Adhesive contact in animal: Morphology, mechanism and bio-inspired application

Many animals possess adhesive pads on their feet, which are able to attach to various substrates while controlling adhesive forces during locomotion. This review article studies the morphology of adhesive devices in animals, and the physical mechanisms of wet adhesion and dry adhesion. The adhesive pads are either ‘smooth’ or densely covered with special adhesive setae. Smooth pads adhere by wet adhesion, which is facilitated by fluid secreted from the pads, whereas hairy pads can adhere by dry adhesion or wet adhesion. Contact area, distance between pad and substrate, viscosity and surface tension of the liquid filling the gap between pad and substrate are the most important factors which determine the wet adhesion. Dry adhesion was found only in hairy pads, which occurs in geckos and spiders. It was demonstrated that van der Waals interaction is the dominant adhesive force in geckos’ adhesion. The bio-inspired applications derived from adhesive pads are also reviewed.