RHEOLOGY OF BIDISPERSE AQUEOUS SILICA SUSPENSIONS - A NEW SCALING METHOD FOR THE BIDISPERSE VISCOSITY

The shear viscosity of electrostatically stabilized bidisperse aqueous silica suspensions has been studied as a function of shear rate, part icle size, total volume fraction of the particles, and volume ratio of small to large particles. The higher limiting relative viscosity (at 1000 s(-1)) of monodisperse suspensions of 0.6, 0.85, and 1.5 mu m sil ica particles was found to be particle size dependent indicating devia tion from hard sphere behavior. It is shown that the flow properties o f bidisperse suspensions of colloidal size particles is not only affec ted by the volume (weight) ratio of the two sizes, but the absolute si ze of the particles and the particle size ratio also play an important role in determining the viscosity of the suspension. Binary mixtures of particles of smaller size ratios show higher viscosities than the s uspensions containing larger size ratio particles. This indicates that the viscosity response of suspensions of colloidal size particles is controlled by both hydrodynamic interactions and colloidal forces. The Farris [Trans. Sec. Rheol. 12, 281 (1968)] model was found to be inad equate when applied to bidisperse suspension of colloidal size particl es. An empirical method is suggested to estimate the viscosity of a bi modal suspension at a given volume fraction of the particles, from the viscosity of monodisperse systems and the viscosity of a bimodal susp ension (at a fixed total volume fraction) as a function of the volume ratio of the two particles. (C) 1998 The Society of Rheology.