A structural model for thixotropy of colloidal dispersions

The present study proposes a structural model to describe the thixotropic behavior of colloidal dispersions. This model uses structural viscosity and elasticity as internal parameters, and predicts the evolution of shear stress when shear rate varies over time. Viscosity represents the structural level of colloidal aggregates and is dependent upon their alignment. Changes in alignment are linked to changes in shear rate, are regarded as instantaneous and are not believed to cause thixotropic behavior. Only changes in the structural level or structural viscosity and in the elastic deformation of colloidal aggregates cause measurable thixotropic behavior. Variations in structural viscosity over time, and at a fixed shear rate, follow a first-order kinetic equation based on two characteristic thixotropic times. This model incorporates several parameters, which are fitted independently by the appropriate rheological tests, in order to avoid any mathematical coupling. This model has been tested in 3 wt% Laponite aqueous dispersions using different ionic strengths.