THE EFFECT OF MANY-BODY INTERACTIONS ON THE SEDIMENTATION OF MONODISPERSE PARTICLE DISPERSIONS

An experimental investigation was made of the sedimentation rate of lo w-charged monodisperse silica and polystyrene latex particle dispersio ns as a function of the particle volume fraction. It was found that th e normalized sedimentation velocity U/U-o, corrected for the effect of the two-body hydrodynamic interaction, increases with the particle vo lume fraction, which indicates that the degree of particle aggregation inside the dispersions increases with the particle volume fraction. T his phenomenon results from attractive many-body hydrodynamic interact ions between colloidal particles. It is reported for the first time th at the many-body hydrodynamic interaction becomes important at the par ticle concentration of 6.5 vol% in monodisperse dispersions, and the m any-body thermodynamic interaction is negligible at a low particle con centration, i.e., less than 15 vol%. The effect of many-body hydrodyna mic interaction on the particle microstructure was also experimentally examined by using a nondestructive Kossel diffraction technique based on the principle of back-light scattering. It was found that the part icle packing structure inside the dispersion initially becomes more or dered with the increase of the particle volume fraction. However, ther e is less increase in the particle ordering structure after 6 vol%. Fu rthermore, after the particle concentration reaches 10 vol%, the parti cle packing structure decreases to a value lower than that of 6 vol% d ue to the increased particle aggregation, as found in the sedimentatio n experiments. Predictions of a statistical thermodynamic model were c ompared with the experimental data on structure factors. It is found t hat particle dimerization occurs around 10 vol%, which agrees with the sedimentation results. (C) 1998 Academic Press.