Mass conservation and anticorrelation effects in the colloidal aggregation of dense solutions

Recent experimental data show that the scattered intensity distribution from dense solutions undergoing diffusion limited aggregation exhibits a peak at a nonzero wave vector. This article presents a simple phenomenological model based on the conjecture that the form factor for a realistic cluster should satisfy local mass conservation and hence exhibit a pronounced depression at zero wave vector. The model introduces two characteristic lengths, the cluster radius R1 and the radius R2 of the depletion region feeding the growing cluster, mass being conserved over the latter length. The intensity distributions observed during the initial stages of the aggregation process are interpreted on the basis of the form factor alone, while the description of the distributions in the later stages, in which scaling occurs, requires in addition the introduction of a structure factor which takes into account steric interactions among the clusters. The model shows remarkably good agreement with the experimental data, and also explains why earlier measurements on dilute solutions failed to exhibit the peak at a nonzero wave vector.