Bryk Taras, Gorelli Federico, Ruocco Giancarlo, Santoro Mario, Scopigno Tullio
Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Street, UA-79011 Lviv, Ukraine and Institute of Applied Mathematics and Fundamental Sciences, Lviv Polytechnic National University, 79013 Lviv, Ukraine.
Istituto Nazionale di Ottica INO-CNR, I-50019 Sesto Fiorentino, Italy and European Laboratory for Non Linear Spectroscopy, LENS, I-50019 Sesto Fiorentino, Italy.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Oct;90(4):042301. doi: 10.1103/PhysRevE.90.042301. Epub 2014 Oct 1.
Despite that the thermodynamic distinction between a liquid and the corresponding gas ceases to exist at the critical point, it has been recently shown that reminiscence of gaslike and liquidlike behavior can be identified in the supercritical fluid region, encoded in the behavior of hypersonic waves dispersion. By using a combination of molecular dynamics simulations and calculations within the approach of generalized collective modes, we provide an accurate determination of the dispersion of longitudinal and transverse collective excitations in soft-sphere fluids. Specifically, we address the decreasing rigidity upon density reduction along an isothermal line, showing that the positive sound dispersion, an excess of sound velocity over the hydrodynamic limit typical for dense liquids, displays a nonmonotonic density dependence strictly correlated to that of thermal diffusivity and kinematic viscosity. This allows rationalizing recent observation parting the supercritical state based on the Widom line, i.e., the extension of the coexistence line. Remarkably, we show here that the extremals of transport properties such as thermal diffusivity and kinematic viscosity provide a robust definition for the boundary between liquidlike and gaslike regions, even in those systems without a liquid-gas binodal line. Finally, we discuss these findings in comparison with recent results for Lennard-Jones model fluid and with the notion of the "rigid-nonrigid" fluid separation lines.
尽管在临界点处液体与相应气体之间的热力学差异不复存在，但最近研究表明，在超临界流体区域中，可以通过高超声速波色散行为识别出类似气体和类似液体行为的痕迹。通过结合分子动力学模拟和广义集体模式方法中的计算，我们精确测定了软球流体中纵向和横向集体激发的色散。具体而言，我们研究了沿等温线密度降低时刚性的下降情况，表明正声色散（即声速超过致密液体典型的流体动力学极限）呈现出与热扩散率和运动粘度严格相关的非单调密度依赖性。这使得基于维登线（即共存线的延伸）对超临界状态的近期观测结果合理化。值得注意的是，我们在此表明，即使在没有液 - 气双节线的系统中，热扩散率和运动粘度等输运性质的极值也为类似液体和类似气体区域之间的边界提供了一个可靠的定义。最后，我们将这些发现与 Lennard - Jones 模型流体的近期结果以及“刚性 - 非刚性”流体分离线的概念进行了比较讨论。
