An accordion model integrating self-cleaning, strong attachment and easy detachment functionalities of gecko adhesion

Recent experiments have shown that gecko adhesion exhibits multifunctionalities including self-cleaning, strong attachment and easy detachment. Currently, there exists no theoretical framework in contact mechanics that is capable of integrating all these features in one unified model. Here we propose an accordion model of adhesion that accomplishes this integration with an adhesive pad consisting of a compliant backbone covered by a foldable hard skin containing hidden adhesives which are initially folded into the structure to avoid adhesion in the stress-free state. At the default, stress-free state, the pad is non-adhesive and self-cleaning. Under a tensile prestress large enough to expose the hidden adhesives, the pad becomes highly stiffened and strongly adhesive. We investigate the adhesion behavior of such a structure and show that strong attachment can be established spontaneously when the pad is dragged on an external surface at a low angle, with prestress trapped in the attached portion of the pad. We then investigate the orientation-dependent peel-off force of an attached accordion pad and show that it can be spontaneously detached above a critical pulling angle, leading to strongly reversible adhesion.