Collective Excitations in Supercritical Fluids
Authors: Bryk T., Gorelli F., Ruocco G., Santoro M., Scopignio T.
Autors Affiliation: Natl Acad Sci Ukraine, Inst Condensed Matter Phys, UA-79011 Lvov, Ukraine; CNR INO, Ist Nazl Ott, I-50019 Sesto Fiorentino, Italy; LENS, I-50019 Sesto Fiorentino, Italy; Univ Roma La Sapienza, Dipartimento Fis, I-00185 Rome, Italy; Ist Italiano Tecnol, Ctr Life Nano Sci Sapienza, I-00161 Rome, Italy
Abstract: Recent progress in theoretical and simulation studies of collective excitations in supercritical fluids is reviewed. We discuss a methodology of fit-free estimation of dispersion of longitudinal and transverse excitations in simple fluids. The issue of vanishing positive sound dispersion-a viscoelastic increase of the speed of sound from adiabatic one to its high-frequency (elastic) value-with reduction of density, as it was observed in inelastic X-ray scattering experiments on supercritical Ar, is discussed from the point of view of finding distinctions in collective dynamics of low-and high-density supercritical fluids. On the basis of several theoretical models within the extended hydrodynamic description of density-density correlations in liquids analytical expressions for positive sound dispersion are obtained and applied for analysis of time correlation functions obtained from molecular dynamics simulations. A location of a crossover from the liquid-like to gas-like types of collective dynamics is discussed based on general findings for spectra of collective excitations in supercritical Ar and soft sphere fluids.
KeyWords: WAVELENGTH SOUND MODES; BINARY-LIQUIDS; THERMAL-DIFFUSIVITY; DYNAMICS; ARGON; FLUCTUATION; DEPENDENCE; METALS; LINE