Abstract
We have performed photon correlation spectroscopy experiments to measure the lifetimes of density fluctuations in liquid salol in both the normal and supercooled regimes down to its glass transition at -55 °C. Two modes were found to exist: a temperature-independent, hydrodynamic mode which we ascribed to entropy fluctuations and called the heat mode, and a strongly temperature-dependent, nonhydrodynamic mode due to internal, structural relaxations. We find that the internal relaxations suppress the heat mode when the heat-mode relaxation time is comparable to or less than the internal-mode relaxation time. This suppression occurs in the same temperature regime as a loss of power-law behavior for the viscosity as predicted by mode-coupling theory. We argue that the phenomena of the glass transition are a consequence of this suppression of the hydrodynamic heat mode by the internal relaxation mode as the temperature falls.