Statically stable stratifications of a less dense solution above a denser one can be destabilized in the gravity field when differential diffusion effects come into play. Specifically, if the stratification involves two solutes that diffuse at different rates, nonmonotonic density profiles can build up. Local convective zones develop then in the areas where, locally, the density decreases along the gravity field. Previous theoretical works have shown that for both double-diffusive (DD) and diffusive-layer-convection (DLC) regimes, the onset time and the mixing velocity of these buoyancy-driven instabilities are controlled by the dynamic density jump across these unstable zones. We provide here an experimental validation of these scalings by analysis of convective dynamics induced by differential diffusion around horizontal stratifications confined within a Hele-Shaw cell. We analyze a wide range of solute combinations and concentrations to vary the two important parameters of the problem which are the diffusion coefficients ratio $\delta$ and the buoyancy ratio $R$. By measuring the mixing length evolution, we quantify in each case the mixing velocity and the instability onset time. We show that, in both DD and DLC regimes, the convective dynamics are indeed governed by the dynamically generated local adverse density difference. In the DD case, the mixing velocity increases linearly with the dynamically induced density difference, while the onset time decreases accordingly. In the DLC regime, these properties are controlled by the local adverse density jump developing independently within each layer. The experimentally obtained scalings confirm the previous theoretical predictions and provide a quantitative framework for controlling buoyancy-driven mixing in stratified two-solute systems.

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• Convection in porous media
• Doubly diffusive convection
• Hele-shaw flows
• Mixing enhancement
• Scaling laws of complex systems