  The interfacial thermal resistance determines condensation-evaporation
processes and thermal transport across material-fluid interfaces. Despite its
importance in transport processes, the interfacial structure responsible for
the thermal resistance is still unknown. By combining non-equilibrium molecular
dynamics simulations and interfacial analyses that remove the interfacial
thermal fluctuations we show that the thermal resistance of liquid-vapor
interfaces is connected to a low density fluid layer that is adsorbed at the
liquid surface. This thermal resistance layer (TRL) defines the boundary where
the thermal transport mechanism changes from that of gases (ballistic) to that
characteristic of dense liquids, dominated by frequent particle collisions
involving very short mean free paths. We show that the thermal conductance is
proportional to the number of atoms adsorbed in the TRL, and hence we explain
the structural origin of the thermal resistance in liquid-vapor interfaces.
