Nature's design of hierarchical superhydrophobic surfaces of a water strider for low adhesion and low energy dissipation

摘要

The mechanics of wet adhesion between water striders' legs and water surface were studied.Firstly, we showed that the nano- to microscale hierarchical surface structure on striders' legs iscrucial for a stable water-repellent properties of the legs. The smallest structure is made for thesake of a stable Cassie state even at harsh environment condition, which sets an upper limit ofthe dimension of the smallest structure. And the maximum stress and the maximum deformationof the surface structures at the contact line are size dependent due to the asymmetric surfacetension, which sets a lower limit for the dimension of the smallest structure. The surface hierarchycan largely reduce the adhesion between the water and the legs by stabilizing the Cassie state,increasing the apparent contact angle, and reducing the contact area and the length of contactline. Secondly, the processes of the legs pressing on and detaching from the water surface wereanalyzed with a two dimensional model. We found that the superhydrophobicity of the legs'surface are critically important for reducing the detaching force and detaching energy. Finally, thedynamic process of the legs striking on the water surface mimicking the maneuver of water striderswas analyzed. We found that the large length of the legs can not only reduce the energy dissipationat the quasi-static pressing and pulling processes, but also enhance the efficiency of energy transferfrom the bio-energy to the kinetic energy in the dynamic process during the maneuver of waterstriders. The mechanical principles found in this study may provide useful guidelines for design ofsuperior water-repellent surfaces and novel aquatic robots.