Number and Size of Primary Particles In Agglomerates From Mass and Mobility Measurements

Number and Size of Primary Particles in Agglomerates from Mass and Mobility Measurements

Real-time characterization of nanoparticles is necessary for continuous monitoring of aerosol manufacturing and airborne pollutant particles, but is still challenging as these particles restructure¹ and sinter or coalesce². Gas-borne nanoparticles tend to cluster and form irregular structures which influence their transport properties, effective surface and density or scattering behavior, to name a few. Here a method is developed to determine the constituent (primary) particle number, n_p, and diameter, d_p, in such structures from their mass and mobility diameter in the free molecular and transition regime. Such data are typically obtained by differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM) measurements of high temperature aerosols encountered in materials processing and engine emissions. Emphasis is placed in determining n_p and d_p in fractal-like agglomerate structures. The method is applied and compared to such measurements, microscopic images, and correlations in the literature. Primary particle size estimations from prior models show a significant overestimation if the agglomerate size is much larger than that of the primary particle. Reasonable agreement between the present method and primary particle diameters from counting electron micrographs is found³. The proposed method allows characterizing nanoparticle agglomerates with respect to primary particle size and number without any fitting together with their structure from DMA-APM measurements. Figure 1 shows such agglomerate consisting of 512 primary particles generated by diffusion-limited cluster-cluster agglomeration (DLCA) with its radius of gyration and mobility radius in the free molecular and transition regime.

Figure 1: The radius of gyration R_g and mobility radius r_m (free molecular and transition regime) of an agglomerate consisting of 512 primary particles generated by DLCA.

1. Eggersdorfer, M.L., Kadau, D., Herrmann, H.J. and Pratsinis, S.E., Fragmentation and restructuring of soft-agglomerates under shear. J. Colloid Interface Sci. 342 (2010) 261-268.

2. Eggersdorfer, M.L., Kadau, D., Herrmann, H.J. and Pratsinis, S.E., Multi-Particle Sintering Dynamics: from Fractal-like Aggregates to Compact Structures. Langmuir in press (2011).

3. Scheckman, J.H., McMurry P.H. and Pratsinis, S.E., Rapid characterization of agglomerate aerosols by in-situ mass-mobility measurements. Langmuir 25 (2009) 8248-8254.