Dynamical Model for Brillouin Scattering Near the Critical Point of a Fluid

A dynamical model for density fluctuations in a one-component fluid near the critical point is proposed and compared with existing measurements in carbon dioxide. The model is described by a set of linearized hydrodynamic equations modified to include a nonlocal pressure density relationship and to include relaxation in the volume viscosity. Parameters for the model are found which are consistent with bulk measurements. With these parameters the model reproduces, within experimental uncertainty, the observed Brillouin spectrum of critical opalescence in CO₂. The low frequency volume viscosity is found to diverge as the - 1/3 power of T - T . An additional modification of the hydrodynamic equations, a frequency dependent thermal conductivity, is considered, but no definite conclusions can be reached as the Brillouin line-width data lack sufficient precision.