Study on the wettability and self-cleaning of butterfly wing surfaces

Many biological surfaces are hydrophobic because of their complicated composition and surface microstructure. Butterflies were selected to study their characteristics by Confocal Light Microscopy, Scanning Electron Microscopy and Contact Angle measurement. The contact angle of the water droplets on the butterfly wings surface consistently measured to be more than 140degrees. The dust on the surface can be easily cleaned by moving spherical droplets when the inclining angle is larger than 3degrees. It can be concluded that the butterfly wing's surface possess a water-repellent, self-cleaning, or "Lotus-effect" characteristic. Contact angle measurement of the wings' surface with and without squamas showed that the water-repellent characteristics are a consequence of the microstructure of the squamas. Each water droplet (diameter 2-3 mm) covers 3050 squamas each with a size of 40x80 microns. The regular riblets with a width of 1000-1500 nm are clearly observed on single squama. Such a nano-structure should play a very important role in the self-cleaning character.