Constructing a measurement system for the nonequilibrium velocity distribution of evaporating water molecules from a liquid-vapor interface*

Multiphase fluid analyses involving phase change require an appropriate boundary condition at the liquid-vapor interface that gives evaporation or condensation mass flux, the interface temperature, etc. The Hertz-Knudsen-Schrage equation is widely adopted for describing the net mass flux. However, the validity of this equation is often questioned because its derivation assumes the Maxwell-Boltzmann velocity distribution for evaporating molecules, which seems inconsistent with the nonequilibrium nature of evaporation/condensation. Molecular dynamics simulations indicated that the velocity distribution of evaporating molecules deviates from the Maxwell-Boltzmann distribution under highly nonequilibrium conditions, whereas such nonequilibrium velocity distributions have not yet been verified by experiments. Here we show an experimental setup for measuring the velocity distribution of evaporating water molecules from a liquid-vapor interface, which is kept in a vacuum using a nanoporous membrane. We discuss the initial result of the nonequilibrium velocity distribution of evaporating water molecules obtained by the time-of-flight method.