Salinity-driven convection mechanisms in stratified brines

Understanding transport and mixing in stratified saline systems is critical for predicting the behavior of brines in natural aquifers, industrial reservoirs, and engineered disposal sites. These multi-ion solutions often exhibit complex instabilities driven by differential diffusion and compositional gradients. The onset and morphology of such convective mixing remain poorly predicted. We investigated double-diffusive (DD) and diffusion-layer convection (DLC) in superimposed aqueous solutions of the salts typical of saline aquifers, sodium chloride (NaCl) and sodium sulphate (Na2SO4). The study combines thermodynamic modeling, optical interferometry experiments, and nonlinear numerical simulations to explore convective instabilities in a ternary system. Our findings reveal a rich variety of convective scenarios depending on salt configuration and concentration ratios. When the faster-diffusing NaCl was placed above Na2SO4, diffusion-layer convection occurred with a delayed and asymmetric onset of instability, an experimentally demonstrated feature not reported previously. In contrast, when the positions were reversed, the system developed double-diffusive fingers that grew slowly due to the small density ratio. These fingers exhibited an unusual morphology, consisting of extremely fine, vertically textured structures that gradually merged away from the interface. This formed a large area of diffuse mixing and suppression of coherent convective structures. In all cases, classical stability criteria failed to fully predict the onset and nature of convection. Instead, we identified the initial position of the system on the stability map, as determined by the full diffusion matrix, as a critical factor.