Evidence for long-range Coulomb effects during formation of nanoparticle agglomerates from pyrolysis and combustion routes

Two processes of the agglomeration of aerosol particles are investigated. The first process involves silicon aerosol formation by silane pyrolysis in a flow reactor. In the secund process, soot aerosol is formed during propane combustion in a Bunsen burner. The agglomerate size and morphology are analyzed by a transmission electron microscope. An imaging system is used to observe agglomerate-agglomerate coagulation and agglomerate sticking to the deposit formed on the surface (tendrils). The movement of agglomerates in the electric field is also studied using the imaging system. It is found that Coulomb interactions are significant during the sticking process and, in particular, they are responsible for the fractal dimension, inferred in the experiments to be significantly lower with respect to thr values obtainable from diffusion limited cluster-cluster aggregation simulations. However, the mechanism of this interaction is different for silicon and soot agglomeration processes. It is found that the silicon agglomerates are dipoles with net charges equal to zero. By contrast, approximately half of the soot agglomerate population is estimated to be charged with a net charge equal to one elementary unit (positive or negative). The major result is that the Coulomb interactions are observed to hold considerable influence during the process of agglomerate-agglomerate sticking, in particular, encouraging the agglomerate mutual sticking at the tips of the single agglomerates.