Homogeneous Nucleation

Abstract

Our outline in the previous three chapters of the equilibrium thermodynamics of the phases of water is insufficient for an understanding of cloud particle formation, since we did not come to grips with the crucial question of how a new phase is initiated. Consider, for example, that on the basis of the Kelvin equation alone, the formation of a water drop from homogeneous water vapor would be precluded because the vapor pressure required to hold a microscopic quantity of newly formed phase in equilibrium would be quite enormous. This expectation is in disagreement with experimental observations which show that a large but finite supersaturation exists above which homogeneous phase change does take place. The reason for this behavior is that the formation of a new phase at the expense of a metastable original phase (‘mother phase’) does not begin in a continuous manner, but rather takes place spontaneously as a result of temperature and density fluctuations (‘heterophase fluctuations’) in the original phase, provided that a critical supersaturation is exceeded. This spontaneous process is called nucleation.