Biological adhesion for locomotion: basic principles

Surface roughness is the main reason why macroscopic solids usually do not adhere to each other with any measurable strength; even a root-mean-square roughness amplitude of approx. 1 mu m is enough to completely remove the adhesion between normal rubber (with an elastic modulus E approximate to 1 MPa) and a hard nominally flat substrate. Strong adhesion between solids with rough surfaces is only possible if at least one of the solids is elastically very soft. Biological adhesive systems used by insects, tree frogs and some lizards for locomotion are built from a relatively stiff material (keratin-like protein with E approximate to 1 GPa). Nevertheless, strong adhesion is possible even to very rough substrate surfaces by using non-compact solid structures consisting of thin fibers, plates and walls. In order to optimize the bonding to rough surfaces while simultaneously avoiding elastic instabilities, e.g., lateral bundling (or clumping) of fibers, Nature uses a hierarchical building principle, where the thickness of the fibers (or walls) decreases as one approaches the outer surface of the attachment pad. Some lizards and spiders are able to utilize dry adhesion to move on rough vertical surfaces, which is possible due to the 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 in the pad-substrate contact area, which generates a relative long-range attractive interaction due to the formation of capillary bridges.