Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ. United Kingdom.
J Chem Phys. 2012 Apr 7;136(13):134705. doi: 10.1063/1.3698601.
Non-equilibrium molecular dynamics simulations of boundary-driven sheared Lennard-Jones liquids at variable pressure up to 5 GPa (for argon) reveal a rich out-of-equilibrium phase behavior with a strong degree of shear localization. At the lowest apparent shear rate considered (wall speed ~1 m s(-1)) the confined region is an homogeneously sheared solid (S) with no slip at the walls. This transforms at higher shear rates to a non-flowing plug with slip at the walls, referred to as the plug slip (PS) state. At higher shear rate a central localized (CL) state formed in which the shear gradient was localized in the center of the film, with the rest of the confined sample in a crystalline state commensurate with the wall lattice. The central zone liquidlike region increased in width with shear rate. A continuous rounded temperature profile across the whole system reflects strong dynamical coupling between the wall and confined region. The temperature rise in the confined film is consistent with the Brinkman number. The transition from the PS to CL states typically occurred at a wall speed near where the shear stress approached a critical value of ~3% of the shear modulus, and also near the peak in the traction coefficient, μ. The peak traction coefficient values computed, ~0.12-0.14 at 1000 MPa agree with those found for traction fluids and occur when the confined liquid is in the PS and CL states. At low wall speeds slip can occur at one wall and stick at the other. Poorly wetting liquids manifest long-lived asymmetries in the confined liquid properties across the system, and a shift in solid-liquid phase co-existence to higher shear rates. A non-equilibrium phase diagram based on these results is proposed. The good agreement of the tribological response of the Lennard-Jones fluid with that of more complicated molecular systems suggests that a corresponding states scaling of the tribological behavior could apply.
非平衡分子动力学模拟在可变压力下边界驱动剪切莱纳德 - 琼斯液体高达 5 GPa（对于氩）揭示了丰富的非平衡相行为与强烈的剪切局部化程度。在考虑的最低表观剪切速率（壁速度1 m s(-1))下，受限区域是均匀剪切的固体（S），在壁处没有滑移。在更高的剪切速率下，它会转变为具有壁滑移的非流动塞，称为塞滑移（PS）状态。在更高的剪切速率下，形成了一个中央局部化（CL）状态，其中剪切梯度集中在薄膜的中心，受限样品的其余部分处于与壁晶格相匹配的结晶状态。中央区域液态区域随剪切速率而增加。整个系统的连续圆形温度分布反映了壁和受限区域之间的强烈动力学耦合。受限薄膜中的温度升高与 Brinkman 数一致。从 PS 状态到 CL 状态的转变通常发生在壁速度接近剪切应力接近剪切模量的临界值3%的位置，并且也接近牵引力系数μ的峰值。计算出的峰值牵引力系数值，~0.12-0.14 在 1000 MPa 时与牵引液相符，并且在 PS 和 CL 状态下发生时，受限液体处于 PS 和 CL 状态。在低壁速度下，滑移可以发生在一个壁上，而在另一个壁上则保持附着。润湿性差的液体在整个系统中表现出受限液体性质的长时间不对称性，以及固态 - 液态共存向更高剪切速率的转变。基于这些结果提出了非平衡相图。莱纳德 - 琼斯流体的摩擦学响应与更复杂的分子系统的响应非常吻合，这表明摩擦学行为可以采用相应状态标度。
