Abstract： Many insects and some animals, such as and crickets, tree frogs, can utilize a thin film of confined liquid between the pad and the smooth surface to facilitate climbing vertical walls and even walking on the ceiling. Our previous experiments based on a ball-disk microtribometer have revealed that a sudden unstable spreading or shrinking of the confined liquid film occurs when the volume reaches a critical value, which respectively corresponds to an unstable increasing or decreasing of interfacial adhesive force. This unstable behavior of the confined liquid film has important implications on the adhesion/releasing mechanism of wet bio-adhesive pad to or from the smooth surface. In this article, we highlight some of the switching mechanisms for capillary adhesion through theoritcal analysis and further experiments. It is found that, the unstable spreading and shrinking of confined liquid film is connected with the instability of the cantilever stiffness, and the maximum interfacial adhesive force decreases with the liquid volume and strongly depends on the film thickness of the central area. Inspired by these results, strategies for a bio-adhesive pad to realize `switchable' adhesive mechanism is analyzed, which will shed light on the design and control of a biomimetic adhesive pad.
bio-adhesive padcapillary forceconfined liquids