Effect of nano- and micro-roughness on adhesion of bioinspired micropatterned surfaces

In this work, the adhesion of biomimetic polydimethylsiloxane (PDMS) pillar arrays with mushroom-shaped tips was studied on nano- and micro-rough surfaces and compared to unpatterned controls. The adhesion strength on nano-rough surfaces invariably decreased with increasing roughness, but pillar arrays retained higher adhesion strengths than unpatterned controls in all cases. The results were analyzed with a model that focuses on the effect on adhesion of depressions in a rough surface. The model fits the data very well, suggesting that the pull-off strength for patterned PDMS is controlled by the deepest dimple-like feature on the rough surface. The lower pull-off strength for unpatterned PDMS may be explained by the initiation of the pull-off process at the edge of the probe, where significant stress concentrates. With micro-rough surfaces, pillar arrays showed maximum adhesion with a certain intermediate roughness, while unpatterned controls did not show any measurable adhesion. This effect can be explained by the inability of micropatterned surfaces to conform to very fine and very large surface asperities.