Study on molecular thermal phenomena in extreme liquid lubrication

利用大尺度分子动力学模拟方法，对平行固体表面间几纳米厚液膜中的热与流体现象进行了分析。当两个固体壁面以不同的恒速运动时，液膜受到剪切作用，在液膜中产生类Couette流动，并发生从宏观流动到热能的能量转换，这在宏观意义上称为粘性加热。液体的温度上升，并且所产生的热能作为热通量被排放回固体壁。本研究的主题是阐明（1）高度非平衡态的发生，（2）固-液界面和液体中能量和动量传递的特征，以及（3）单独的固-固、固-液、液-液分子间传递的热和流体现象和机制的细节，分析了几种固体壁面与液体组分的组合体系，固体壁面表面具有不同的分子尺度结构，液体组分与固体壁面之间的相互作用是固体壁面与液体组分之间相互作用的结果。液膜由单原子分子、线性分子、水或链分子中的一种组成。分析了各自由度的分子运动对总能流的贡献，发现在固液界面附近的分子运动对总能流的贡献与体相分子运动的贡献的比例不同。研究结果不仅为纳米尺度下的热与流体现象提供了新的认识，而且对于构建以界面现象为主要特征的纳米润滑系统和设计纳米器件也具有重要意义。

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.