Langevin Simulation of Aggregate Formation in the Transition Regime

Abstract

We study the aggregation of monomers in an aerosol via non-dimensional Langevin simulations, in which particles remain in point contact upon collision, and report the hydrodynamic radii and projected areas of the formed aggregates with less than 300 primary particles. Unique from prior studies, in examining aggregation we monitor the evolution of the distributions of two Knudsen numbers: the traditional Knudsen number (Kn) and the diffusive Knudsen number (KnD), which both shift to smaller mean values as aggregation proceeds. As Kn transitions from large to small values, momentum transfer changes from a free molecular to a continuum process; analogously, as KnD decreases, aggregation is altered from occurring ballistically to diffusively in a dilute system. During simulations, the change in drag coefficient with both changing Kn and changing aggregate structure is accounted for. We find that as compared to completely coalescing particles (spheres), non-coalescing aggregates with the same initial Kn and KnD have KnD values, which decrease more rapidly due to aggregation; hence, aggregates are more likely to collide with one another diffusively when compared with their spherical counterparts of the same Kn distribution. Further, we find that aggregation with evolving Knudsen numbers does not lead to strong scaling between the number of monomers in a formed aggregate and the aggregate radius of gyration for aggregates composed of 300 or fewer primary particles. In spite of this, aggregate hydrodynamic radii and orientationally averaged projected areas are found to scale well with the number of monomers per aggregate.Copyright 2015 American Association for Aerosol Research