The effect of primary particle polydispersity on the morphology and mobility diameter of the fractal agglomerates in different flow regimes

• Radius of gyration, surface area, and mass increase with monomer polydispersity.
• Free-molecular mobility diameter increases with primary particle polydispersity.
• Continuum mobility diameter increases with primary particle polydispersity.
• Mobility diameters are correlated to number, diameter and distribution of monomers.
• Polydispersity within individual particles and ensembles of particles are different.

Properties of colloidal and aerosol agglomerates depend on their morphology. Accurate estimation of the mobility-equivalent diameter dmdm in different flow regimes is essential in many industrial processes and measurements. Previous work on the hydrodynamic properties of clusters focussed on agglomerates composed of monodisperse primary particles. However aggregates formed in real processes, e.g. soot particles, are usually formed from polydisperse monomers. Using numerically-generated agglomerates it is shown here that the radius of gyration, surface area, and mass of the agglomerates increase with primary particle polydispersity (given constant geometric mean primary particle size dpgdpg). Here, dmdm is taken as the projected area-equivalent diameter for the free molecular regime; Stokesian Dynamics is used to compute dmdm in the continuum flow regime. For fixed number of primaries and dpgdpg, dmdm increases with polydispersity in both free molecular and continuum regimes (>20% for large particles at high polydispersity). Considering an aerosol population with polydisperse primary particles, this increase is found to depend on whether the variations in primary particle size occur within aggregates or between aggregates; this can be important in the interpretation of measurements. Finally, mobility diameters are correlated with total number, median diameter and its geometric standard deviation of the primary particles.