Buoyancy-driven instabilities induced by a neutralization reaction in immiscible fluids

Buoyancy-driven instabilities triggered by neutralization reaction in an immiscible two-layer system placed in a vertically oriented narrow cell were studied experimentally. The initial density of the layers was always set to exclude the development of the Rayleigh-Taylor instability. The problem was examined for the surface-inactive species, namely hydrochloric acid and sodium hydroxide, which allowed studying a convective instability development solely due to buoyancy-driven mechanisms, excluding Marangoni effects. We show that one of two global scenarios develops in the system right after the layers came into contact. These scenarios, called by us diffusion-controlled and convection-controlled, essentially differ in the prevailing mechanism of mass transfer and, therefore, in the rate of reaction-diffusion-convection processes. The authors introduce the nondimensional parameter, the value of which determines the border between the two scenarios in the regime map.