Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India.
Phys Rev E. 2018 Jan;97(1-1):012131. doi: 10.1103/PhysRevE.97.012131.
The existence of the Frenkel line in the supercritical regime of a Lennard-Jones (LJ) fluid shown through molecular dynamics (MD) simulations initially and later corroborated by experiments on argon opens up possibilities of understanding the structure and dynamics of supercritical fluids in general and of the Frenkel line in particular. The location of the Frenkel line, which demarcates two distinct physical states, liquidlike and gaslike within the supercritical regime, has been established through MD simulations of the velocity autocorrelation (VACF) and radial distribution function (RDF). We, in this article, explore the changes in the structural features of supercritical LJ fluid under partial confinement using atomistic walls. The study is carried out across the Frenkel line through a series of MD simulations considering a set of thermodynamics states in the supercritical regime (P=5000 bar, 240K≤T≤1500K) of argon well above the critical point. Confinement is partial, with atomistic walls located normal to z and extending to "infinity" along the x and y directions. In the "liquidlike" regime of the supercritical phase, particles are found to be distributed in distinct layers along the z axis with layer spacing less than one atomic diameter and the lateral RDF showing amorphous-like structure for specific spacings (packing frustration) and non-amorphous-like structure for other spacings. Increasing the rigidity of the atomistic walls is found to lead to stronger layering and increased structural order. For confinement with reflective walls, layers are found to form with one atomic diameter spacing and the lateral RDF showing close-packed structure for the smaller confinements. Translational order parameter and excess entropy assessment confirms the ordering taking place for atomistic wall and reflective wall confinements. In the "gaslike" regime of the supercritical phase, particle distribution along the spacing and the lateral RDF exhibit features not significantly different from that due to normal gas regime. The heterogeneity across the Frenkel line, found to be present both in bulk and confined systems, might cause the breakdown of the universal scaling between structure and dynamics of fluids necessitating the determination of a unique relationship between them.
分子动力学（MD）模拟最初显示，在莱纳德-琼斯（LJ）流体的超临界状态下存在弗伦克尔线，后来实验也证实了这一点，这为理解超临界流体的结构和动力学提供了可能性，特别是弗伦克尔线。通过 MD 模拟速度自相关（VACF）和径向分布函数（RDF），确定了超临界状态下区分液态和气态两种截然不同物理状态的弗伦克尔线的位置。在本文中，我们使用原子壁探索超临界 LJ 流体在部分约束下的结构特征变化。该研究通过一系列 MD 模拟在超临界区（P=5000 巴，240K≤T≤1500K）跨越弗伦克尔线进行，考虑了一组热力学状态，氩气的超临界区远高于临界点。约束是部分的，原子壁垂直于 z 方向，沿 x 和 y 方向延伸至“无穷大”。在超临界相的“液态”区域，发现粒子沿 z 轴分布在不同的层中，层间距小于一个原子直径，并且对于特定间距（包装挫折），侧向 RDF 显示出无定形结构，对于其他间距，侧向 RDF 显示出非无定形结构。增加原子壁的刚性会导致更强的分层和增加的结构有序性。对于具有反射壁的约束，发现层形成具有一个原子直径的间距，并且对于较小的约束，侧向 RDF 显示紧密堆积的结构。平移有序参数和过剩熵评估证实了原子壁和反射壁约束下的有序化。在超临界相的“气态”区域，粒子沿间距的分布和侧向 RDF 表现出与正常气体区域没有显著不同的特征。在体相和约束系统中都发现的弗伦克尔线的异质性可能导致流体结构和动力学之间的通用缩放关系的破裂，需要确定它们之间的独特关系。
