Failure in a soft gel: Delayed failure and the dynamic yield stress

A soft thermo-reversible protein gel was studied with respect to failure. Flow curves recorded at constant shear-rates revealed a dynamic yield stress sigma(y), seen as a stress plateau below about 10s(-1). When a shear stress below sigma(y) was applied to fractured gels, they reformed after a time that increased with increasing stress and diverged at sigma(y). Application of shear stress to fresh gels led to an initial elastic response followed by creep. Following this creep regime, the strain stagnated below the dynamic yield stress sigma(y), while for sigma > sigma(y) failure was observed after a time that increased with decreasing stress and diverged at sigma = sigma(y). The time-to-failure dependence on the stress for sigma > sigma(y), with two distinct exponential scaling regimes, agrees with existing proposed theories for the fracture of colloidal strands. (C) 2013 Elsevier B.V. All rights reserved.