SETTLING BY CLUSTER DEPOSITION IN AGGREGATING COLLOIDAL SUSPENSIONS

At low volume fractions, calcium carbonate colloidal suspensions under go cluster deposition: aggregates grow in the suspension, settle indiv idually when their size becomes large enough, and deposit at the botto m of the container, forming a voluminous sediment. We have measured th e settling velocity of individual aggregates as a function of their ra dius. We show that the clusters are self-similar with a fractal dimens ion, D = 2.2 +/- 0.1, which is significantly larger than that in diffu sion-limited aggregation, and that they do not grow above a limiting r adius, R(max) congruent to 0.5 mm. These results are discussed in rela tion with the mechanical properties of colloidal fractal aggregates. ( C) 1996 Academic Press, Inc.