Settling by cluster deposition in aggregating colloidal suspensions

At low volume fractions, calcium carbonate colloidal suspensions undergo cluster deposition: aggregates grow in the suspension, settle individually when their size becomes large enough, and deposit at the bottom of the container, forming a voluminous sediment. We have measured the settling velocity of individual aggregates as a function of their radius. We show that the clusters are self-similar with a fractal dimension, D = 2.2 +/- 0.1, which is significantly larger than that in diffusion-limited aggregation, and that they do not grow above a limiting radius, R(max) congruent to 0.5 mm. These results are discussed in relation with the mechanical properties of colloidal fractal aggregates. (C) 1996 Academic Press, Inc.