The direct measurement of the nucleation rate constants

Abstract

In previous work the method of Laplace was applied to the evaluation of the nucleation integral with a rate given by the expression J = A exp (- B/ T 3 ln 2 S) to yield the duration of the nucleation pulse as a function of the time dependent temperature T and supersaturation S. We now demonstrate the measurement of the nucleation rate and the constants A and B that apply to the nucleation of subcooled water droplets formed in the presence of argon. These measurements are possible as a result of the development of a novel use of hot wire anemometry to yield the temperature history of the supersaturated vapor during the expansion process. The rate constants so obtained are based solely on experimental observations and, in particular, their derivation does not require a knowledge of droplet growth rate. The high supersaturations achieved, the high nucleation rates and particle concentrations suggest that homogeneous nucleation is being observed. The results clearly depict high sensitivity of the final particle concentration, the nucleation pulse duration, and the nucleation rate upon the rate of expansion as well as to the initial thermodynamic state of the vapor—inert gas mixture.