LIGHT-SCATTERING STUDY OF FRACTAL CLUSTER AGGREGATION NEAR THE FREE MOLECULAR REGIME

This paper presents light scattering measurements of the aggregation r ate for an aerosol of fractal aggregates in a rarefied gas. A premixed ethylene/oxygen flame was used to create a hot, fractal aggregate aer osol of carbonaceous soot. Static light scattering involving absolute scattering, extinction, and optical structure factor measurements was used to measure the soot cluster morphological parameters, cluster rad ius of gyration, fractal dimension, monomers per aggregate and monomer size, all as a function of height above burner. Laser Doppler velocim etry was used to convert height to time. Above a height of 7 mm, it wa s established that aggregation was the dominant growth mechanism. The aggregation cluster growth kinetics are compared to the Smoluchowski e quation prediction using an aggregation kernel that accounts for the f ractal nature of the aggregates. The kinetics of this system ranged fr om the free molecular to the Epstein regime. Comparison of experiment to theory shows the theory to be from 15 to 100% too low which may be compared to an experimental uncertainty of a factor of two due largely to soot particle refractive index uncertainty. The discrepancy could be due to attractive dispersive forces between aerosol particles. Thes e discrepancies are small, however, compared to the range of the aggre gation kernel. Hence this work establishes the validity of the theoret ical form of the kernel for fractal aggregates in rarefied gases. (C) 1997 Elsevier Science Ltd.