Abstract: Wet adhesion is widely adopted in biological adhesion systems in nature, and it is beneficial to design new materials with desired properties based on the underlying physics of wet adhesion. The aim of this work is to develop a design criterion to regulate the wet adhesion. The effects of different contact shapes (flat and sphere) and morphologies of the substrate (smooth, microstructure and nanostructure) on the adhesion force are investigated. Combining with the theoretical models,  the dominated factors in the separation process and isolating the viscous contributions from the capillary interactions are evaluated. The results demonstrate that the adhesion mechanisms depend significantly on the capillary numbers of the interstitial liquid and the contact geometry, and the ratio of capillary force to viscous force is a key to regulate the wet adhesion mechanism. These findings can not only explain some phenomena of wet adhesion to organisms, but also provide some inspirations to design new adhesion technology for robotic fingers that can grasp objects in wet environments.

Key words: wet adhesion, capillary force, viscous force, adhesion mechanism