2. Nonequilibrium Effects on the Phase Interface

The description of intense phase changes calls for the solution to the flow problem in the ambient spaces of evaporating (condensing) matter, as described by gas dynamic equations. The specific feature of an intense phase change lies in the formation near the condensed-phase surface (CPS) of the Knudsen layer of thickness of order of the mean free path of molecules. The existence of the Knudsen layer depends on the nonequilibrium character of evaporation (condensation) resulting in the anisotropy of the velocity distribution function (DF) near CPS. In this setting, the gas dynamic description becomes unjustified—the phenomenological gas parameters (temperature, pressure, density, velocity), as defined according to the conventional rules of statistical averaging, lose their macroscopic sense. Such a situation can be described at a simplified level with the help of an “imaginary experiment”. Assume that the Knudsen layer has hypothetical micrometers of pressure and temperature. Then their readings will not agree with the statistically averaged values, but will rather depend on the structure of a micrometer. Such anomalies disappear beyond the Knudsen layer, in the outer region, where the Navier–Stokes equations hold. The outer region is also called the “Navier–Stokes region”.