Contribution of intermolecular energy transfer to heat conduction in a simple liquid

Abstract

Heat conduction in a low-temperature liquid is mainly caused by interaction between molecules. The intermolecular energy transfer (IET), which contributes to macroscopic heat conduction flux, is discussed in the present paper. Intermolecular energy transfer is examined using the results of molecular dynamics (MD) simulations for a simple liquid, which is modeled using the Lennard-Jones (LJ) (12-6) potential. The intermolecular energy exchange rate (IEER) is defined as the product of the intermolecular forces acting between two molecules and the velocities of the molecules. A probability distribution of the magnitude of the IEER was obtained in an ordinary equilibrium MD simulation; such a distribution was well correlated to the intermolecular distances. The substantial contribution of the IET to heat conduction flux, which is called the intermolecular energy transfer rate (IETR) here, is given by a time average of the IEER. A model is introduced to evaluate the contribution of the IET to heat conduction flux, based on an assumption that the IETR is proportional to the magnitude of the IEER itself. To verify this model, MD simulations of heat conduction in the LJ liquid under a constant temperature gradient were performed and the IETR was obtained directly. It was found that the IETR had a strong correlation with the IEER. The direct observation of the IETR by the simulation of heat conduction also showed that energy transfer in the direction opposite to the macroscopic heat flux appears in certain regions of intermolecular distances periodically. The negative energy transfer is observed between a center molecule and molecules in the inner half shell of the second and further neighbor shells The appearance of negative energy transfer reduces the net contribution of energy transfer between molecules separated by longer distances, and consequently heat conduction in a liquid is effected only by the IET between first neighbor molecules.