Mesoscale structure of diffusion-limited aggregates of colloidal rods and disks

We explore the dependence of the non-universal mesoscale structure of diffusion-limited aggregates upon the shape of their constituent particles. Using random-walker simulations that model anisotropic diffusion in a viscous fluid, we study diffusion-limited aggregation (DLA) of right-circular cylinders having a wide range of length-to-diameter aspect ratios. This single-parameter family of particle shapes allows us to determine the role of particle quasi-dimensionality on the structure of DLA clusters that are composed of effectively one-dimensional thin rods and two-dimensional thin plates. We compare these clusters to those formed by traditional DLA of compact objects by studying the local nature of the interparticle contacts (end-end, end-body, or body-body), the distribution of the number of interparticle contacts, and the wavevector-dependent structure factor S(q) of the resulting clusters. We find that clusters of rods are less dense than those of disks or compact objects of equal volume, yet the long length-scale structure of the DLA clusters conforms to the expected DLA scaling relations. However, the local structure at the particle scale, including the nearest-neighbor distribution functions and dominant collision types, depend strongly on the particle's quasi- dimensionality. We explain the non-universal local structure by introducing the concept of a diffusing particle's 'touch space', which incorporates both the particle's geometry and its anisotropic diffusion.