Thixotropy in yield stress fluids as a limit of viscoelasticity

Thixotropic yield stress fluids have a yield stress and viscosity which change slowly over time as the fluid undergoes structural changes. This article reviews recent work which shows how such behaviour can arise as a limit of viscoelastic flow when a relaxation time becomes large. The large relaxation time introduces a small parameter which can be used as a basis for singular perturbation studies. A simple prototype for a viscoelastic model will be used to illustrate the analysis. We review the behaviour in startup of shear flow, oscillatory shear flow and elongational flow and identify potential directions for future research.