Anti-thixotropic dynamics in attractive colloidal dispersions: a shear restructuring driven by elastic stresses

TL;DR

This study uncovers the mechanisms behind anti-thixotropic behavior in attractive colloidal dispersions, revealing how shear-induced structural rearrangements driven by elastic stresses lead to viscosity decrease and steady flow states.

Contribution

It provides a detailed mechanistic understanding of anti-thixotropy in colloidal dispersions, linking structural dynamics to elastic stresses and flow behavior, which was previously poorly understood.

Findings

Viscosity decrease is due to shear-induced structural rearrangements.

Anti-thixotropic time diverges at a critical shear rate where elastic stresses dominate.

Steady state microstructure exhibits nearly zero yield stress, indicating loss of flow memory.

Abstract

Due to rich rheological properties, dispersions of attractive colloidal particles are ubiquitous in industries. Specifically, upon experiencing a sudden reduction in shear rate, these dispersions may exhibit transient behaviors such as thixotropy-where viscosity increases over time-and anti-thixotropy, 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 anti-thixotropy remain poorly understood. Here, we investigate the anti-thixotropic dynamics of carbon black particles dispersed in oil-a system known for exhibiting anti-thixotropy-through flow step-down experiments. Using a multi-technique 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 anti-thixotropic time $\tau$, a steady flow is reached. This time $\tau$ diverges with increasing shear rate at a critical value corresponding to a Mason number of one, indicating that anti-thixotropy occurs only when colloidal attraction outweighs viscous forces. More precisely, we show that the structural rearrangement underpinning the viscosity decrease is mediated by elastic stresses $\sigma_e$, such that $\tau \propto \sigma_e^{-3}$. 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 provide a mechanism for anti-thixotropy and suggest pathways for controlling viscosity and yield stress in attractive colloidal dispersions.