Local order in two-dimensional colloidal aggregation

Diffusion limited cluster-cluster aggregation in two dimensions has been studied both experimentally and in computer simulation. In both cases the structure function at late times exhibits hitherto unsuspected sixfold symmetry. The triangular structure of the system in real space is not very pronounced, resembling the local order of a liquid. The six peaks in the structure function lie at the wave number of the characteristic spatial modulation previously found for this system. It is argued that both phenomena arise due to mutually exclusive depletion zones which form around clusters in diffusion limited aggregation, providing an effective long-range intercluster repulsion.