Failure precursors in colloidal and biopolymer gels*

Material failure is widespread and occurs on vastly different lengthscales, from earthquakes to the atomic level. It often involves sudden and unpredictable events, with little or no macroscopically detectable precursors. A better understanding of the mechanisms leading to failure is however highly desirable and would have deep implications, possibly paving the way to predicting failure.
We will present experiments probing failure precursors in two types of gel: colloidal and biopolymer gels. Colloidal gels can be regarded as model network forming materials, are brittle and exhibit a power law linear viscoelasticity, whereas biopolymer gels are more complex, show strain-hardening and develop negative normal stresses under shear.
We use original set-ups that simultaneously probe the macroscopic rheological response and the microscopic structure and dynamics of a gel submitted to a constant shear stress. We show that the failure is systematicaly precedented by qualitative and quantitative change of the dynamics, from reversible particle displacement to a burst of irreversible plastic rearrangements with a complex spatial evolution in the sample. Those plastic events are failure precursors of the materials and may be regarded as a novel tool to understand the sudden failure of solids.