Antithixotropic dynamics in attractive colloidal dispersions: A shear restructuring driven by elastic stresses

Due to rich rheological properties, dispersions of attractive colloidal particles are ubiquitous in industries. Specifically, upon experiencing a sudden reduction in the shear rate, these dispersions may exhibit transient behaviors such as thixotropy—where viscosity increases over time—and its antonym, antithixotropy, characterized by an initial viscosity decrease before reaching a steady state. While thixotropy has been described as a competition between structure buildup and disruption, the mechanisms of antithixotropy remain poorly understood. Here, we investigate the antithixotropic dynamics of carbon black particles dispersed in oil—a system known for exhibiting antithixotropy—through flow step-down experiments. Using a multitechnique approach combining rheology with velocimetry and structural characterizations, we show that viscosity decrease results from a decrease in wall slip concomitant to shear-induced structural rearrangements, indicating a transition from a dynamical network of fractal clusters into a network of loosely connected dense agglomerates. Additionally, after a characteristic antithixotropic time τ, a steady flow is reached. This time τ diverges with increasing shear rates at a critical value corresponding to a Mason number of one, indicating that antithixotropy occurs only when colloidal attraction outweighs viscous forces. More precisely, we show that the structural rearrangement underpinning the viscosity decrease is mediated by initial elastic stresses σe, such that τ∝σe−3. Finally, on long time scales, the steady state is linked to a microstructure with nearly zero yield stress, indicating a loss of flow memory. These findings clarify the mechanics of antithixotropy and its distinction from thixotropy, providing a better understanding of both processes in attractive colloidal dispersions.