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

A flow reaction chamber was employed to study the interaction of soot aerosol particles with water droplets. To investigate the influence of the hygroscopicity of soot aerosol particles on these processes, comparative measurements between pure soot aerosol and soot particles pretreated with n-butanol vapor were carried out. The amount of the interaction between soot particles and water droplets was evaluated from the size distribution changes determined with an aerodynamic particle sizer before and after the interaction. The size distribution measurements in the diameter range between 0.5 and 15 μm for both fractions, particles and droplets, displayed significant changes of their disperse characteristics. The obtained results reveal two different mechanisms for a redistribution of the initial size distribution: the first is the scavenging of the soot particles by being captured by the droplets, the second is caused by the surface coagulation of the soot particles leading to new size modes emerging in the final size distribution. The efficiency of each mechanism appears to depend strongly on the wettability of the soot particles. Electron-microscopic analysis showed that the size distribution of initially pure soot aerosol did not shift significantly after the interaction with water droplets, while the size distribution of n-butanol-processed soot aerosol shifted to smaller sizes: the mean size was reduced by factor of three. This size redistribution could be explained by the assumption that wettable particles penetrate inside a water droplet during the coagulation, and, by being uniformly squeezed by capillary forces and subsequent water evaporation, become more compact and consequently smaller. The structure analysis data have shown, that the fractal dimension of soot aggregates increased from 1.72 to 1.87 as a result of such densification of the microstructure.