Observations of nascent soot: Molecular deposition and particle morphology

Abstract

A complete understanding of soot particle formation is critical for accurate combustion models. Limited details are known about the molecules and physical processes involved in the formation of nascent soot. The objective of this study was to discover molecular size distribution and particle morphology from freshly nucleated particles. To accomplish this, particles created in a homogeneous, high temperature (1600–2000 K) and pressure (10 atm) environment were collected at well-resolved times throughout the early stages of soot formation and were imaged with a high-resolution transmission electron microscope (HRTEM). Image post processing revealed that the molecules deposited on both nascent and aged soot particles had a constant size distribution, and that the molecules were likely to be polycyclic aromatic hydrocarbon (PAH) molecules. Additionally, the images revealed particle morphology. At low temperatures, PAH molecules coagulated to form fairly amorphous particle cores. At high temperatures the particle cores became comprised of agglomerated 2–8 nm diameter particles. Eventually all particle cores became surrounded by layers of molecules while the small particles no longer attached as growth species. The imaged transition from small particle agglomeration at the particle core to surface growth was replicated in a coagulation model. The model quantified the increase in small particle agglomeration that was attributed to an increase in the coagulating molecule’s rate of production.