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

Daniel Bonn
Laboratoire de Physique Statistique
Ecole Normale Superieure

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. I will argue that most of the difficulties disappear when taking the thixotropy of yield stress fluids into account, and will demonstrate an experimental protocol that allows to obtain reproducible results on the critical stress necessary for flow of these fluids. As a bonus, I will show that the interplay of yield stress and thixotropy allows one to account for the ubiquitous localization of shear in these materials. The price to pay, however, is high: due to the thixotropy the yield stress is no longer a material property, since it depends on the (shear) history of the sample.