(418l) 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.