Towards Understanding Why a Superhydrophobic Coating Is Needed by Water Striders

Abstract

have discovered that the legsof water striders have superhydrophobic coatings that bearhierarchical structures of many oriented tiny hairs with finenanogrooves. The superhydrophobic coatings were believedto be responsible for the water striders’ floating. However,since many other kinds of insects, such as mosquitoes, canfloat on the water surface with only normal hydrophobic coat-ings, it is not clear why superhydrophobic coatings are re-quired for the water strider’s legs. In the work reported here,using gold threads that are modified with normal hydrophobicor superhydrophobic coatings as model systems, we studied towhat extent the hydrophobicity contributes to the goldthread’s floating and movement on the water surface. In addi-tion, combining experimental results and theoretical forceanalysis, we report, for the first time, that deformation of thegold thread contributes to the total supporting force of thewater on the floating gold thread. For easily deformedfloatingobjects, such as very thin gold threads or legs of water striders,the contribution of deformation is significant compared to theother components of the supporting force. Our results suggestthat the superhydrophobiccoating of a water strider’s legs cannot only provide a larger supporting force, but also help to de-crease the insect’s total density beneath the water surface andallow it to move faster on the water surface. Therefore thesuperhydrophobic coating is indispensable, although the nor-mal hydrophobic coating provides enough supporting forcefor the water strider merely to float. We anticipate that ourresults provide a deeper understanding of the mechanism ofaquatic insects’ floating, and hope that it may lead to furtherresearch on drag-reducing materials and rapid underwatermotion.In an attempt to understand why a surperhydrophobic sur-face is needed by water striders, we modified gold threadswith superhydrophobic coatings (contact angle >150°) ornormally hydrophobic coatings (contact angle ≈110°).