A structural kinetics model for thixotropy

Abstract

A general structural kinetics model is presented to describe the flow behaviour of thixotropic systems based on inelastic suspending media. The total stress is divided in structure-dependent elastic and viscous contributions. The kinetic equation for the structure parameter takes into account the effect of shear on structure breakdown and build-up, as well as the effect of Brownian motion on build-up. The relaxation and deformation of the flocs is also included. Both the kinetic and the relaxation equations contain a distribution of time constants. The predictions of this model, as well as those of two representative models from the literature, are compared with experimental data using an objective method for parameter estimation. Model validation is based on stress transients resulting from sudden changes in shear rate, including both structure build-up and breakdown. The predictions of the viscous and elastic components are evaluated separately using stress jump experiments with steady and non-steady state starting conditions.