Low-shear viscosity of isotropic dispersions of (Brownian) rods and fibres; A review of theory and experiments

Abstract

A review is presented of the low-shear rheology of isotropic rod dispersions. Various theoretical descriptions are compared, and trends in experiments on a variety of rods and rodlike polymers are identified and discussed. For dilute dispersions, the intrinsic viscosity [η] is only a function of the rod dimensions and hardly depends on specific system characteristics like chain flexibility, weak colloidal attractions, Brownian motion and surface roughness. For a large range of aspect ratios (20< r<1000), the asymptotic result of Onsager: [η]≈( 4r 2 15 ln r) predicts intrinsic viscosities surprisingly well. At higher particle concentrations though, the shear viscosity is dominated by specific interactions between the rods. For Brownian rods, the semi-empirical “Krieger-Dougherty” equation predicts the viscosity fairly well. The dramatic effect of deviations from hard-rod interactions can be modelled by adapting the isotropic volume fraction at which the low-shear viscosity diverges. The viscosity data of non-Brownian fibres are better described by the “Quemada” or “Maron-Pierce” equation. Experiments on well-defined (inorganic) model colloids are needed to quantify the concentration-dependence of the low-shear viscosity for rod dispersions. Theoretical calculations are still incapable of predicting this dependence.