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 g enerated in an ethylene-air diffusion flame, is investigated. In contrast t o the predictions from the Mie theory for isolated spheres, experimental ev idence is provided here about the fact that the size of spherules (some ten s of nanometers), which stick together to form agglomerates (some hundreds of nanometers), can be recognized from polarization ratio measurements. Val idation of the proposed scattering technique is obtained by first performin g standard measurements of the primary particle size by SEM analysis of soo t samples taken on quartz inserted directly in the flame along the burner a xis. Then, the polarization ratio P(theta) = sigma(HH)/sigma(VV) of scatter ed light is measured at the same locations and for the same flame condition s for different polar scattering angles theta. As major result, evidence is provided of a linear relationship existing between the primary sizes, obta ined independently by SEM analysis, and the measurements of the polarizatio n ratio P(90 degrees). Finally, a procedure is reported and applied to retr ieve the absolute spherule size from the direct observation of the transiti on between the power-law and Pored's scattering regimes, which correspond t o the domains of long-range (fractal) and short-range (not fractal) interac tions between primary particles, respectively.