Transition from a simple yield stress fluid to a thixotropic material

TL;DR

This study demonstrates how adding colloidal particles to a simple yield stress fluid transforms it into a thixotropic material, exhibiting time-dependent viscosity increase and flow stoppage below a critical shear rate.

Contribution

It introduces a new understanding of how colloidal particles induce thixotropy in yield stress fluids and presents a model for this transition.

Findings

Loaded emulsion shows a critical shear rate for steady flow.

Viscosity increases over time under sub-yield stress conditions.

A model based on droplet linking explains the thixotropic behavior.

Abstract

From MRI rheometry we show that a pure emulsion can be turned from a simple yield stress fluid to a thixotropic material by adding a small fraction of colloidal particles. The two fluids have the same behavior in the liquid regime but the loaded emulsion exhibits a critical shear rate below which no steady flows can be observed. For a stress below the yield stress, the pure emulsion abruptly stops flowing, whereas the viscosity of the loaded emulsion continuously increases in time, which leads to an apparent flow stoppage. This phenomenon can be very well represented by a model assuming a progressive increase of the number of droplet links via colloidal particles.