Nucleation and growth in freely expanding gases

A model is developed for the description of clustering processes in first-order phase transformations in freely expanding gases. The theory results in a set of first-order differential equations describing the evolution of the cluster size distribution, the time dependence of the thermodynamic parameters of the system like particle density, pressure, temperature as well as of the rate of expansion of the gas. The model considerations do not require as a basic preposition the assumption of an isentropic expansion of the gas. Therefore, based on the theoretical approach developed, the effect of clustering on the entropy and other thermodynamic state functions of the system can be studied in detail. Solutions of the set of equations are given and possible applications (expansion of atomic, molecular and nucleonic gases) are discussed.