Collective excitations in soft-sphere fluids.
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.
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.
Taras
T
Bryk
Istituto Nazionale di Ottica INO-CNR, I-50019 Sesto Fiorentino, Italy and European Laboratory for Non Linear Spectroscopy, LENS, I-50019 Sesto Fiorentino, Italy.
Federico
F
Gorelli
Dipartimento di Fisica, Universita di Roma La Sapienza, I-00185, Roma, Italy and Center for Life Nano Science @Sapienza, Istituto Italiano di Tecnologia, 295 Viale Regina Elena, I-00161, Roma, Italy.
Giancarlo
G
Ruocco
Istituto Nazionale di Ottica INO-CNR, I-50019 Sesto Fiorentino, Italy and European Laboratory for Non Linear Spectroscopy, LENS, I-50019 Sesto Fiorentino, Italy.
Mario
M
Santoro
Dipartimento di Fisica, Universita di Roma La Sapienza, I-00185, Roma, Italy and Center for Life Nano Science @Sapienza, Istituto Italiano di Tecnologia, 295 Viale Regina Elena, I-00161, Roma, Italy.
Tullio
T
Scopigno
D004058:Diffusion; D057446:Hydrodynamics; D056004:Molecular Dynamics Simulation; D013816:Thermodynamics; D014783:Viscosity