Collective excitations in soft-sphere fluids

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.

Collective Excitations in Supercritical Fluids

“…Interestingly, that for model fluids of soft spheres, which do not have liquid-gas coexistence, the density dependence of collective dynamics was found very similar as in the supercritical Ar [35]. The apparent speed of sound calculated for soft sphere fluids [35] gives evidence of a strong positive dispersion for the dense fluids and its reduction with the decrease of density reaching almost zero-PSD state at some density.…”

“…The coupling to heat waves causes the "residual PSD" far outside the hydrodynamic region. If the PSD was solely caused by the structural relaxation it would vanish as it was shown in [34,35] within the viscoelastic (no coupling with thermal processes) analysis of PSD. In Fig.…”

“…The apparent speed of sound calculated for soft sphere fluids [35] gives evidence of a strong positive dispersion for the dense fluids and its reduction with the decrease of density reaching almost zero-PSD state at some density. Further decrease of density for soft sphere fluids resulted again in non-zero PSD, that gives evidence of very similar scenario of behaviour of PSD as it was observed in the case of Lennard-Jones fluids [5,59].…”

“…Note, that the high-frequency speed of sound is always higher than the adiabatic one c s [34,35], which in its turn is ffiffi ffi c p times higher than c T .…”

“…Here cðkÞ is the wavenumber-dependent ratio of specific heats which easily can be expressed via correlators used in the GCM approach [20,52]. To date this is the most precise methodology of calculations of adiabatic speed of sound from classical [35,48] and ab initio [53][54][55] simulations. A comparison of the calculated adiabatic speed of sound for supercritical Ar with the NIST database showed almost perfect agreement in the whole density range, see [48].…”