SEDIMENTATION OF BIDISPERSE, UNCHARGED COLLOIDAL SPHERE SUSPENSIONS -INFLUENCE OF VISCOSITY AND IRREGULAR SURFACES

The sedimentation velocity of uncharged, nonaggregated silica spheres under gravity is strongly reduced after addition of small amounts of n onsedimenting small spheres. This reduction is largely due to surface irregularities on a nanoscale of the large spheres at which a limited number of small spheres adsorbs, leading to an increase of the hydrody namic friction per particle. This adsorption also screens effectively any weak Van der Waals attraction between the large spheres, which des pite its weakness, significantly influences the concentration dependen ce of settling in a pure solvent. The concentration dependence and mag nitude of the large-sphere sedimentation velocity in a more concentrat ed dispersion of small spheres agree with the prediction by Batchelor that the small particles mainly manifest themselves as an effective vi scosity increase. (C) 1996 Academic Press, Inc.