LOW-SHEAR VISCOSITY OF ISOTROPIC DISPERSIONS OF (BROWNIAN) RODS AND FIBERS - A REVIEW OF THEORY AND EXPERIMENTS

A review is presented of the low-shear theology of isotropic rod dispe rsions. Various theoretical descriptions are compared, and trends in e xperiments on a variety of rods and rodlike polymers are identified an d discussed. For dilute dispersions, the intrinsic viscosity [eta] is only a function of the rod dimensions and hardly depends on specific s ystem characteristics like chain flexibility, weak colloidal attractio ns, Brownian motion and surface roughness. For a large range of aspect ratios (20 < r < 1000), the asymptotic result of Onsager: [eta] appro ximate to (4r(2)/15 In r) predicts intrinsic viscosities surprisingly well. At higher particle concentrations though, the shear viscosity is dominated by specific interactions between the rods. For Brownian rod s, the semi-empirical ''Krieger-Dougherty'' equation predicts the visc osity fairly well. The dramatic effect of deviations from hard-rod int eractions can be modelled by adapting the isotropic volume fraction at which the low-shear viscosity diverges. The viscosity data of non-Bro wnian fibres are better described by the ''Quemada'' or ''Maron-Pierce '' equation. Experiments on well-defined (inorganic) model colloids ar e needed to quantify the concentration-dependence of the low-shear vis cosity for rod dispersions. Theoretical calculations are still incapab le of predicting this dependence. (C) 1998 Elsevier Science B.V.