Molecular-scale heat transfer in liquids and at liquid-solid interfaces: Toward the quality evaluation of heat flux

Molecular mechanism of heat conduction in liquids and at solid-liquid interfaces, which is getting important recently especially in the field of micro/nanofluidics and biodevices, is discussed based on the results of the authors' molecular dynamics (MD) simulations. The concept of intermolecular energy transfer (IET) is introduced as a view of molecular-scale mechanism of heat conduction in liquids focusing attention on the liquid structure that transfers thermal energy. Characteristics of various types of IET are examined. Heat conduction flux is a sum of IET and in that sense, heat conduction flux is a product assembled with various types of IET as "parts." In case of heat conduction in highly nonequilibrium thermal state, only some types of IET constitute particular contribution to macroscopic heat conduction flux. A case of a solid-liquid interface through which high heat flux exists is reported as an example. In this case, a type of IET passes through the interface with a low thermal boundary resistance and other types do with a high resistance. The overall thermal boundary resistance of a solid-liquid interface depends on the types of IET of which the heat flux consists. For example, high thermal resistance is resulted if the heat flux consists of "high-resistance type" IET. On the other hand, heat flux consisting of "low-resistance type" IET can pass through the interface with a small temperature drop. A sort of "quality" of heat flux, which is determined by IET as its "parts" can thus be defined.