Scavenging of Aerosol Particles by a Falling Water Droplet

Abstract

This is a study of the collection efficiency of water droplets (0.71–2.54 mm radius) freely falling through a silver chloride aerosol consisting of a relatively uniform distribution of spheres with radii in the range of several tenths of a micron. The droplets accelerated for a distance of 30 cm prior to entering the 130-cm aerosol chamber. The particle size distribution of the aerosol was determined by light scattering. The amount of AgCl scavenged by the droplet was analyzed by a very sensitive colorimetric technique. Measured collection efficiencies E were extremely low, dropping off sharply from a value of 3.0 × 10−4 for a 2.54-inm droplet to a minimum of 7.0 × 10−6 for a 0.94-mm droplet; they appear to rise again for smaller droplets. There was no evidence of any significant trends with particle radius which ranged from 0.2–0.5 µm. For the three smallest droplets the data correlate well with the Péclet number, E = 1.68 Pé−⅔, suggesting that the mechanism of collection is Brownian diffusion through the streamlines. For larger droplets there was no correlation with either the Péclet or the Stokes numbers. In view of the large Reynolds numbers for these cases, the mechanism may involve eddies in the wake of the droplet with enhanced collection on the backside.