Molecular Mechanisms of Self-mated Hydrogel Friction

Abstract

Hydrogel-like structures are responsible for the low friction experienced by our joints when we walk or by our eyelids when we blink. At low loads, hydrogel contacts show extremely low friction that rises with velocity beyond a threshold speed. Here we combine mesoscopic simulations and experiments to test the polymer-relaxation hypothesis for this velocity dependence, where a velocity-dependent regime emerges when the perturbation of interfacial polymer chains occurs faster than their relaxation at high velocity. Our simulations quantitatively match the experimental findings, with a friction coefficient that rises with velocity to some power of order unity in the velocity-dependent regime. We show that the velocity-dependent regime is characterized by reorientation and stretching of polymer chains in the direction of shear, leading to an entropic stress that can be quantitatively related to the shear response. The detailed exponent of the power law in the velocity-dependent regime depends on how chains interact: We observe a power close to 1/2 for chains that can stretch, while pure reorientation leads to a power of unity. These results show that the friction of hydrogel interfaces can be engineered by tuning the morphology of near-surface chains.