NONEQUILIBRIUM FLUCTUATIONS IN LIQUID-MIXTURES UNDER THE INFLUENCE OF GRAVITY

The theory of fluctuations in liquid mixtures that are subjected to a stationary temperature gradient, while still being kept in a quiescent state, is extended to include the influence of gravity. Specifically, we evaluate the effects of a temperature gradient combined with gravity on the Rayleigh component of the light-scattering spectrum of liquid mixtures. For the case that the liquid system is heated from below we analyze the crossover from one-component-like to mixture-like behavior of the critical slowing down of the fluctuations near the convective instability. When the liquid mixture is heated from above, propagating modes appear for sufficiently large temperature gradients; a phenomenon similar to that previously encountered in one-component liquids. By retaining all the couplings between the temperature and concentration fluctuations in the hydrodynamic equations, we have also derived a new expression for the Rayleigh number R in liquid mixtures.