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.
To understand the mechanical interactions when geckos move on ceiling and to obtain an inspiration on the controlling strategy of gecko-like robot,we measured the ceiling reaction force(CRF) of freely moving geckos on ceiling substrate by a 3-dimensional force measuring array and simultaneously recorded the locomotion behaviors by a high speed camera.CRF and the preload force(FP) generated by the geckos were obtained and the functions and the differences between forces generated by fore-and hind-feet were discussed.The results showed that the speed of gecko moving on the ceiling was 0.17-0.48 m/s,all of the fore-and hind-legs pulled toward the body center.When geckos attached on the ceiling incipiently,the feet generated a very small incipient FP and this fine FP could bring about enough adhesive normal force and tangential force to make the gecko moving on ceiling safely.The FP of the fore-feet is larger than that of the hind-feet.The lateral CRF of the fore-feet is almost the same as that of the hind-feet's.The fore-aft CRF generated by the fore-feet directed to the motion direction and drove their locomotion,but the force generated by the hind-feet directed against the motion direction.The normal CRF of fore-and hind-feet accounted for 73.4% and 60.6% of the body weight respectively.Measurements show that the fore-aft CRF is obviously lager than the lateral and normal CRF and plays a major role in promoting the fore-feet,while the hind-feet of the main role are to provide a smooth movement.The results indicate that due to the differences of the locomotion function of each foot between different surfaces,the gecko can freely move on ceiling surfaces,which inspires the structure designing,gait planning and control developing for gecko-like robot.
