Adhesion of gecko on vertical surfaces with different roughness

Abstract

Locomotion is the basis of predation, escape, reproduction for animals. Geckos, spiders and insects run and climb with exceptional speed, strength and agility for their size, representing in many respects an ideal model system for the study of terrestrial locomotion. Biologists make comparative biology study with animals to develop a deeper understanding of the foundmental biomechanical design rules common to all legged organisms. Engineers look to animals’ locomotion for design principles to improve the performance of legged robots including wall-climbing robots and other complex systems. Claws and setae are two kinds of adhesive devices in geckos’ toe pads. These two devices can adhere on surfaces depending on different attachment mechanism. Synergistic action between these two devices makes geckos freely move on vertical walls and ceilings with arbitrary roughness. The interaction of claws with a substrate is determined by the roughness of the substrate, the friction coefficient between the claw and the substrate, and the relative dimension between the claw and the substrate. The stability of the interaction depends on the mechanism of mechanical inter-locking. The adhesion of geckos’ setae is thought to be due to dry adhesion, which is generated by van der Waals force. The key parameters for the setae adhesion here is the distance between the terminal part of the setae and the substrate, which is in the range of a few nanometers in order to generate van der Waals forces. In addition that the microstructure of gecko setae affects to the adhesion performance, the morphology of substrate surface also plays an important role on the adhesion performance. Geckos with claws amputation were placed on 14 grades of vertical sandpapers to study setae’ adhesive ability. The ratio of particles’ diameter to distance on the surface of sandpapers was calculated. According to the ratios, 14 grades of sandpapers were classified into three new grades by the Q cluster analysis method which principle is that the difference between two neighbouring grades is the smallest among all grades. On the surface of sandpaper which the ratio is relatively smaller, geckos will slip down and try to make transverse adduction, even adjust its posture to keep adhering on the substrate. The maximum friction force between gecko spatula and sandpaper was calculated by the contact area. Then the relationship between the maximum friction force and adhesive force was also deduced. The results show that slipping velocity, adduction speed of geckos on all grades of sandpapers surface are different. Slipping velocity decreases with the ratio of particles’ diameter to their distance on the surface of sandpaper increasing. The result from calculation is consistent with the conclusion from the experiments. The adhesive ability of geckos is determined by the contact area between the surface of the setae on the distal tip and sandpaper, and surface roughness is not a key factor. The study of adhesive performance of geckos setae can contribute to the design of wall climbing robot based on the adhesion of gecko-inspired setae.