Coagulation kinetics and aggregate morphology in the intermediate regimes between diffusion-limited and reaction-limited cluster aggregation

We investigate the kinetics and fractal morphology of aggregating polystyrene latex in an intermediate region between diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). The measurements are made by means of a low-angle elastic-light-scattering setup covering two decades in scattering wave vectors. For each aggregation run, the salt concentration c is changed and the samples are prepared under isopycnic conditions, to avoid differential sedimentation. The average cluster mass is found to grow according to a power law with an exponent z varying in a continuous fashion in the range of salt concentration used. At high c (DLCA), we find for the fractal morphology ${\mathit{d}}_{\mathit{f}}$=1.65. For lower concentrations, ${\mathit{d}}_{\mathit{f}}$ is at first larger than 2 and therefore close to the typical RLCA fractal dimension, but then it gradually reverts to lower values not far from those typical of DLCA. This behavior is also exhibited at the lowest values of c explored, although the decay to DLCA is much slower. An interpretation of the results is presented on the basis of the available theories and simulation work on the cluster morphology in the intermediate regimes.