Salt-Controlled Hyperaging, Viscosity Bifurcation, and Stress Buildup in Aqueous Clay Dispersions

Abstract In this work, we examine stress relaxation followed by stress buildup behavior, subsequent to the cessation of preshear in aqueous clay dispersions with varying concentrations of sodium chloride (NaCl). We observe that the temporal evolution of elastic moduli shifts to lower aging times at higher concentrations of the salt. The static yield stress evolves with time and is observed to increase with increasing salt concentrations. We find that the system displays hyperaging dynamics, wherein the relaxation time grows faster than linearly with the time elapsed since shear cessation. Interestingly, after flow cessation, the system initially exhibits a stress decay, but this is followed by a stress buildup after reaching a minimum. We used a simple time-dependent linear Maxwell model framework that qualitatively predicts the experimentally measured stress response. We attribute the observed stress buildup response to a combination of hyperaging dynamics and time-dependent evolution of the elastic modulus. The mechanism underlying this stress relaxation and subsequent buildup behavior is obtained by showing a correlation between the logarithmic rate of stress buildup and the logarithmic rate of microstructural aging that is observed to be of the same order of magnitude over the range of explored salt concentrations.