Onset of depature from linearized hydrodynamic behavior in argon gas studied with neutron Brillouin scattering

Abstract

We have investigated the dynamic structure factor of room-temperature $^{36}\mathrm{Ar}$ at two densities (0.25 and 0.63 times the critical) and at very low momentum transfer (0.31.3 ${\mathrm{nm}}^{\mathrm{\ensuremath{-}}1}$). Linearized hydrodynamics agrees with the experimental data only at the higher density and \ensuremath{\kappa}1 ${\mathrm{nm}}^{\mathrm{\ensuremath{-}}1}$. However, the description in terms of three extended hydrodynamic modes is fully satisfactory at both densities over the whole \ensuremath{\kappa} range. We provide the first experimental evidence that the concept of extended hydrodynamic modes is valid for the whole range from hydrodynamics up into the kinetic regime.