Evaporation into half-space: Experiments with water at the molecular mean free path scale

The temperature at the liquid–vapor phase change interface is measured at the molecular mean free path scale. The water layer is locally heated from below and evaporates in an environment of pure water vapor at reduced pressure. The study establishes the equilibrium conditions under which gas and liquid phases are maintained at a constant temperature without a temperature jump at the interface. It also examined how applying heat to the liquid causes the interface to enter a non-equilibrium state. The first experimental validation of the long-standing inverted temperature gradient paradox is also provided. Nonlinear kinetic theory is shown to be in quantitative agreement with the experimental results.