Impact of nanometer-thin stiff layer on adhesion to rough surfaces

Abstract

Adhesives require molecular contact, which is governed by roughness, modulus, and load. Here, we measured adhesion for stiff glassy polymer layers of varying thickness on top of a soft elastomer with rough substrates. We found that a 90-nm-thick PMMA layer on a softer elastic block was sufficient to drop macroscopic adhesion to almost zero during the loading cycle. This drop in adhesion for bilayers follows the modified Persson-Tosatti model, where the elastic energy for conformal contact depends on the thickness and modulus of the bilayer. In contrast, we observed no dependence on thickness of the PMMA layer on the work of adhesion calculated using the pull-off forces. Understanding how mechanical gradients (like bilayers) affect adhesion is critical for areas such as adhesion, friction, and colloidal and granular physics.