Shear-mode contact splitting for a microtextured elastomer film.

Texturing the surface of a PDMS fi lm with microscale pillars has been shown to enhance shear adhesion to a rigid substrate by an order of magnitude when compared to untextured control fi lms. In this study, it is shown that shear load is distributed uniformly among the contacts rather than concentrated on the contacts nearest the edge of the fi lm-substrate interface, which is in contrast to the Kendall peel model for shearing substrates. The total shear strength of adhesion is thus estimated as the combined shear strength of each individual microcontact. Shear-mode contact splitting is confi rmed optically with experiments in which shear failure is observed through a glass coverslip. Recent literature has suggested that texturing the surface of an elastic fi lm may enhance adhesion to a rigid substrate. Theoretical studies have shown that introducing surface microstructures allows interfacial stress to be distributed to the edge of every feature, rather than concentrated at the blunt edge of the fi lm-substrate interface. [ 1 , 2 ] For the case of a solitary surface contact, irregularities initiate crack propagation throughout the entire interface. In dividing the contact into fi ner sub-contacts, there is an increased tolerance of defects at individual contacts. [ 3–5 ] Contact splitting in a peel-test, for which one substrate is incision-patterned, has shown that a new crack must reinitiate at every incision. [ 6–8 ] Crack initiation from a fi lm defect occurs at a load much higher than that required for propagation on a smooth unpatterned surface, yielding intermittent propagation for multiple defects and an overall increase in normal adhesion. Experimentally, this property has been manifested in fi brillar samples showing larger pull-off stress per unit contact area than non-fi brillar controls. [ 9–15 ] Furthermore, the role of fi brillar contact shape on adhesion of split contact surfaces has been shown to be notable. [ 14 , 15 ] An increased tip size, relative to the fi bril base, seems to distribute stress singularities along the edge of the contact interface, delaying failure in adhesion. [ 13 ]