A convective instability mechanism for quasistatic crack branching in a hydrogel

TL;DR

This paper uncovers a new crack branching instability in hydrogels caused by wetting the crack tip with a less viscous solvent, modeled through a balance of growth and advection rates in a non-linear elastic region.

Contribution

It introduces a minimal predictive model linking non-linear elasticity and process zone physics to crack instability in hydrogels, supported by experimental observations.

Findings

Wetting the crack tip triggers unstable branching.

The instability is governed by a balance between crack growth and advection.

Non-linear elasticity is crucial for understanding crack front instabilities.

Abstract

Experiments on quasistatic crack propagation in gelatin hydrogels reveal a new branching instability triggered by wetting the tip opening with a drop of aqueous solvent less viscous than the bulk one. We show that the emergence of unstable branches results from a balance between the rate of secondary crack growth and the rate of advection away from a non-linear elastic region of size $\mathcal G/E$ where $\mathcal G$ is the fracture energy and $E$ the small strain Young modulus. We build a minimal, predictive model that combines mechanical characteristics of this mesoscopic region and physical features of the process zone. It accounts for the details of the stability diagram and lends support to the idea that non-linear elasticity plays a critical role in crack front instabilities.