Yield stress and thixotropy: on the difficulty of measuring yield stresses in practice

TLDR

Yield stress determination is experimentally challenging, sparking debate over the usefulness of the yield stress concept in yield stress fluids. The authors argue that incorporating thixotropy eliminates these difficulties and propose a reproducible protocol to measure the critical flow stress, also demonstrating that yield stress–thixotropy interplay explains ubiquitous shear localization. Their protocol measures the critical stress for flow by accounting for thixotropic effects, yielding reproducible data. They find that yield stress is history‑dependent and therefore not a material property.

Abstract

The yield stress of many yield stress fluids has turned out to be difficult to determine experimentally. This has led to various discussions in the literature about those experimental difficulties, and the usefulness and pertinence of the concept of yield stress fluids. We argue here that most of the difficulties disappear when taking the thixotropy of yield stress fluids into account, and will demonstrate an experimental protocol that allows reproducible data to be obtained for the critical stress necessary for flow of these fluids. As a bonus, we will show that the interplay of yield stress and thixotropy allows one to account for the ubiquitous shear localization observed in these materials. However, due to the thixotropy the yield stress is no longer a material property, since it depends on the (shear) history of the sample.