Numerical investigation of plate edge and slot size effects in low yield stress measurements with a slotted plate device

There is a need for accurate yield stress measurements, especially in the case of low yield stress complex materials such as biological samples. This task cannot be accomplished with conventional rotational rheometers due to significant wall slip effects and the necessity to operate the device at very low shear rates, often beyond the limit that such rheometers can achieve. In this paper, we focused on the slotted plate method proposed recently for low yield stress measurements. Using computational fluid dynamics, we studied the effects of plate geometry on the measurement accuracy of the slotted plate method. Results of this study indicate that both wall slip effects and pressure drag force can be substantially reduced by adopting a thin plate with sharp front and rear edges, high slot area ratio, and large number of slots. If the plate has 30° triangular edges, a slot area ratio of 80%, and 12 slots, the slotted plate method overpredicts the yield stress of a 0.09 wt.% Carbopol dispersion (yield stress of 9.17 Pa) by only 8.4% under no-slip conditions and underpredicts the yield stress by 12.3% under free-slip conditions. Similar results were obtained for human saliva characterized by a very low yield stress (0.073 Pa).