3D simulations of the flow of thixotropic fluids, in large-gap Couette and vane-cup geometries

Models of the vane-cup and Couette rheometers are compared using computational fluid dynamics as well as approximate solutions. Thixotropy of the fluid is incorporated by means of a model based on experimental data for various toothpastes. Parameters of the model are calculated by fitting the results of step-shear tests in Couette geometry and are subsequently used to predict torque for similar tests in vane-cup geometry. Calculations for the thixotropic model were compared to the calculations for a time-independent equilibrium model with the same steady-shear rheology. There are two important findings in this work. First, at constant angular velocity, torque in the vane-cup rheometer for the thixotropic model turns out greater than for the equilibrium model, because the structure of the thixotropic fluid has not had enough time to reach equilibrium. This has implications for rheometry and the modeling of mixing operations. In both cases, torques may be underestimated if the standard equilibrium model is used in calculations. The second finding is relevant to rheometry. As is well known, equilibrium model predicts a lower torque value for a vane-cup geometry than for an equivalent Couette geometry. We found that taking thixotropy into account either makes the difference less pronounced or in some cases actually makes torque for vane-cup higher than for Couette. End effects are also analyzed.