The collisional growth of soot particles at high temperatures

Abstract

Recently, we have shown that the light-scattering behaviour of soot aerosols in incident shock flows (duration ≈ 3 ms) at ≈1800 K is consistent with the predictions of free-molecule coagulation theory. However, agreement between theory and experiment is obtained only if it is assumed that the collisions between soot particles are coalescent in that the collision partner fuse completely after each collision to form a new spherical particle. The aim of the present study is to determine as directly as possible, whether the collisions that occur in the first 1–2 milliseconds following nucleation are indeed coalescent or chain forming. This was achieved from observations on soot aerosols generated in cycloheptatriene/argon shock-flow at ≈1750 K using a laser light-scattering technique in which the polarization of the incident beam was modulated at ≈20 kHz. The ratios of the absorbances of the aerosol at 488 nm and 3.39 μm were also measured to assess the relative proportions of true soot (I.R. absorbing) and polynuclear aromatics (I.R. transparent) in the condensed phase. The polarization modulation permitted the variation in the ratio of the differential light-scattering cross-sections of the aerosols, for light-scattered perpendicular and parallel respectively to the polarization direction of the linearly polarized incident beam, to be measured during the same shock flow. This ratio was observed to increase throughout the flow, showing that the scattering properties of the particles become more and more like those of isotropic spheres as time proceeds. This behaviour is the opposite of that predicted for chain-forming collisions. The small but significant scattering that is observed along the polarization direction of the incident beam is attributed to an intrinsic, rather than a shape anisotropy, that reflects the highly anisotropic nature of the polynuclear aromatics that are present in the particles.