Predicting and assessing rupture in protein gels under oscillatory shear

TL;DR

This study reveals that protein gels rupture abruptly under oscillatory shear, and introduces an empirical criterion to predict rupture points based on harmonic mode analysis, supported by ultrasonic imaging of local properties.

Contribution

It provides a new empirical criterion for predicting rupture in protein gels under oscillatory stress, linking harmonic mode buildup to failure, and uses ultrasonic imaging to analyze local mechanical changes.

Findings

Rupture occurs abruptly and brittle-like in protein gels.

Harmonic modes can be rescaled to predict rupture points.

Local mechanical properties show phase separation at rupture.

Abstract

Soft materials may break irreversibly upon applying sufficiently large shear oscillations, a process which physical mechanism remains largely elusive. In this work, the rupture of protein gels made of sodium caseinate under an oscillatory stress is shown to occur in an abrupt, brittle-like manner. Upon increasing the stress amplitude, the build-up of harmonic modes in the strain response can be rescaled for all gel concentrations. This rescaling yields an empirical criterion to predict the rupture point way before the samples are significantly damaged. "Fatigue" experiments under stress oscillations of constant amplitude can be mapped onto the former results, which indicates that rupture is independent of the temporal pathway in which strain and damage accumulate. Finally, using ultrasonic imaging, we measure the local mechanical properties of the gels before, during and after breakdown, showing that the strain field remains perfectly homogeneous up to rupture but suddenly gives way to a solid-fluid phase separation upon breakdown.