Soot Structure

Electron micrographs of soot have been obtained in diffusion flames and in the engine combustion chamber. Soot is seen to be aggregates of primary spherical particles. The primary soot particles are mostly in the 15 to 30 nm size range.  Typical electron micrograph of soot obtained from acetylene-air diffusion flame is shown in Fig. 2.22.   Primary soot particles are seen as spheroids on the micrograph. Primary particles of about 30 nm size and branched chainlike soot aggregates are clearly observed in this micrograph at 80000 magnifications.
A single spherical soot particle contains 10⁵ to 10⁶ carbon atoms. Initially, the combustion-generated soot particles have one hydrogen atom to about 8 carbon atoms, (C₈H)_n with a density of approx. 1800 kg/m³.. The primary soot particles form aggregates in the combustion chamber of 100-200 nm in size containing  generally 20 to over 100 primary particles. These aggregates may further agglomerate to particles as large as 1 μ.  The aggregates usually resemble a cluster of spheres in branched or chain like structure.