Ewen J P, Gattinoni C, Zhang J, Heyes D M, Spikes H A, Dini D
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, England, UK.
Phys Chem Chem Phys. 2017 Jul 21;19(27):17883-17894. doi: 10.1039/c7cp01895a. Epub 2017 Jun 29.
A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5-2.0 GPa) and strain rates (10-10 s) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it.
深入了解受限流体的行为对于一系列工业应用至关重要，例如控制工程部件中的摩擦。在本研究中，结合摩擦学实验和受限非平衡分子动力学模拟来研究基础流体分子结构对非平衡相行为和摩擦的影响。一项广泛的参数研究，包括对几种润滑剂和牵引流体分子施加弹性流体动力润滑 regime 典型的压力（0.5 - 2.0 GPa）和应变率（10 - 10 s），揭示了摩擦与流动行为之间的明确关系。润滑剂是柔性的、大致呈线性的分子，在实验和模拟中都给出低摩擦系数，且摩擦系数随应变率和压力增加。相反，基于刚性脂环族基团的牵引流体给出高摩擦系数，且仅微弱地依赖于应变率和压力。模拟中观察到的牵引流体更强的剪切局部化现象可以合理解释所观察到的摩擦行为差异。更高的压力导致更明显的剪切局部化，而增加的应变率导致剪切区域变宽。本研究中使用的方法阐明了重要受限流体行为的物理机制，并在改进弹性流体动力摩擦预测和开发新分子以控制摩擦方面都显示出巨大潜力。
