Search

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. [ABSTRACT FROM AUTHOR]

To predict the complex transient rheology of thixotropic elasto-viscoplastic (TEVP) fluids, we generalize a previous scalar thixotropic-only "multilambda" (ML) model [Wei et al., J. Rheol. 60(6), 1301-1315 (2016)] and combine it with the isotropic kinematic hardening (IKH) model [C. J. Dimitriou and G. H. Mckinley, Soft Matter 10(35), 6619-6644 (2014)]. This new constitutive equation, which we call ML-IKH model, has the following features: (1) Multiple thixotropic structure parameters that collectively exhibit a stretch-exponential thixotropic relaxation in step tests; (2) nonlinear thixotropic kinetic equations in both the shear rate or stress and the structure parameters; (3) incorporation of the Armstrong-Frederick kinematic hardening rule [C. O. Frederick and P. Armstrong, Mater. High Temp. 24(1), 1-26 (2007)] for the evolution of yield stress; and (4) viscoelasticity. We evaluate this 12-parameter model, discuss its four key features, and compare its predictions with those of the ML, IKH, and modified Delaware thixotropic models [Armstrong et al., J. Rheol. 60(3), 433-450 (2016)] for two sets of experimental data [Wei et al., J. Rheol. 60(6), 1301-1315 (2016); Armstrong et al., J. Rheol. 60(3), 433-450 (2016)] of a TEVP fumed silica suspension. The shear-rate histories include steady state, step shear rate, step stress, intermittent shear, flow reversal, and large amplitude oscillatory shear (LAOS). We show that in step tests the thixotropic and viscoelastic evolutions are dominant, while intermittent shear tests the multiple thixotropic timescales, and flow reversal tests the viscoelastic and plastic evolutions. The rheological responses in LAOS tests are more complex and involve all aspects of TEVP rheology. The four features quantitatively capture different aspects of TEVP rheology. We also provide a tensorial formulation of the ML-IKH model that is frame-invariant, obeys the second law of thermodynamics, and can reproduce the predictions of the scalar version. [ABSTRACT FROM AUTHOR]