Structural crossover in dense irreversibly aggregating particulate systems

Cluster-cluster aggregation has been simulated by off-lattice Monte Carlo methods for diffusion-limited (DLCA), ballistic-limited, and reaction-limited cluster aggregation classes. We find that as the system evolves and becomes dense, the largest cluster develops a hybrid structure with mass fractal dimension $D_f\simeq 2.6$ over large length scales, while at smaller length scales, the early time dilute-limit fractal structure is frozen in. The largest cluster is thus an aggregate of smaller aggregates with a different fractal dimension, and we call it a “superaggregate.” The crossover length separating the two morphologies, which we call the critical radius of gyration, can be calculated based on a simple theory that assumes a monodisperse cluster size distribution. This agrees well with simulation results for DLCA. However, for other classes we find that the increasing polydispersity in cluster size pushes the simulated crossover length radius of gyration to values systematically larger than the predicted value.