Light Scattering Shape Diagnostics for Nano-Agglomerates

摘要

Motivated by light scattering experiments showing enhanced intensity of electric field aligned nano-agglomerates versus randomly oriented nano-agglomerates, we address the theoretical basis for this effect by applying the theory of small angle Rayleigh-Gans-Debye light scattering to oriented nano-clusters generated by classical diffusion-limited cluster-cluster aggregation (DLCA). We show that the shape anisotropy of these clusters is related to the ratio of small angle light scattering for partially aligned and randomly oriented clusters. 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 of clusters with 30 to 1000 nano-monomers. For large q, it is shown from the simulations that S(q) for the partially aligned nano-clusters is not proportional to q~(-D) f, where D_f is the fractal dimension, as it is for randomly oriented clusters. Nano-clusters with a fixed orientation are shown to result in a structure factor with multiple peaks, which might be used to obtain more detailed information about particle structure than shape anisotropy. The measurements reported in the literature 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.