ONSET OF DEPARTURE FROM LINEARIZED HYDRODYNAMIC BEHAVIOR IN ARGON GAS STUDIED WITH NEUTRON BRILLOUIN-SCATTERING

We have investigated the dynamic structure factor of room-temperature Ar36 at two densities (0.25 and 0.63 times the critical) and at very low momentum transfer (0.3< <1.3 nm-1). Linearized hydrodynamics agrees with the experimental data only at the higher density and <1 nm-1. However, the description in terms of three extended hydrodynamic modes is fully satisfactory at both densities over the whole 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. © 1990 The American Physical Society.