Insight into the Viscous and Adhesive Contributions to Hydrogel Friction

Investigation of the mechanisms underlying hydrogel lubrication is pivotal in understanding the complexity of biolubrication. In this work, the frictional characteristics of poly(acrylamide) hydrogels with varying composition have been studied over a wide range of sliding velocities and normal loads by colloidal probe lateral force microscopy. The results show that the friction force between the hydrogel and the colloid increases with velocity at sliding velocities above a transition value (V^∗), while the friction force at slower sliding velocities depends on the composition, and it can either increase or decrease with velocity. Based on the viscoelastic behavior of hydrogels, we model hydrogel friction as the combination of viscous dissipation and the energy dissipated through the rupture of the transient adhesive bridges across the interface. The model parameters depend on relaxation characteristics of the confined polymer network at the interface and on the (bulk) viscoelastic behavior of the hydrogel and are sensitive to the compressive stress. We observe a collapse of the experimental data (at different loads and velocities and for hydrogels with different compositions) in a non-monotonic master curve with a minimum friction force at the transition velocity. Furthermore, a simple relation for the transition velocity V^∗ is derived from theory, thereby demonstrating the competing effect of both the adhesive and the viscous contributions to friction, which helps to reconcile discrepancies between previous studies of hydrogel friction.