Rayleigh–Brillouin scattering and structural relaxation of a viscoelastic liquid

Abstract

The linear response theory using a complete set of dynamic variables involving density, velocity, and energy fluctuations is used to analyze the Rayleigh–Brillouin spectrum of a viscoelastic liquid. The result is discussed in the fast and slow relaxation limits. In the former case, the Rayleigh–Brillouin spectrum is identical to that predicted by the classical hydrodynamic equations; whereas in the latter case a new structural central peak is found, in addition to the anomalous dispersion and relaxation effects present in the frequency and linewidth data associated with the Brillouin peak. The evolution of structural relaxation and its effect on the entire Rayleigh–Brillouin spectrum is described. The structural central peak is most pronounced when the frequency dispersion and the linewidth maximum are present. The theoretical result has been used to calculate the Rayleigh–Brillouin spectra of polypropylene glycol at various temperatures. The results of the frequency shift, the spectral linewidth, and the Landau–Placzek ratio are in good agreement with the experiment.