Experimental study of colloidal aggregation in two dimensions. III. Structural dynamics

Time-dependent aspects of the structural self-similarity of colloidal aggregates formed in a two-dimensional system at a liquid-air interface are considered. The aggregation was induced by the addition of ${\mathrm{CaCl}}_2$ to the aqueous phase, the electrolyte concentration governing the growth conditions. At all subphase molarities large clusters, necessarily formed late in the aggregation process, showed evidence of internal reorganization, the fractal dimension of the cores of the clusters being significantly larger than that for the global cluster structure. Early in the aggregation process, during a regime of slow cluster growth, the overall cluster morphology resembled that of reaction-limited cluster-cluster aggregation for all ${\mathrm{CaCl}}_2$ concentrations. However, during the later, rapid growth phase the fractal dimensions of the clusters fell at high subphase molarities, becoming compatible with expectation for diffusion-limited aggregation. It is shown that all of these observations are in accord with the hypothesis that the intrinsic particle-particle reaction probability, while experimentally variable, was small under all conditions. A possible explanation for this hypothesis is discussed.