Experiments on depolarized optical scattering to sense in-situ the onset of early agglomeration between nano-size soot particles

Soot aerosol particles generated in hydrocarbon flames are investigated in this paper by laser light scattering techniques. The contribution of co-polarized and cross-polarized scattered light, each for vertical and horizontal linear polarization states of the incident laser light at 514.5 nm are measured at variable polar angles theta. Main results here reported are the following: (1) the vertical depolarization ratio rho_v measured against the scattering angle is very low and flat in the case of smaller chain-like (aggregate fractal dimension Df~1.3), whereas, in the case of larger branched-chain aggregates (Df~1.7), rho_v exhibits a maximum at about 90°. (2) in contrast to the reciprocity theorem, the measured ratio I_HV / I_VH between the two depolarized contributes for each of the two possible linear (Vertical and Horizontal) polarization states of the incident light, is found to be different than unity when first sticking of the primary particles occurs. Therefore, the measurement of the reciprocity ratio I_HV / I_VH seems a powerful and reliable tool to establish the onset of the early aggregation mechanism between nano-size soot particles formed in a flame.