Role of normal stress in the creep dynamics and failure of a biopolymer gel

TL;DR

This study explores how normal stresses influence the delayed rupture of biopolymer gels under shear, revealing microscopic precursors and dynamic signatures that predict macroscopic failure.

Contribution

It uncovers the role of normal stress relaxation during gelation and links microscopic dynamics to eventual macroscopic fracture in biopolymer gels.

Findings

Normal stress relaxation correlates with gel weakening before fracture.

Microscopic dynamics accelerate and fluctuate prior to failure.

Localized plastic regions nucleate and expand leading to rupture.

Abstract

We investigate the delayed rupture of biopolymer gels under a constant shear load by simultaneous dynamic light scattering and rheology measurements. We unveil the crucial role of normal stresses built up during gelation: all samples that eventually fracture self-weaken during the gelation process, as revealed by a partial relaxation of the normal stress concomitant to a burst of microscopic plastic rearrangements. Upon applying a shear stress, weakened gels exhibit in the creep regime distinctive signatures in their microscopic dynamics, which anticipate macroscopic fracture by up to thousands of seconds. The dynamics in fracturing gels are faster than those of non-fracturing gels and exhibit large spatio-temporal fluctuations. A spatially localized region with significant plasticity eventually nucleates, expands progressively, and finally invades the whole sample triggering macroscopic failure.