Load-independent hydrogel friction

Abstract

Biology largely manages tribological challenges either by eliminating sliding altogether or by protecting sliding interfaces with soft aqueous gels. In the body, aqueous gels are often thin (thickness, t < 100 μm), soft (elastic modulus, E < 10 kPa), lubricious (friction coefficients, μ < 0.01), and cover compliant surfaces, including cell membranes, pleura, cartilage, and the eye. These characteristics provide a natural defense against wide ranges of applied loads. In this work, hydrogel samples (7.5 wt% polyacrylamide, 0.3 wt% N,N′-methylenebisacrylamide) were prepared with spherically-capped shell probe geometries, which have been previously determined to provide constant contact pressures during indentation measurements against flat hydrogel disks. In a self-mated (“Gemini”) sliding configuration, this geometry is capable of load-independent friction over a range of low normal loads spanning 0.5 to 2.0 mN. This friction behavior is consistent with da Vinci-Amontons' friction law (F_f = μF_n) due to the large compliance of the spherically-capped shell probe geometry enabling the area of contact to increase in proportion with the applied load and due to low shear stresses reacted across the sliding interface for high water content aqueous gels. Future bio-inspired lubrication strategies involving aqueous gels may benefit from leveraging contact geometry for constant, load-independent friction.