Collective Excitations in Supercritical Fluids

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.