NONEQUILIBRIUM FLUCTUATIONS IN LIQUID-MIXTURES UNDER THE INFLUENCE OFGRAVITY

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 gravi ty on the Rayleigh component of the light-scattering spectrum of liqui d mixtures. For the case that the liquid system is heated from below w e analyze the crossover from one-component-like to mixture-like behavi or of the critical slowing down of the fluctuations near the convectiv e instability. When the liquid mixture is heated from above, propagati ng modes appear for sufficiently large temperature gradients; a phenom enon similar to that previously encountered in one-component liquids. By retaining all the couplings between the temperature and concentrati on fluctuations in the hydrodynamic equations, we have also derived a new expression for the Rayleigh number R in liquid mixtures.