Observation of restructuring of nanoparticle soot aggregates in a diffusion flame by static light scattering

Static light scattering experiments are performed within an ethylene-air diffusion flame for different flow-rates of the fuel gas. Direct evidence is here given about the fact that soot agglomerates, at the early stages of the agglomeration process, first grow as elongated chain-like structures and then, at later stages, assume more compact morphologies with significantly smaller size. This mechanism is observed to occur for the first time in a diffusion flame along the flame axis in the range of low Heights-Above-Burner (HABs less than one fourth of the total flame length). Thus, branched elongated aggregates grow up at lower HABs to assume a radius of gyration Rg~380 nm while the fractal dimension is practically constant Df*~1.3. Thereafter, at larger HABs, they self-reorganise in much shorter clusters (Rg~160 nm), with Df* increasing up to 1.9. Explanation is proposed for the observed rearrangement of soot fractal aggregates in terms of local heat transfer balance at cluster surface, which could cause, in the case of chain-like aggregates, a supplementary oxidation inside the pores of soot with possible breaking of the graphitic layers that act as bridges between primary particles within an aggregate. The relevance of depolarised scattered light is discussed with regard to some distinguishing features of clusters that share the same measured fractal dimension.