-structure of locust wing surface was established, and the coupling mechanism of hydrophobicity on locust wing surface was discussed. The wing surfaces of four butterfly species tested display regular micrometer-rough structure. Pillar gibbosities distribute on fore wing and hind wing surfaces of Oedaleus infernalis, Atractomorpha heteroptera, Aiolopus tamulus, and Oxya chinensis, the diameter of gibbosity is 1.6~2.3 μm, the height of gibbosity is 2.3~3.2 μm, the spacing of gibbosity is 6.8~7.6 μm. Locust wing surface is a wax layer composed by long chain hydrocarbon, tallate and fatty-acid alcohol, which is a natural hydrophobic surface and has higher hydrophobicity (contact angle 139.0°~150.4°). Composite contact occurs between droplet and wing surface, fitting Cassie equation. The high hydrophobicity of wing surface is the result of cooperative effect by waxy crystal (material coupling elements) and micrometer-rough structure (structure coupling elements).