RAYLEIGH-TAYLOR INSTABILITY IN A PHASE-SEPARATING MIXTURE UNDER A CONCENTRATION GRADIENT

In order to investigate the onset of hows induced by gravity during ph ase separation of fluids, the behavior of a density-matched liquid mix ture is studied after a temperature quench. phase transition is invest igated in the presence of a weak linear concentration (and thus, densi ty) gradient. We observe a ''fast'' growth zone for critical and near critical concentrations where phase separation proceeds by coalescence of interconnected domains. A ''slow'' growth zone surrounds the fast growth zone for off-critical concentrations where phase separation occ urs through coalescence of spherical droplets. We observe at late time s the development of a pattern of convective rolls, due to a Rayleigh- Taylor instability between the fast and slow zones. The rate of evolut ion of the instability is several orders of magnitude lower than expec ted. This is because when the flows develop, the adjacent phases are l ikely to have a density difference much smaller than that expected bet ween the coexisting phases at equilibrium.