Brownian coagulation and reshaping of nucleation mode ultrafine carbon soot in flames

Several experimental studies performed by ex-situ, in-situ and on-line techniques suggest that the carbon units born in combustion systems the so-called primary particles (20-60 nm), which constitute the fractal soot aggregates (~0.1 micron), are actually constituted by clusters of smaller sub-units, termed sub-primary particles (typically in the size range 3 to 8 nm). These last undergo, at the very early stage of soot formation, an agglomeration process that occurs contemporarily to the addition on the surface of fresh condensed matter. The net result is that graphitic nuclei are detected at relatively low residence times. Thus, sub-primary particles are included in compact clusters with the incoming fresh matter acting as a glue to tight them and form the primary particles. The present contribution is aimed to demonstrate that a simple Brownian coagulation algorithm is able to simulate and predict the clustering process of sub-primaries with the formation of rather compact entities (that later on are observed as primary particles which agglomerate to fractal soot). Both the coagulation constant and the fractal dimension are found out in simulations to critically depend on the size of the smaller carbon bricks which are considered in the agglomeration process at typical combustion aerosol conditions. In particular, simulations predict the tendency of smaller particles (<10 nm) to form a relatively numerous clusters with a low number of sub-units (3-4). Vice versa, the larger particles (>20 nm) aggregate more frequently in clusters with higher number of bricks (6-8). The net result is that the mean gyration radius is skewed towards lower and higher values in the case of smaller and larger monomers, respectively. This results seems to be coherent with the SAXS observation that sub-primaries with radius 3-4 nm undergo a fast agglomeration process to form larger units with the addition of condensable matter which constitute the primary particles of fractal soot aggregates.