Structure and anisotropy of colloid aggregates

Abstract

We study the complex, disordered structure of clusters formed by several kinetic aggregation processes. By expanding the structure factor into spherical harmonics, detailed information about both the structure and anisotropy of the clusters at all length scales is obtained. The scaling of the expansion terms with cluster mass and order of the expansion is investigated. For cluster-cluster aggregates, we find that the fractal dimension, which reflects the scaling of the orientationally averaged structure, also describes the scaling of the spherical harmonics, which reflect the anisotropies of the structure. By contrast, clusters formed by single-particle diffusion-limited aggregation do not appear to exhibit the same degree of scale invariance. Cluster-cluster aggregates are found to exhibit considerably more anisotropy than those formed by single-particle diffusion. The consequences of the anisotropies for quasielastic light scattering experiments are discussed.