Abstract

The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially important for applications and their determination by computational simulations is very desirable. In this study, we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations, which are computationally demanding but also have the potential to be most accurate. We used the 9,10-dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and nonequilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that, by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully; while at 40 degrees Celsius, EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields.

摘要

润滑剂在操作条件下（例如在高压下）的行为是一个具有重大工业意义的主题。特别是，粘度和粘度-压力关系对于应用尤其重要，并且非常需要通过计算模拟来确定它们。在这项研究中，我们评估了基于完全原子分子动力学模拟来计算这些量的方法，这些方法在计算上要求很高，但也有可能是最准确的。我们使用 9,10-二甲基十八烷分子（PAO-2 基础油的主要成分）作为我们测试的润滑剂。用于粘度模拟的方法是 Green-Kubo 平衡分子动力学 (EMD-GK) 和非平衡分子动力学 (NEMD)，在高达 1.0 GPa 的压力和各种温度（40-150 摄氏度）下。我们介绍了这些方法背后的理论，并研究了模拟参数如何影响获得的结果，以确保粘度与模拟间隔和所有其他参数的收敛。我们表明，通过在其适用范围内使用每种方法，我们可以在模拟值和测量值之间取得良好的一致性。高压下的 NEMD 模拟成功捕获了零剪切粘度；而在 40 摄氏度时，EMD-GK 仅适用于高达 0.3 GPa 的压力，其中粘度较低。在 NEMD 中，更长和多次重复的模拟提高了粘度的置信区间，这在较低压力下是必不可少的。这些方法的另一方面是选择用于原子相互作用的力场。这是通过选择两个不同的常用力场来研究的。