Rayleigh-Taylor Instability in a Phase Separating Mixture under a Concentration Gradient

In order to investigate the onset of flows induced by gravity during phase separation of fluids, the behavior of a density-matched liquid mixture is studied after a temperature quench. Phase transition is investigated in the presence of a weak linear concentration (and thus, density) 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 occurs through coalescence of spherical droplets. We observe at late times the development of a pattern of convective rolls, due to a Rayleigh-taylor instability between the fast and slow zones. The rate of evolution of the instability is several orders of magnitude lower than expected. This is because when the flows develop, the adjacent phases are likely to have a density difference much smaller than that expected between the coexisting phases at equilibrium