Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the frictional-adhesion behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a grip state (high adhesion and friction forces after shearing along the tilt of the fibers, F-ad(+) and F-parallel to(+)) and a release state (low adhesion and friction after shearing against the tilt of the fibers, F-ad(-) and F-parallel to(-)). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces F-parallel to(+) and F-parallel to(-) and the release state adhesion force F-ad(-) exhibit a maximum at intermediate relative humidity between 40% and 75%.