Rheology of Thixotropic Dispersions: Transient Phenomena upon a Rising Shear Rate

A structural rheological model is used to study a thixotropic disperse system of silica in oil. The model includes a kinetic equation delineating the formation and destruction of particle aggregates and a rheological equation with a structural parameter (the number of aggregated particles within a unit volume). The equilibrium plastic flow corresponding to experimental flow curves is considered. The coefficients of the rheological equation are determined. The transition upon a stepwise rise in the shear rate from \({{\dot {\gamma }}_{1}}\) to \({{\dot {\gamma }}_{2}}\) is regarded as a transition from one equilibrium state of flow to another state through a certain non-equilibrium state of flow. The coefficient of deviating from equilibrium is introduced, and its temporal dependence \(\xi (t)\) is calculated using exponential functions. The limit value of \({{\xi }_{0}}\) is determined, and transition dependences \({{\tau }^{{1/2}}}(t)\) are approximated.