Pretransitional effects near the convective instability in binary mixtures

Abstract

The pretransitional fluctuations of the Rayleigh-Bénard or convective instability are obtained for a binary mixture and compared to the one component case. There are now enhanced concentration fluctuations and although the velocity fluctuations are critically enhanced, they are weaker by a factor of D/χ. The behaviour of the critical (soft) mode is compared to the fluctuations in the order parameter obtained for a classical analysis of a phase transition. The region around the instability where the classical treatment of the order parameter fluctuations is invalid is increased in the binary case by a factor of (χ/D)1/2 over that of the pure fluid Rayleigh-Bénard instability.