Structural and entropic modes in supercooled liquids: an experimental and theoretical study

We studied the relaxation processes of glycerol by a transient grating experiment with polarization selectivity and optical heterodyne detection. The density response of supercooled glycerol, for a wavevector q = 0.63 mum(-1), has been studied in that temperature range (T = 200-340 K), where the rearrangement of the structure (alpha-relaxation) and thermal diffusion occur on the same timescale. A strong interaction between the two modes is observed, manifesting in a dip in the T dependence of the apparent thermal conductivity and in a flattening of the apparent alpha-relaxation time upon cooling. A parameter-free thermohydrodynamic model for the long-time response is developed. The model is capable of quantitatively reproducing the data and of explaining the observed phenomenology.