| Summary: | The effect of surface charges on the hindered settling of colloidal particles was studied using chemically precipitated mono-dispersed 0.6-micron silica spheres. Settling rates were measured in tubes of 11 mm inside diameter containing distilled and deionised water that had been adjusted with sodium hydroxide to alter the zeta potentials on the particles, with sodium chloride to alter the non-dimensional electrokinetic radius and with hydrochloric acid to suppress electrostatic interactions between the particles at a pH of about 3, which is close to the iso-electric point of the silica spheres. The particles were dispersed by means of a Brinkman homogeniser and the zeta potentials were measured by the method of micro-electrophoresis. The volume fraction c of particles in the dispersions was varied from 0.005 to 0.26, while the zeta potential and the non-dimensional electrokinetic radius were progressively raised to as high as -81 millivolts and 240, respectively. The settling rates measured for various combinations of particle concentration c, zeta potential ξ and electrokinetic radius were virtually in all cases lower than for the uncharged spheres, thus attesting qualitatively to the retarding effect of the Dorn potentials and throwing some light on anomalous sedimentation results for fine particles in the literature. The experimental data for the uncharged spheres were compared with models in the literature for sedimentation of fine particles and found to fall between the theoretical prediction of Batchelor for dilute dispersions and that of Stauffer and Clavin for more concentrated dispersions. However, comparison of the settling data for the charged spheres with the theory of Levine et al. on the retarding effect of the Dorn potentials in concentrated dispersions showed this very restrictive theory to be quite inadequate to the present data. === Applied Science, Faculty of === Chemical and Biological Engineering, Department of === Graduate
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