Condensation and evaporation of metallic aerosols

Abstract

The heat-pulse cloud chamber has been used to study the condensation of metallic aerosols in the presence of purified argon. Multiplication, growth and evaporation of Ca, Cd, Pb and Zn particles vary with the background temperature in the chamber. The volatile Cd and Zn resemble the alkali halides in that growth occurs readily in suspension when a sufficient vapour pressure is maintained, and particles are formed that settle out at appreciable speeds. These particles fall from the cloud independently.

With Pb, the vapour pressure in the vicinity of the melting point is much lower, and nucleation in the vapour at the high temperatures close to its point of generation is followed by the rapid arrest of growth and evaporation as the particles move away into the chamber. This results in the freezingin of large numbers of minute particles, and a smoke is formed in which there is little evidence of further change. When the temperature of the chamber is reduced to room temperature, the particles are exceedingly fine and numerous when first condensed but the smoke thins out, apparently by agglomeration. Observable motion in the smoke, apart from the Brownian motion, is dependent on convection in the supporting gas; the particles move by streaming and do not fall out.