Molecular mechanisms of self-mated hydrogel friction

TL;DR

This study combines simulations and experiments to understand how polymer chain dynamics at hydrogel interfaces influence velocity-dependent friction, revealing chain reorientation and stretching as key factors.

Contribution

It provides the first quantitative link between polymer chain morphology and velocity-dependent hydrogel friction through combined simulations and experiments.

Findings

Friction is velocity-independent at low speeds.

Friction increases with velocity following a power law.

Chain reorientation and stretching drive the velocity dependence.

Abstract

Self-mated hydrogel contacts show extremely small friction coefficients at low loads but a distinct velocity dependence. 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 reproduce the experimental findings, with speed-independent friction at low velocity, followed by a friction coefficient that rises with velocity to some power of order unity. 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. Our simulations quantitatively match experiments and show that the velocity dependence of hydrogel friction at low loads can be firmly traced back to the morphology of near-surface chains.