Size distribution and morphology of nascent soot in premixed ethylene flames with and without benzene doping

Particle size distribution functions of nascent soot formed in four burner-stabilized, premixed ethylene-oxygen-argon flames were studied in a spatially resolved manner by online sampling/scanning mobility particle sizer. Particle morphology was analyzed by atomic force microscopy (AFM) of substrate-deposited samples. Two of the four flames were doped with benzene. An aerosol electrometer is introduced to extend the lower detection limit to around 1.5 nm in diameter. The results show that the bimodal behavior of particle size is applicable to all premixed ethylene flames studied. The variation of the size distribution from flame to flame is conclusively attributed to flame temperature variation. Under the condition of an equal carbon concentration, benzene doping leads to negligible changes in the characteristics of the size distribution. For all flames studied, AFM observations show that nascent soot is liquid-like and spreads extensively upon impact on a substrate surface.