Self-healing slip pulses and the friction of gelatin gels

TL;DR

This study investigates the frictional behavior of gelatin gels on glass, revealing self-healing slip pulses at low velocities and homogeneous sliding governed by shear-thinning rheology at higher velocities, with implications for understanding interfacial aging.

Contribution

It provides a detailed experimental and scaling analysis of gelatin gel friction, highlighting the mechanisms of slip pulse propagation and the role of interfacial rheology and aging.

Findings

Propagation of self-healing slip pulses at low velocities.

Friction governed by shear-thinning rheology above critical velocity.

Observation of static friction aging and velocity-weakening regime.

Abstract

We present an extensive experimental study and scaling analysis of friction of gelatin gels on glass. At low driving velocities, sliding occurs via propagation of periodic self-healing slip pulses whose velocity is limited by collective diffusion of the gel network. Healing can be attributed to a frictional instability occurring at the slip velocity $v = V_c$. For $v > V_c$, sliding is homogeneous and friction is ruled by the shear-thinning rheology of an interfacial layer of thickness of order the (nanometric) mesh size, containing a semi-dilute solution of polymer chain ends hanging from the network. Inspite of its high degree of confinement, the rheology of this system does not differ qualitatively from known bulk ones. The observed ageing of the static friction threshold reveals the slow increase of adhesive bonding between chain ends and glass. Such structural ageing is compatible with the existence of a velocity-weakening regime at velocities smaller than $V_c$, hence with the existence of the healing instability.