Molecular adsorption induces normal stresses at frictional interfaces of hydrogels

TL;DR

This study reveals that molecular adsorption at hydrogel interfaces not only causes friction but also induces a normal force pulling the sphere into the hydrogel, supported by experiments and a theoretical model.

Contribution

It demonstrates for the first time that molecular adsorption mechanisms cause normal stresses at hydrogel interfaces, supported by experimental evidence and a thermal bond model.

Findings

Adsorption causes a normal force pulling the sphere into the hydrogel.

The normal force and friction relationship are consistent across various conditions.

A theoretical model explains the normal force via thermal activation of molecular bonds.

Abstract

Friction experiments were conducted on hydrogel thin films sliding against a rigid sphere in a low velocity regime where molecular adsorption at the sliding interface sets the friction force, through a dissipative adsorption-stretching-desorption mechanism initially postulated by Schallamach. By carefully imaging the contact from the initial indentation step of the sphere into the hydrogel to steady state sliding, we evidence for the first time that this very same adsorption mechanism also results in a normal force pulling the sphere further into the hydrogel. Observations of this tangential-normal coupling is made on a variety of chemically modified silica spheres, over 3 decades in velocity and at varied normal load, thereby demonstrating its robustness. Quantitative measurements of the extra normal force and of the friction-velocity relationship versus normal load are well rationalized within a theoretical model based on the thermal actuation of molecular bonds. To do so, we account for the finite non-zero thickness of the sliding interface at which molecular adsorption and stretching events produce an out-of-plane force responsible for both friction and normal pull-in.