Soot Restructuring and Compaction: Mechanisms and Resulting Effective Density

JOEL CORBIN, Timothy Sipkens, Robin Modini, Martin Gysel, National Research Council Canada

     Abstract Number: 609
     Working Group: Combustion

Abstract
Recent work has identified the specific mechanisms by which soot restructuring causes compaction (Corbin et al., 2023). That work alternated between solid and liquid coatings to demonstrate that compaction was caused by both condensation and evaporation. Based on a review of dozens of other studies, in both the aerosol and surface-science fields, that work also identified specific mechanisms of compaction: capillary compaction (at junctures between soot spherules), nanodroplet activation (following heterogeneous nucleation) and droplet-evaporation compaction (occurring during evaporation, rather than condensation). The work of Corbin et al. (2023) discussed the implications of these mechanisms for climate and health in detail but did not evaluate the quantitative impacts of these mechanisms on the physical properties of soot.

Here, we quantitatively evaluate data from published studies of soot compaction. We focus on mobility data, with the goal of determining the range of effective densities that may be observed for soot. We also focus on cases of “complete” compaction, which we define as data from studies that show that the addition of more coating material did not cause more compaction. This constraint allows an apples-to-apples comparison to be made between studies. We find that the variability in the available data sets of complete compaction is largely explained by variation in primary particle diameter (and therefore number). Additional variability is due to two apparently unique compaction mechanisms. Importantly, the data show that commonly cited sources of uncertainty, such as surface tension and viscosity, are minor. We summarize these conclusions in terms of the resulting uncertainty in the effective density of compacted soot.