Wet adhesion with application to tree frog adhesive toe pads and tires

Strong adhesion between solids with rough surfaces is only possible if at least one of the solids is elastically very soft. Some lizards and spiders are able to adhere (dry adhesion) and move on very rough vertical surfaces due to very compliant surface layers on their attachment pads. Flies, bugs, grasshoppers and tree frogs have less compliant pad surface layers, and in these cases adhesion to rough surfaces is only possible because the animals inject a wetting liquid into the pad - substrate contact area, which generates a relative long-range attractive interaction due to the formation of capillary bridges. In this presentation I will discuss some aspects of wet adhesion for tree frogs and give some comments related to tire applications