Experimental analysis of density fingering instability modified by precipitation

We analyze the effect of precipitate formation on the development of density induced hydrodynamic instabilities. In this case, the precipitate is BaCO₃, obtained by reaction of CO₂ with aqueous BaCl₂. CO₂(g) dissolution increases the local density of the aqueous phase, triggering Rayleigh-Taylor instabilities and BaCO₃ formation. It was observed that at first the precipitate was formed at the finger front. As the particles became bigger, they began to fall down from the front. These particles were used as tracers using PIV technique to visualize the particle streamlines and to obtain the velocity of that movement. This falling produced a downward flow that might increase the mixing zone. Contrary to expectations, it was observed that the finger length decreased, indicating that for the mixing zone development, the consumption of CO₂ to form the precipitate is more important than the downward flow. The mixing zone length was recovered by increasing the availability of the reactant (higher CO₂ partial pressure), compensating the CO₂ used for BaCO₃ formation. Mixing zone development rates reached constant values at shorter times when the precipitate is absent than when it is present. An analysis of the nonlinear regime with and without the precipitate is performed.