Adhesion of a Fibrillar Interface on Wet and Rough Surfaces

We have studied the adhesion of a biomimetic fibrillar interface terminated by a thin film in aqueous medium and against dry rough surfaces to simulate more realistic environmental conditions. We consistently observed enhancement of adhesion under water (compared with measurements in air) against hydrophobic surfaces for all samples. In most cases, adhesion could be represented by a multiplicative coupling between crack-trapping due to the architecture and the intrinsic work of adhesion. With increasing inter-fibrillar separation, adhesion first increases and then usually decreases. The decrease in adhesion for large interfibrillar separations can be explained by a transition in the controlling mechanism for interfacial separation from crack-trapping to stress-controlled nucleation of interfacial voids. Against rough surfaces, as roughness was increased, all samples showed a reduction in adhesion but fibrillar surfaces retained significant adhesion even when flat surfaces had almost none.