Ultraviolet and visible Brillouin scattering study of viscous relaxation in 3- methylpentane down to the glass transition

"Brillouin light scattering spectra from transverse and longitudinal acoustic waves in liquid and supercooled. 3-methylpentane have been collected from room temperature down to 80 K, just above. the glass transition. Spectra at different wave vectors have been obtained using 532 nm and 266 nm. excitation. We found evidence of a shear relaxation with a characteristic time of 100 s at the glass. transition which only partly accounts for the relaxation observed in the propagation and attenuation. of the longitudinal modes. The inclusion of a relaxing bulk viscosity contribution with a relaxation. time of the order of 102 ns at the glass transition is found to adequately reproduce the experimental. data including transient grating data at a much lower frequency. A consistent picture of relaxed shear. and bulk moduli as a function of temperature is derived. These two quantities are found to be related. by a linear relation suggesting that a Cauchy-like relation holds also above the glass transition."