Control of Rayleigh-Taylor instability onset time and convective velocity by differential diffusion effects

Fingering instabilities of a miscible interface between two fluids in a gravitational field can develop due to adverse density gradients as in the well-known Rayleigh-Taylor (RT) and double-diffusive (DD) instabilities. In the absence of differential diffusion, the mixing rate and the onset time of the RT instability developing when a denser solution of a given solute A overlies a less dense solution of a solute B are respectively proportional and inversely proportional to the initial density difference Delta rho(0) between the two superposed layers. We show here both experimentally and theoretically for porous media flows that when the mechanisms of RT and DD instabilities are combined, the properties of the convective growth of the fingers are controlled by the dynamic density jump Delta rho(m) of the nonmonotonic density profile induced by the differential diffusion effects. In particular, the onset time and mixing rate can be controlled by varying the ratio of the diffusion coefficients of the solutes.