Light scattering from nanoparticle agglomerates

Accounting for the morphology of fractal-like particles is important for their in-situ characterization by optical diagnostics. Here, the formation of carbonaceous aerosol (soot) fractal-like particles by surface growth and agglomeration is investigated experimentally and numerically accounting for necking and multiple scattering between polydisperse primary particles (PPs) by Discrete Element Modeling (DEM) coupled with the Discrete Dipole Approximation (DDA). The DEM-derived number of constituent PPs, soot agglomerate effective density and differential scattering cross-sections for vertically-, C-v, and horizontally-polarized incident light, C-h, are in excellent agreement with those measured here in premixed ethylene flames. In contrast, using the Rayleigh-Debye-Gans (RDG) theory and neglecting necking (aggregation) and polydispersity of constituent PPs underestimates the above C-v and C-h up to 50 %. A DEM-derived scaling law for PP aggregation and polydispersity is developed. Using this law in the RDG theory, the soot scattering cross-sections are in agreement with those estimated by rigorous DDA. Thus, both DEM-DDA and revised RDG theory can be used to optimize optical diagnostics for soot or carbon black characterization and selective sensing by fire (smoke) detectors. (C) 2019 Elsevier B.V. All rights reserved.