Estimation of zero-shear viscosity of polymer solutions from falling sphere data

Abstract

Extrapolation methods for the determination of zero-shear viscosity from falling sphere tests are compared with each other and in particular with dicrect viscometric measurements of this parameter. It is found that all methods of extrapolation overestimate the true zero-shear viscosity and that the discrepancy depends on the degree of shear thinning encountered by the falling spheres. Falling sphere tests only yield the true zero-shear viscosity when the spheres fall in the lower Newtonian region of fluid behaviour. In most instances a suitable combination of sphere properties to achieve this can only be found in the case of very viscous fluids which can in any case also be characterized by direct viscometric measurements in this region. If sphere fall data must be extrapolated, methods based on shear rate rather than shear stress appear preferable since they generally yield lower values of zero-shear viscosity, which are therefore nearer to the true value.