Hyperaging and stress buildup in soft colloidal gels

In this work, we investigate the transient rheological behavior of two soft glassy materials: a clay dispersion and a silica gel, emphasizing their unconventional shear stress buildup behavior under conditions of constant imposed strain. For both materials, the elastic modulus and static yield stress undergo time-dependent evolution or aging. In addition, following an intense period of preshearing (i.e., shear rejuvenation or destructuration), the material relaxation time is observed to show a stronger than linear dependence on the sample age, indicative of hyperaging dynamics. We show that these features are consistent with nonmonotonic steady-state flow curves characterized by a local stress minimum. When a steady shearing flow is suddenly ceased, and the total imposed sample strain is held constant, both materials show an initial relaxation of the shear stress, followed by a period of shear stress buildup, resulting in a local minimum in the evolution of shear stress with time. For the clay dispersion, the intensity of these effects increases at higher preshear rates, whereas for the silica gel, the effects are largely independent of the preshear rate. We also propose a simple time-dependent linear Maxwell model that qualitatively predicts the experimentally observed trends in which the shear stress buildup is directly related to a monotonic increase in the elastic modulus with time, giving keen insight into this peculiar phenomenon.