In this article, we explore the development and mixing that results from Rayleigh–Taylor instability between two layers. Unlike the classical problem, one of the two layers initially has a stable density stratification, while the density of the fluid in the other is uniform. The results presented here are from low Atwood number experiments with incompressible fluids, although there is an analogy between this case and low accelerations of compressible fluids that result in hydrostatic gradients that differ significantly. In both scenarios, the instability growth is asymmetric. For the present case, mixing will occur throughout the region initially occupied by the layer of uniform density but, depending on the strength of the stratification compared with the density difference across the initial Rayleigh–Taylor-unstable interface, the mixing may be arrested by the stratification before it reaches the lower boundary of the domain. We propose a simple model for the growth of the instability and see that it captures the qualitative behaviour. The final state comprises a weakly stratified layer bounded on one side by the remnant of the initially stratified layer. The structure of this weak stratification is seen to be consistent with the idea of maximal configurational entropy for the mixing zone. Ultimately, the efficiency of this mixing process is found to follow the pattern expected for ‘perfect mixing’ equivalent, but achieves only 94% of the corresponding value
