Friction through molecular adsorption at the sliding interface of hydrogels: Theory and experiments

TL;DR

This study combines experiments and theory to investigate how molecular adsorption influences friction at hydrogel interfaces, revealing a velocity-dependent frictional behavior sensitive to surface chemistry.

Contribution

It introduces a molecular model linking friction to thermally activated polymer adsorption and provides experimental data on adhesion energies and adsorption times.

Findings

Friction varies logarithmically with sliding velocity within a specific range.

Surface chemistry significantly affects the frictional response.

Experimental data align with the molecular adsorption model.

Abstract

We report on the frictional properties of thin ($\approx \mu m$) poly(dimethylacrylamide) hydrogel films within contacts with spherical silica probes. In order to focus on the contribution to friction of interfacial dissipation, a dedicated rotational setup is designed which allows to suppress poroelastic flows while ensuring an uniform velocity field at the sliding interface. The physical-chemistry of the interface is varied from the grafting of various silanes on the silica probes. Remarkably, we identify a velocity range in which the average frictional stress systematically varies with the logarithm of the sliding velocity. This dependency is found to be sensitive to the physical-chemistry of the silica surfaces. Experimental observations are discussed in the light of a molecular model where friction arises from thermally activated adsorption of polymer chains at the sliding interface, their elastic stretching and subsequent desorption. From this theoretical description, our experimental data provide us with adhesion energies and characteristic times for molecular adsorption that are found consistent with the physico-chemistry of the chemically-modified silica surfaces.