LOW-SHEAR VISCOSITIES OF DILUTE DISPERSIONS OF COLLOIDAL RODLIKE SILICA PARTICLES IN CYCLOHEXANE

Low-shear viscosities have been studied for isotropic dispersions of u ncharged colloidal silica rods in cyclohexane as a function of volume fraction Phi and aspect ratio r(p). Intrinsic viscosities in the limit Phi-->O agree with theoretical results [Brenner, H., Int. J. Multipha se Flow 1, 195 (1974)] for colloidal cylinders. The Phi(2) term in the low-shear viscosity, accounting for two-particle interactions, is con siderably larger than predicted by D. H. Berry and W. B. Russel [J. Fl uid Mech. 180, 475 (1987)] for dilute hard-rod dispersions. The discre pancy is very likely due to weak interparticle attractions, which also account for the novel observation of an isotropic-isotropic phase sep aration in the silica rod dispersions. Our results demonstrate that, i n comparison to spheres, high aspect ratios markedly increase both the shear viscosity and its sensitivity to even minor attractions. (C) 19 96 Academic Press, Inc.