Spontaneous Condensation of Water Vapour in Expansion Chamber Experiments

When clean, saturated air is subjected to a sufficiently large adiabatic expansion, small water droplets arise spontaneously from the vapour. An equation is derived for the growth rate of a droplet in an atmosphere of given supersaturation and temperature. This is used in conjunction with the Becker-Döring theory of nucleation to calculate the supersaturation and the concentration of droplets at successive time intervals during a cloud-chamber expansion, allowance being made for warming of the air by liberation of the latent heat of condensation. For large expansion ratios, the supersaturation attains a maximum and the droplet concentration a sensibly constant value before the end of the expansion. The computed value of this maximum droplet concentration is in acceptable agreement with the recent observations of Frey. The experimental results appear incompatible with Tolman's recent theory concerning the variation of surface tension with droplet radius. The predicted rates of droplet growth agree well with recent measurements made by Hazen.