A review of thixotropy and its rheological modeling

We review thixotropy, its attributes, and accompanying rheological phenomena, such as yielding, hysteresis in shear-rate ramps, the influence of rest time and viscosity bifurcation, and the prevalence and importance of thixotropy in common fluids. While older work is reviewed in brief, the major emphasis is on recent developments, including nonmonotonic responses of stress to changes in strain rate, viscosity bifurcation, shear banding, and kinematic hardening. The major categories of phenomenological constitutive models are reviewed; these can include viscoelasticity and aging, plasticity, kinematic hardening, and thixotropy; and distinctions between these phenomena and thixotropy are discussed. A few available microstructural models are also reviewed, including population balance models and mesoscopic simulations. We end by highlighting important future work that is needed, including further development of microscopic models and their connection to phenomenological constitutive equations, detailed measurements of microstructures and flow fields with bands, and the investigation of flows other than simple shear. (C) 2019 The Society of Rheology.