Interaction of soot aerosol particles with water droplets: Influence of surface hydrophilicity

The interaction between soot particles and water droplets in a flow reaction chamber was studied. A comparative analysis was performed for systems of two types differing in the degree of particle hydrophilicity: pure carbon black was used in one case; and soot particles treated by butanol vapor, in the other. The interaction effect was estimated by comparing the APS size spectra of the droplet and soot components before and after their interaction. Measurements in the size range 0.5-15 mum revealed that interactions between soot particles and water droplets change the disperse characteristics of both fractions. The results showed that two basic mechanisms are responsible for transformations of the initial distributions in a mixed disperse system. The first mechanism is soot-particle capture by water droplets, and the second is surface coagulation. The efficiency of each mechanism was found to depend strongly on the hygroscopic properties of soot particles. Specifically, an analysis of electron microscopy data showed that, on the whole, the particle size spectrum for pure carbon black changes slightly after soot-droplet interactions, while a considerable shift to smaller sizes is observed for butanol-treated soot particles: the mean size of the aggregates decreases by a factor of 3. This change in particle sizes is explained by the fact that hydrophilic particles penetrate into water droplets when coalescing with the latter, and, as the droplet sizes reduce due to evaporation, capillary forces cause overall compression and consolidation of initially sparse aggregates. Structure analysis data showed that the fractal dimension of hydrophilic aggregates increases from 1.72 to 1.87 due to consolidation.