Morphology of nascent soot in ethylene flames

Because of their significant impact on climate, environment and health, reducing the emission of soot from combustion processes remains a problem that requires detailed understanding of its formation as well as of the principles that govern how the result in terms of size, morphology, and chemical reactivity of soot particles depends upon the formation process. Especially very small, below ten nanometer-sized, particles in the early stages of the soot nucleation process are interesting targets for more detailed inspection, to reveal useful insight and to guide further model development. In this study, Helium-ion microscopy (HIM) is applied as an imaging technique new to combustion studies to analyze the morphology of soot particles >2 nm and to determine their geometrical characteristics. For this analysis, a series of premixed ethylene flames are investigated. Mobility size measurements from an earlier investigation have been compared with the particle sizes determined by HIM. Observed particle shapes and geometrical statistics suggest that in all flames under investigation, nascent soot possesses no well-defined morphologies. Additionally, investigations have been made using X-ray photoelectron spectroscopy (XPS) to obtain more information on the chemical characteristics of these particles.