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

The mechanics of wet adhesion between a water strider's legs and a water surface was studied. First, we showed that the nanoscale to microscale hierarchical surface structure on striders' legs is crucial to the stable water-repellent properties of the legs. The smallest structure is made for the sake of a stable Cassie state even under harsh environment conditions, which sets an upper limit for the dimension of the smallest structure. The maximum stress and the maximum deformation of the surface structures at the contact line are size-dependent because of the asymmetric surface tension, which sets a lower limit for the dimension of the smallest structure. The surface hierarchy can 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 the contact line. Second, the processes of the legs pressing on and detaching from the water surface were analyzed with a 2D model. We found that the superhydrophobicity of the legs' surface is critically important to reducing the detaching force and detaching energy. Finally, the dynamic process of the legs striking the water surface, mimicking the maneuvering of water striders, was analyzed. We found that the large length of the legs not only reduces the energy dissipation in the quasi-static pressing and pulling processes but also enhances the efficiency of energy transfer from bioenergy to kinetic energy in the dynamic process during the maneuvering of the water striders. The mechanical principles found in this study may provide useful guidelines for the design of superior water-repellent surfaces and novel aquatic robots.