Scaling Effect of Dry Adhesion for Microfibrils and Transition Size to Flaw Insensitivity

The mechanics of detachment (e.g., a cylindrical fibril separating from a dissimilar substrate) has been treated in the perspectives of contact mechanics and fracture mechanics theory along with numerical simulations, but systematic experimental studies on the adhesion of an individual microfibrils is still scarce. In this work, we conducted detailed experiment on the adhesion tests of individual cylindrical microfibrils within a large range of varying diameters from 4 to 400 mm made of three different polyurethanes with moduli among ~1-40 MPa. We confirmed the scaling effect of an individual microfibril, i.e. the adhesion sad of the individual fibril scales with fibrillar diameters D with an exponent of ~ -0.4 to -0.45. As the fibrillar diameter is reduced below 10 mm, the adhesion becomes unchanged and size-insensitive. This result is in good agreement with the theoretical predictions. Furthermore, the effects of the Young's modulus and retraction rates during the adhesion tests on the adhesion strength were also investigated. Our experimental work will provide a guide for optimal design of the micron-scale surfaces with improved adhesion.