  We propose a simple, self-consistent kinetic model for the evolution of a
mixture of droplets and vapor expanding adiabatically in vacuum after rapid,
almost isochoric heating. We study the evolution of the two-phase fluid at
intermediate times between the molecular and the hydrodynamic scales, focusing
on out-of-equilibrium and surface effects. We use the van der Waals equation of
state as a test bed to implement our model and study the phenomenology of the
upcoming second neutralized drift compression experiment (NDCX-II) at Lawrence
Berkeley National Laboratory (LBNL) that uses ion beams for target heating.We
find an approximate expression for the temperature difference between the
droplets and the expanding gas and we check it with numerical calculations. The
formula provides a useful criterion to distinguish the thermalized and
nonthermalized regimes of expansion. In the thermalized case, the liquid
fraction grows in a proportion that we estimate analytically, whereas, in case
of too rapid expansion, a strict limit for the evaporation of droplets is
derived. The range of experimental situations is discussed.
