Collective excitations in soft-sphere fluids

Year: 2014

Authors: Bryk T., Gorelli F., Ruocco G., Santoro M., Scopigno T.

Autors Affiliation: Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, 1 Svientsitskii Street, Lviv, Ukraine; Institute of Applied Mathematics and Fundamental Sciences, Lviv Polytechnic National University, Lviv, Ukraine; Istituto Nazionale di Ottica INO-CNR, Sesto Fiorentino, Italy; European Laboratory for Non Linear Spectroscopy, LENS, Sesto Fiorentino, Italy e Dipartimento di Fisica, Università di Roma la Sapienza, Rome, Italy; Center for Life Nano Science Sapienza, Istituto Italiano di Tecnologia, 295 Viale Regina Elena, Rome, Italy

Abstract: 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.


Volume: 90 (4)      Pages from: 042301  to: 042301

KeyWords: collective excitations
DOI: 10.1103/PhysRevE.90.042301

Citations: 21
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