The strength of bioinspired adhesive microstructures with interfacial defects

Bioinspired dry adhesives have exhibited statistical variation in local strength across arrays of micropatterned fibrillar sub-contacts, due to interfacial defects which form in the presence of surface roughness, contaminants, and imperfections. To understand the origins of statistical variation, this work examines the adhesive strength of three fibril geometries on substrates with pre-existing defects through cohesive zone finite element simulations. A cylindrical-punch-shaped fibril is compared to the prevalent mushroom-shaped fibril geometry, with and without a fillet where the stalk meets the flange. Results for the punch-shaped fibril show that even in the presence of defects in the contact center, the edge controls propagation of detachment. Only when defects are large and close to the contact edge does the adhesive strength differ from the defect-free case. For the mushroom-shaped fibrils, there is a strong dependence upon defect size and position. The transition to defect-insensitivity in the flange is affected by the presence of the fillet, as well as the intrinsic adhesive strength of the interface. Together, these results suggest that for a distribution of defects at the interface, across an array of fibrillar sub-contacts, the local statistical variation will strongly depend upon the details of fibril geometry close to the interface.