High-frequency structural relaxation in supercooled liquids

Abstract

High-frequency structural relaxation in glass forming solutions of LiCl in water was studied using complex conductivity measurements in the frequency range 0.5 MHz to 32 GHz for temperatures between 300 and 160 K. The conductivity data show a low-frequency plateau, and a strong frequency dependence above a characteristic frequency. Both the low-frequency conductivity and the characteristic frequency decrease strongly with temperature, scaling approximately as the inverse of the viscosity, suggesting that hydrodynamic processes govern structural relaxation. The ionic conductivity can be related to the mechanical properties using the Stokes–Einstein relation. The frequency dependence cannot be understood in terms of a single viscoelastic relaxation time. A semiphenomenological version of the mode coupling theory of supercooled liquids, using the experimentally determined temperature dependence of the static viscosity as a parameter describes the frequency dependence of the data reasonably well.