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

The effect of microscale contact of rough surfaces on adhesion and friction under negative normal forces is experimentally investigated. The adhesion do not vary with the normal forces on the condition of single point contact - sapphire ball to flat polyurethane (PU). The adhesion increased with the normal forces according to logarithmic relation in the circumstances of rough surface contact, which was assumed as a great number of point contacts. Under negative normal forces - adhesion, the tangential forces increased with the decrease of negative normal forces linearly. The results reveal why a gecko’s toe must slide slightly on the target surface when it contacted the ceiling and suggest how the biomimetic gecko foot is designed.