Viscous and adhesive contribution to hydrogel friction
Tooba Shoaib and Rosa M. Espinosa-Marzal
Abstract
Hydrogel lubrication, while pivotal in the advancement of biotribology, is requires fundamental understanding of the underlying mechanisms. The frictional characteristics of poly(acrylamide) hydrogels with varying monomer concentration has been studied as a function of the normal load and over a wide range of sliding speeds by colloidal probe lateral force microscopy. The results show that 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 strongly on the composition, and can be either velocity strengthening (for 12 wt%) or velocity weakening (for 4% and 6%). Based on the viscoelastic behavior of hydrogels, and inspired by previous works on rubber friction, we model hydrogel friction as the combination of viscous dissipation upon shear of the hydrogel and the energy dissipated through the rupture of the transient adhesive bridges across the interface, while accounting for hydrogel’s poroelastic behavior. The model parameters depend on the shear thinning behavior of the hydrogel and on the relaxation characteristics of the confined polymer and are sensitive to the applied load. Furthermore, a simple relation for the transition velocity V^* is derived from theory, thereby demonstrating the competing effects of the adhesive elastic and the viscous dissipation contributions to friction, which helps to reconcile the discrepancies between previous hydrogel studies. This work provides a significant advancement in understanding the molecular mechanisms underlying hydrogel friction and lubrication.