Rheological characterization of flow inception of thixotropic yield stress fluids using vane and T-bar geometries

In this work, two geometries are studied, the vane and the T-bar, which are best suited for assessing the start-up flow of thixotropic yield stress fluids because they minimize the sample disturbance. Based on step-shear measurements with the vane geometry at different angular velocities and on a wide range of products, mostly commercial toothpastes, we calculate the torque on the T-bar using computational fluid dynamics (CFD). The results are compared to the previously suggested approximate theory by Anderson and Meeten (AMT) and extensive original experiments. It turns out that the agreement between CFD, AMT, and the experimental data depends primarily on the shape of the flow curve which may be quantified by the fluid flow index, N, defined in the shear rate range which represents the flow around the rotating rod of the T-bar. While the CFD and AMT predictions agree well with each other (R-2 = 0.98), they both underestimate the experimental data although the experimental-to-predicted ratio also correlates to N (R-2 = 0.84) going up from 1 to around 2 as N increases from 0.1 to 0.5. This suggests that when using the T-bar for viscosity measurements, the user needs to take into account the flow index to which end a simple estimate of the effective shear rate is suggested also being a function of N.