Study on molecular thermal phenomena in extreme liquid lubrication

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Thermal and fluid phenomena in a few-nanometer-thick liquid film confined between parallel solid surfaces have been analyzed by large-scale molecular dynamics simulations. As shear was given to the liquid film by moving the two solid walls in different constant speeds, a Couette-like flow is induced in the liquid film and the energy conversion from the macroscopic flow to the thermal energy occurs, which is called as viscous heating in a macroscopic sense. The temperature of the liquid rises up, and the generated thermal energy is discharged back to the solid walls as heat flux. The subject of this study is to clarify the detail of thermal and fluid phenomena and mechanisms of (1) the occurrence of highly non-equilibrium state, (2) characteristics of energy and momentum transfer at solid-liquid interface as well as in liquid, and (3) individual solid-solid, solid-liquid, liquid-liquid intermolecular transfer, which is the governing factor of (2).The analysis was done for various systems that were the combinations of several kinds of solid walls and liquid species ; solid walls had different molecular-scale structures on the surface ; liquid film was consisted of one of monatomic molecules, linear molecules, water, or chain molecules. Contributions of the molecular motions in each degree of freedom to the total energy flux were analyzed and it was revealed that these contributions in the region close to the solid-liquid interface are different in ratio from those in bulk. Also, it was revealed that the interfacial resistance against energy and momentum transfer depends on the molecular-scale configuration of the solid surface that contacts the liquid.The findings of the study did not only bring a new knowledge in thermal and fluid phenomena in nanoscale, but are also applicable for constructing a desired nanoscale lubrication system, and designing nano-devices where interfacial phenomena govern the major characteristics.