Coagulation rate coefficients for fractal-like agglomerates in the diffusive and ballistic limits

Abstract

Predicting agglomeration dynamics in aerosol or colloidal systems requires accurate models for the agglomerate collision rate. In the present work, numerical simulations cover the ballistic and the diffusive collision regimes as limiting cases and are used to derive coagulation rate coefficients for a large number of agglomerate sizes and morphologies. The simulations account for both the translational and rotational motion of agglomerates and allow for the monitoring of individual collision events. The results suggest that the power law suffices to characterize the clusters’ morphologies and that representative collision radii of agglomerates are mainly controlled by two parameters: (i) the radius of gyration, Rg, and (ii) the morphology characterized by the fractal dimension Df. Collision radii for the diffusive regime are up to 10% above the corresponding ballistic results and cluster rotation also increases the effective collision radius. The latter effect can lead to differences of up to 16% in the diffusive regime. In contrast, interpenetration effects, i.e. a small cluster passing through a larger cluster without collision, are less pronounced in the diffusive limit. The results provide strong correlations between collision radii and key parameters for both regimes such that improved analytical expressions for the coagulation rate coefficients can be suggested. Compared to existing models for the ballistic and diffusive regimes, the proposed correlations offer reduced complexity across an extended range of the relevant parameter space.