Rheological model to predict the thixotropic behaviour of colloidal dispersions

We propose a rheological structural model to describe the thixotropic behaviour of colloidal dispersions. The model shows a certain connection between the structural changes of colloidal dispersions with viscosity. Viscosity depends on the size, shape and orientation of colloidal aggregates. Instantaneous shear rate changes cause instantaneous changes in aggregate shape and orientation, but non-instantaneous changes in aggregate size. The kinetic process in changing aggregate size determines the thixotropic behaviour of colloidal dispersions. The model differentiates two characteristic thixotropic times: one for structure formation and other for structure destruction. The elastic effects due to non-instantaneous changes of aggregate shape and orientation are also included in the model. The model has the following parameters: (i) three parameters to determine equilibrium or steady-state rheology; (ii) two parameters to set the viscosity for a given aggregate size at a given shear rate; (iii) two characteristic thixotropic times, which depend on shear rate; (iv) a characteristic elastic modulus. Each group of parameters are fitted independently with appropriate rheological tests and coupling among them is prevented. The model has been tested with consecutive lineal increases and decreases of the shear rates with time levels of maximum shear rates. Carbopol solutions of different molecular weights have been analysed with good agreement between experimental and predicted viscosity.