Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces

TL;DR

This study investigates the frictional behavior of hydrogels on smooth surfaces, revealing three regimes influenced by pore size, velocity, and interfacial dynamics, with implications for tunable low-friction applications.

Contribution

It provides a detailed microscopic understanding of hydrogel friction, identifying distinct regimes and the influence of pore size and interfacial shear thinning effects.

Findings

Friction at low velocities is governed by hydrodynamic flow inversely related to pore size.

High-velocity friction involves elastohydrodynamic lubrication with a liquid film.

Between regimes, friction decreases significantly and relaxes over minutes, affected by confinement and polymer shear thinning.

Abstract

Hydrogel consists of a crosslinked polymer matrix imbibed with a solvent such as water at volume fractions that can exceed 90\%. They are important in many scientific and engineering applications due to their tunable physiochemical properties, bio-compatibility, and ultra-low friction. Their multiphase structure leads to a complex interfacial rheology, yet a detailed, microscopic understanding of hydrogel friction is still emerging. Using a custom-built tribometer, here we identify three distinct regimes of frictional behavior for polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on smooth surfaces. We find that at low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network, and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms between the gel and surface that obeys elastohydrodynamic theory. Between these regimes, the frictional force decreases by an order of magnitude and displays slow relaxation over several minutes. Our results can be interpreted as an interfacial shear thinning of the polymers with an increasing relaxation time due to the confinement of entanglements. This transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.