SMALL-ANGLE RAYLEIGH-SCATTERING FROM NONEQUILIBRIUM FLUCTUATIONS IN LIQUIDS AND LIQUID-MIXTURES

We have developed an improved experimental arrangement for measuring s mall-angle Rayleigh scattering in liquid systems subjected to a statio nary temperature gradient. This improved arrangement has enabled us to measure nonequilibrium fluctuations with a simple helium-neon laser. We have used this technique to measure long-range nonequilibrium fluct uations in n-hexane and in several mixtures of n-hexane with toluene, thus complementing measurements for the equimolar mixture obtained pre viously. The experiments confirm that all diffusive nonequilibrium flu ctuations vary with (del T)2/k4, where del T is the imposed temperatur e gradient and where k is the wave number of the fluctuations. A quant itative comparison of the strength of these nonequilibrium fluctuation s with the theoretical predictions is also presented.