Adhesion enhancement in a biomimetic fibrillar interface

Two important putative functions of the fibrillar contact interfaces commonly found in lizards and insects are to provide contact compliance and enhanced adhesion. To explore the question of whether a fibrillar architecture inherently enhances adhesion, we constructed model structures consisting of thin sheets of poly(vinyl butyral) (PVB) bonded on one of their thin sides to glass plates. The PVB samples had two flat, unstructured regions interrupted by a central fibrillar region along the bonded interface. The effect of the fibrillar geometry on the performance of the adhesive bond was tested using a tensile pull-off test, in which failure occurred by interfacial crack propagation, starting at an end where a crack initiator was introduced. We observed that fibrils in all samples fail consistently at the same critical stress, which is consistent with a previous theoretical result we have determined for flaw insensitivity during fibrillar pull-off. In addition, we measured the energy release rate required to fail the interface in the fibrillar region to be about an order of magnitude greater than that in the non-fibrillar region. We present experimental evidence demonstrating that this increase results partly from dissipation of strain energy stored in the fibrils.