The Structure of Agglomerates consisting of Polydisperse Particles.

Agglomeration is encountered in many natural or industrial processes, like growth of aerosol particles in the atmosphere and during material synthesis or even flocculation of suspensions, granulation, crystallization and with colloidal particle processing. These particles collide by different mechanisms and stick together forming irregular or fractal-like agglomerates. Typically, the structure of these agglomerates is characterized with the fractal dimension, Df , and pre-exponential factor, kn , of simulated agglomerates of monodisperse primary particles (PP) for ballistic or diffusion-limited particle-cluster and cluster-cluster collision mechanisms. Here, the effect of PP polydispersity on Df and kn is investigated with agglomerates consisting of 16 - 1024 PP with closely controlled size distribution (geometric standard deviation, σ g = 1-3). These simulations are in excellent agreement with the classic structure (Df and kn ) of agglomerates consisting of monodisperse PPs made by four different collision mechanisms as well as with agglomerates of bi-, tri-disperse and normally distributed PPs. Broadening the PP size distribution of agglomerates decreases monotonically their Df and for sufficiently broad PP distributions (σ g > 2.5) the Df reaches about 1.5 and kn about 1 regardless of collision mechanism. Furthermore with increasing PP polydispersity, the corresponding projected area exponent, Dα , and pre-exponential factor, ka , decrease monotonically from their standard values for agglomerates with monodisperse PPs. So Df as well as Dα and ka can be an indication for PP polydispersity in mass-mobility and light scattering measurements, if the dominant agglomeration mechanism is known, like diffusion-limited and/or ballistic cluster-cluster coagulation in aerosols.