Non-preserving cluster size distributions in the initial stages of orthokinetic aggregation

Cluster size distributions for aggregating suspensions of electrostatically destabilised polystyrene microspheres have been explicitly derived as a function of time, using photon correlation spectroscopy (PCS). Latex spheres of two diameters, 264 and 303 nm, were studied. Size distributions were determined both for aggregation due entirely to diffusional processes, and also for growth induced by a combination of diffusion and applied shear, with gamma = 20-200 s(-1). Whilst size distributions for diffusion-limited aggregation are of a self-preserving, log-normal form, those generated when shear was applied to the suspension show considerable deviation from a self-preserving shape; only after an extended period is the log-normal distribution re-established. Certain parameters, including the mean cluster diameter and moments of the distributions, indicate that shear-induced aggregation is less efficient at higher shear rates, and also when the clusters are small. Temporal changes in total particle number density were compared with Smoluchowski's equations for peri- and ortho-kinetic aggregation. Rapid rates of increase in cluster size, in the initial stages of aggregation, may be explained, and to a certain extent predicted, by considerable, short-term, increases in the skewness of the size distribution.