The vane‐in‐cup as a novel rheometer geometry for shear thinning and thixotropic materials

Abstract

We have addressed the question of the suitability of the vane‐in‐cup as a rheometer geometry. A numerical simulation of this geometry was conducted for a power‐law fluid and the results compared with a similar study for a conventional bob‐in‐cup geometry. The comparison indicates that for a sufficiently shear‐thinning fluid (of shear‐thinning index less than 0.5) the fluid within the periphery of the vane blades is essentially trapped there and turns with the vane as a solid body. Calculation of the shear stress at the cup wall indicates that this quantity is equal in both geometries for a given rotational rate of the spindle. Thus the torque required to turn the spindle would be the same and identical flow curves would be predicted. This prediction was tested on two fluids thought to possess a yield stress: a 5.5% sodium carboxymethylcellulose (CMC) solution and a 4.2% Veegum PRO clay suspension. Equivalent flow curves were obtained at very low stresses/shear rates but a sudden, catastrophic viscosity loss was found for both fluids with the bob at shear rates which were still quite low. Such a loss was observed with the vane as well, but at much higher shear rates. It is suggested that this phenomenon is a form of apparent slip due to the formation of a thixotropic layer at the bob/vane surface. The much flatter stress profile obtained in the vane geometry is reasoned to postpone the formation of this layer. Rheological data obtained with the vane appear to be a faithful representation of these materials and show the absence of a true yield stress.