Effect of Microscale Contact State of Polyurethane Surface on Adhesion and Friction

The effect of microscale contact of rough surfaces on the adhesion and friction under negative normal forces was experimentally investigated. The adhesive force of single point contact — sapphire ball to flat polyurethane did not vary with the normal force. With rough surface contact, which was assumed to be a great number of point contacts, the adhesive force increased logarithmically with the normal force. Under negative normal force adhesive state, the tangential force (more than hundred mN) were much larger than the negative normal force (several mN) and increased with the linear decrease of negative normal force. The results reveal why the gecko's toe must slide slightly on the target surface when it makes contact on a surface and suggest how a biomimetic gecko foot might be designed.