AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Light Scattering Shape Diagnostics for Agglomerates
GEORGE MULHOLLAND, Lei Zhou, Michael Zachariah, William Heinson, Chris Sorensen, Amit Chakrabarti, University of Maryland
Abstract Number: 39 Working Group: Aerosol Physics
Abstract Motivated by light scattering experiments showing enhanced intensity of electric field aligned agglomerates vs. randomly oriented agglomerates, we address the theoretical basis for this effect by extending the theory of small angle scattering of clusters to oriented clusters and then applying the theory to clusters generated by classical diffusion–limited cluster-cluster aggregation (DLCA). Based on clusters with 30 monomers and with 100 monomers, the ratio of the slopes based on Zimm plots for the partially aligned (aligned along the major axis but free to rotate about that axis) and randomly oriented clusters is well correlated with a linear fit to the shape anisotropy, defined as the ratio of the square of the major to minor principle radii of gyration. It is also shown that state of the art small-angle aerosol scattering measurements would have the angular resolution required to measure the shape anisotropy for clusters with 30 to 1000 monomers for a size parameter of the monomer of about 0.15.
For large q for clusters with 30 to1000 monomers, it is shown from the simulations that S(q) for the partially aligned clusters does not follow the power law behavior observed for the randomly oriented clusters. Clusters with a fixed orientation are shown to result in a structure factor with multiple peaks, which could be used to obtain more detailed information about particle structure than shape anisotropy.
The measurements showing enhanced scattering for partially aligned soot agglomerates were for angle integrated measurements. Calculation of the integrated light scattering cross section for the same range of angles and polarization direction as the experiments indicate a significant enhancement of 70 % and 120 % for two representative aspect ratios. The smaller value overlaps with measured values of the scattering enhancement for oriented soot agglomerates in an electric field.