2007-05-10
10:15 at HCI J 574

Heat and mass transfer into and through interfaces

Dick Bedeaux

Institute of Physical Chemistry, Norwegian University of Science and Technology, Trondheim, Norway

Experiments done in the last ten years have found temperature differences across the liquid-vapour interface of up to 8 degrees Celsius during evaporation. These temperature differences are much larger than expected on the basis of kinetic theory. The challenge is to describe these results in a clear macroscopic context, on the one hand, and to understand them on a microscopic level, on the other hand. We will show that non-equilibrium thermodynamics provides the needed macroscopic description. Questions like: how do we describe the surface and is the surface in local equilibrium, need to be answered. The description chosen uses, as Gibbs did for equilibrium interfaces, excess densities. This implies that the surface is a separate thermodynamic system. When the system is not in equilibrium the surface will even have a temperature different from the temperatures of the adjacent phases. Away from equilibrium we extend Gibbs’ analysis by the introduction of excess fluxes along the interface. Using balance equations and the Gibbs relation we obtain the excess entropy production and are thereby able to give force-flux relations and boundary conditions for the surface. In order to elucidate all these questions we simulated such transports using molecular dynamics. On the basis of these simulations we were then able to verify that the description using non-equilibrium thermodynamics is correct and that the surface is in local equilibrium during evaporation and condensation. Results from molecular dynamics simulations of heat and mass transport through the surface during condensation will be presented and compared with the results from experiments and kinetic theory.