Feasibility of an optical experimental method for the sizing of primary spherules in sub-micron agglomerates by polarized light scattering

The possibility of inferring by a non-invasive experimental method the size of primary particles (spherules), which constitute the agglomerated soot generated in an ethylene–air diffusion flame, is investigated. In contrast to the predictions from the Mie theory for isolated spheres, experimental evidence is provided here about the fact that the size of spherules (some tens of nanometers), which stick together to form agglomerates (some hundreds of nanometers), can be recognized from polarization ratio measurements. Validation of the proposed scattering technique is obtained by first performing standard measurements of the primary particle size by SEM analysis of soot samples taken on quartz inserted directly in the flame along the burner axis. Then, the polarization ratio P(theta)=sigma(HH)/sigma(VV) of scattered light is measured at the same locations and for the same flame conditions for different polar scattering angles theta. As major result, evidence is provided of a linear relationship existing between the primary sizes, obtained independently by SEM analysis, and the measurements of the polarization ratio P(90°). Finally, a procedure is reported and applied to retrieve the absolute spherule size from the direct observation of the transition between the power-law and Porod’s scattering regimes, which correspond to the domains of long-range (fractal) and short-range (not fractal) interactions between primary particles, respectively.