Shear and elongation flow properties of kaolin suspensions

The elongation viscosity of kaolin pigment suspensions (71 wt %-49 vol %) was measured with a technique developed in our laboratory. Elongation thickening became apparent at rates of 10(3) s(-1) while shear thickening occurred at a shear rate an order of magnitude larger. It is not entirely clear if elongation thickening was in fact due to shear within the elongation flow geometry, however, possible parasitic effects have been considered and it is believed the observed elongation thickening is close to a true material property. Torsional rheometer stress measurements at lower shear rates allowed the total stress to be deconvoluted into the viscous (hydrodynamic) and elastic (thermodynamic or structural) components using the stress jump technique. The total stress was equal to that obtained with the capillary rheometer lending some confidence in the experimental technique. Furthermore, it was found that the stress or viscosity) at higher shear rates was dominated by the viscous component. Thus, particle hydrodynamics play a key role in shear thickening as well as in elongation thickening (should it be present). The ratio of the elongation to shear viscosity (Trouton ratio) was found to be a function of the particle size and quite remarkably had values of the order of 50-100, in line with those values frequently quoted for some polymer solutions and melts. (C) 2002 The Society of Rheology.