10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Simulations of Light Scattering from Aligned and Randomly Oriented Polydisperse Aggregates for Determining Aggregate Shape
GEORGE MULHOLLAND, James Corson, Michael Zachariah, University of Maryland
Abstract Number: 423 Working Group: Aerosol Physics
Abstract This study focuses on the simulation of the light scattered by aligned and randomly oriented fractal aggregates of touching spheres. We find from the simulations that a shape parameter for a population of aggregates can be determined from these scattering calculations. The dependence of the structure factor, S, on the magnitude of the scattering wave vector, q, has been an important tool in the study of the size, the fractal dimension, and the pair distribution function for randomly oriented aggregates. However, such measurements do not provide a measurement of the shape of the aggregate. As discussed in a previous paper (Mulholland, G.W. et al., 2013), an aggregate aligned by an electric field in the direction of the largest principal radii of gyration will scatter more intensely in the forward direction than in other orientations because there is less destructive interference. The scattering plane is oriented perpendicular to the electric field. This paper also showed that a ratio of slopes computed from small angle light scattering intensities is related to the shape parameter A31, which is the ratio of the largest to the smallest principle radii of gyration of the inertia tensor. This measure of shape anisotropy may be an important property regarding the transport of the aggregates, the aggregation rate, the collection efficiency by a filter, and for the characteristics of manufactured aggregates. Since A31 is unity for a sphere, a measurement of A31 can also be a sensitive diagnostic for the presence of aggregates in an aerosol such as an atmospheric cloud.
In the present study, aggregates were generated to simulate the range of aggregate sizes (30 to 2000), primary sphere radii (10 nm – 23 nm), and size distributions for soot from buoyant turbulent flames at long residence times for a range of hydrocarbon fuels(Koylu, U.O.et al., 1992). More nearly monodisperse aggregates were also generated to simulate the size distributions obtained via mobility or mass classification. The aggregates were generated for a fractal dimension characteristic of classical diffusion–limited cluster-cluster aggregation (DLCA) using an algorithm developed by Mackowski (2006). The value of S(q) was computed for the aligned (but free to rotate about the alignment axis) and randomly oriented clusters. The aggregates were aligned by the electric field in the direction yielding the minimum energy associated with the polarizability of the aggregate. This direction was computed using an algorithm developed by Mansfield et al. (2001).
Results on the correlation between the ratio {S(q)}a/ {S(q)}r for the monodisperse and polydisperse clusters and the average value of A31 indicate that light scattering measurements in the q range of fractal behavior would be feasible for shape characterization. Also for small q (Guinier regime), the strong correlation between A31 and the ratio of slopes of S(q)-1 vs. q2 previously obtained for single aggregate simulations is also valid for polydisperse aggregates. This suggests that small angle light scattering measurements would provide shape information.
The fundamental differences in the light scattering and the pair correlation function for the aligned and randomly oriented clusters will also be discussed.