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Reactive molecular dynamics (MD) simulations are employed to investigate the coalescence of incipient soot clusters. Initially, one thousand acetylene molecules collide and react with each other, allowing bond breakage and new bond formation upon collision, leading to various species (e.g., linear hydrocarbons, branched polyaromatic hydrocarbons) up to the formation of nascent soot clusters with diameter of up to 3.5 nm. The structure and composition of the formed soot clusters are quantified by the packing density and carbon-to-hydrogen (C/H) ratio, respectively, during nucleation and up to the formation of large nascent soot nanoparticles. Then, the nucleated incipient soot clusters are isolated from the surrounding reactive species and are allowed to coalesce with each other isothermally to investigate soot coalescence. The coalescence between incipient soot clusters of different sizes is elucidated at various process temperatures, ranging from 800 to 1800 K. The characteristic coalescence time of nascent soot is quantified by tracking the evolution of the particle surface area, for the first time. Soot clusters consisting of up to 760 atoms coalesce instantly (within 0.1 ns), especially at relatively low temperatures (i.e., 800–1000 K). At higher temperatures (1200–1600 K), incipient soot clusters are less prone to coalescence due to the larger fraction of constituent aromatic rings leading to more rigid particles. Large clusters consisting of more than 1300 atoms do not coalesce within the time scales investigated here (i.e., up to 5 ns). The employed reactive MD approach gives significant insight into fundamental soot formation and growth mechanisms, which are typically treated semi-empirically, facilitating a better understanding and more efficient control of soot in combustion processes.


Accepted version. Carbon, Vol. 180 (August 15, 2021): 215-225. DOI. © 2021 Elsevier. Used with permission.

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