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

Abstract

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 and F) and a "release state" (low adhesion and friction after shearing against the tilt of the fibers, F and F). 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 and F and the release state adhesion force F exhibit a maximum at intermediate relative humidity between 40% and 75%.

The effect of substrate wettability and modulus on gecko and gecko-inspired synthetic adhesion in variable temperature and humidity

“…GSA adhesion are consistent and match adhesion behavior of other polymer GSAs in variable RH [46][47][48] . Using a GSA as a control for material softening and surface chemistry (i.e., GSAs experience little relative change in modulus and surface chemistry as function of water 49 ), it is clear both systems are significantly influenced by capillary adhesion on hydrophilic glass (i.e., adhesion is higher at 70% RH than at 30% RH), similar to studies testing single setae and setal tips (spatula) 16,17 .…”

Switchable Adhesives for Multifunctional Interfaces

“…[189] On the other hand, capillary bridges can be the only basic physics needed in order to create adhesion between two objects and is therefore an interesting target for the development of adhesion switching platforms. If a fluid interface is exposed to air (or another fluid), there will be an interfacial tension associated with the interface.…”

“…Experimenters using fluids must be careful to avoid (or account for) such "capillary bridge" forces when reporting adhesion measurements. [189] On the other hand, capillary bridges can be the only basic physics needed in order to create adhesion between two objects and is therefore an interesting target for the development of adhesion switching platforms. Vogel and Steen demonstrate such a device.…”

Strong Wet and Dry Adhesion by Cupped Microstructures

“…10−13 Among the tip designs, mushroom-shaped ends have been shown to enhance the adhesion force by over 5–10 times compared to simple flat tips by reducing the stress singularity near the cap edge. 14−16 Yet, when mushroom-shaped microstructures are fully immersed in water, their remarkable adhesion is drastically weakened, 17−19 especially when the fabrication materials and/or the substrate are hydrophilic. 20 Investigating the detachment behavior of mushroom-shaped tips, Heepe et al 18 proposed that a thin water layer in the contact interface led to lower pull-off forces because of the lower Hamaker constant in water.…”