Thermodynamic description of depolarized light scattering from viscoelastic liquids

A thermodynamic calculation of the depolarized scattering spectrum of a viscoelastic liquid is presented. We introduce the stress tensor and the polarizability density as additional state variables. By using extended irreversible thermodynamics, we derive time evolution equations for all the independent variables. The temperature dependence of the spectrum is calculated and we show that it reproduces the general observed features of the spectrum. We also show that from our description we recover, as special cases, the results of other phenomenological approaches that are in good agreement with experiment and microscopic theories. The implications and limitations of our approach are discussed.