Underwater Adhesion of Mushroom‐shaped Adhesive Microstructure: An Air‐entrapment Effect

In this work, we revisited the underwater adhesion of mushroom‐shaped adhesive microstructure (MSAMS) which has come a long way from the inspiration of attachment devices evolved in beetles to a large‐scale industrial production. It was shown that MSAMS has an about twice higher pull‐off force compared to smooth control made from the same material measured in air. Enhanced adhesion was found to be a combination of intermolecular van der Waals forces and particular crack‐trapping geometry. Recently, it was shown that underwater adhesion of MSAMS was even higher than that in air (M. Varenberg, and S. N. Gorb, J. R. Soc. Interface 5, 383, 2008). It was a surprising result, since van der Waals interactions should theoretically be strongly reduced, when the surfaces are submerged in water. It was hypothesized that each individual MSAMS acts as a passive suction device. The present study aims at an experimental investigation of an alternative hypothesis, namely the effect of air‐entrapment. For this purpose, we compared pull‐off forces of MSAMS, measured under water with a layer of entrapped air with those completely wetted by water. In the presence of an air layer pull‐off forces were distinctly higher compared to measurements performed in air. When MSAMS was completely wetted by water, lower pull‐off forces compared to those measured in air were obtained. These data suggest that air retaining capabilities of MSAMS, when submerged in water, may also explain their enhanced underwater adhesion.