The optimal shape of elastomer mushroom-like fibers for high and robust adhesion

Over the last decade, significant effort has been put into mimicking the ability of the gecko lizard to strongly and reversibly cling to surfaces, by using synthetic structures. Among these structures, mushroom-like elastomer fiber arrays have demonstrated promising performance on smooth surfaces matching the adhesive strengths obtained with the natural gecko foot-pads. It is possible to improve the already impressive adhesive performance of mushroom-like fibers provided that the underlying adhesion mechanism is understood. Here, the adhesion mechanism of bio-inspired mushroom-like fibers is investigated by implementing the Dugdale-Barenblatt cohesive zone model into finite elements simulations. It is found that the magnitude of pull-off stress depends on the edge angle theta and the ratio of the tip radius to the stalk radius beta of the mushroom-like fiber. Pull-off stress is also found to depend on a dimensionless parameter chi, the ratio of the fiber radius to a length-scale related to the dominance of adhesive stress. As an estimate, the optimal parameters are found to be beta = 1.1 and theta = 45 degrees. Further, the location of crack initiation is found to depend on chi for given beta and theta. An analytical model for pull-off stress, which depends on the location of crack initiation as well as on theta and beta, is proposed and found to agree with the simulation results. Results obtained in this work provide a geometrical guideline for designing robust bio-inspired dry fibrillar adhesives.