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

A flow reaction chamber was employed to study the interaction of soot aeros ol particles with water droplets. To investigate the influence of the hygro scopicity of soot aerosol particles on these processes, comparative measure ments between pure soot aerosol and soot particles pretreated with n-butano l vapor were carried out. The amount of the interaction between soot partic les and water droplets was evaluated from the size distribution changes det ermined with an aerodynamic particle sizer before and after the interaction . The size distribution measurements in the diameter range between 0.5 and 15 mum for both fractions, particles and droplets, displayed significant ch anges of their disperse characteristics. The obtained results reveal two di fferent mechanisms for a redistribution of the initial size distribution: t he first is the scavenging of the soot particles by being captured by the d roplets, the second is caused by the surface coagulation of the soot partic les 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. Electronmicroscopic analysis showed that the size di stribution of initially pure soot aerosol did not shift significantly after the interaction with water droplets, while the size distribution of n-buta nol-processed soot aerosol shifted to smaller sizes: the mean size was redu ced 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 s ubsequent 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. (C) 2001 Elsevier Science Ltd. All rights reserved.