Modifying the anti-wetting property of butterfly wings and water strider legs by atomiclayer deposition coating: surface materials versus geometry

Abstract

Although butterfly wings and water strider legs have an anti-wetting property,their working conditions are quite different. Water striders, for example, live in awet environment and their legs need to support their weight and bear the highpressure during motion. In this work, we have focused on the importance of thesurface geometrical structures in determining their performance. We haveapplied an atomic layer deposition technique to coat the surfaces of bothbutterfly wings and water strider legs with a uniform 30 nm thick hydrophilicAl2O3 film. By keeping the surface material the same, we have studied the effect of differentsurface roughness/structure on their hydrophobic property. After the surface coating, thebutterfly wings changed to become hydrophilic, while the water strider legs still remainedsuper-hydrophobic. We suggest that the super-hydrophobic property of the water strider isdue to the special shape of the long inclining spindly cone-shaped setae at the surface. Theroughness in the surface can enhance the natural tendency to be hydrophobic orhydrophilic, while the roughness in the normal direction of the surface is favorable forforming a composite interface.