' INTRODUCTIONIn reactive systems, forced convection is an efficient way to mix reactants and hence increase the reaction rate. This is of particular importance in chemical engineering processes. Conversely, one can address the question: how can chemical reactions influence natural convection or even be at the very source of hydrodynamic motion? These issues are at the heart of numerous applications in combustion, 1,2 polymer processing, 3,4 extraction techniques, 5,6 microfluidic devices, 7À9 bioconvection, 10 traveling fronts, 11À13 and CO 2 sequestration, 14,15 to name a few.To answer such questions, experimental studies have for instance investigated chemically driven convective mixing and enhanced extraction from one phase to another, induced by reactions between reactants initially contained separately in immiscible solvents. 5,16À18 In that case, it has been shown that the flow around the interface and within the bulk solutions result from (i) the coupling between transfer of chemical species at the interface, (ii) changes by the reaction of the density of the solutions which can trigger buoyancy-driven convective motions, and (iii) reaction-induced Marangoni effects, that is, fluid motion generated by surface tension changes at the immiscible interface. The situation is therefore quite complex, and even if theoretical studies 19À21 provide some help in understanding the influence of the various parameters, there is a need to gain insight also into simpler situations where some of the various effects are isolated. In this regard, the use of miscible solvents removes the influence of both transfer rate and Marangoni effects and allows one to separately analyze the influence of purely buoyancy-driven convection.For such miscible solvents, it has been shown experimentally that putting in contact aqueous solutions of an acid and of a base in the gravity field allows one to observe a wealth of beautiful convective patterns and instabilities. 22À25 More specifically, ascending plumes can develop above the reaction front when a solution of hydrochloric acid is put on top of a denser miscible equimolar aqueous solution of sodium hydroxide. 23 The patterns are different in presence of a color indicator, 22 indicating that this species is not neutral to the convective dynamics. 24 In this context, it is of interest to analyze such miscible systems in which a simple acidÀbase reaction takes place to understand the various possible buoyancy-driven instabilities induced by the presence, in aqueous solutions, of the neutralization reaction H + + OH À f H 2 O. To do so, we study experimentally chemically driven convective motions arising when putting in contact an aqueous solution of a strong acid on top of a denser aqueous solution of a strong base in the gravity field. We explain the influence on the dynamics of changing the type of reactants used and their concentrations. In a first part, we vary the type of counterion in the basic solution at fixed concentrations. We next vary the ratio in concentrations between th...

We report the hydrodynamic instabilities found in a simple exothermic neutralization reaction. Although the heavier aqueous NaOH solution was put below the lighter layer of aqueous HCl solution, fingering at the interface in a Hele-Shaw cell was observed. The reaction front, which propagates downward, becomes buoyantly unstable in the gravity field. The mixing zone length and wave number depend on the reactant concentrations. The mixing zone length increases and the wave number decreases when the reactant concentrations decrease.

We consider the buoyancy driven Rayleigh-Taylor instability of reaction-diffusion acidity fronts in a vertical Hele-Shaw cell using the chlorite-tetrathionate ͑CT͒ reaction as a model system. The acid autocatalysis of the CT reaction coupled to molecular diffusion yields isothermal planar reaction-diffusion fronts separating the two miscible reactants and products solutions. The reaction is triggered at the top of the Hele-Shaw cell and the resulting front propagates downwards, invading the fresh reactants, leaving the product of the reaction behind it. The density of the product solution is higher than that of the reactant solution, and hence a hydrodynamic instability develops due to unfavorable density stratification. We examine the linear stability of the isothermal traveling wavefront with respect to disturbances in the spanwise direction and demonstrate the existence of a preferred wavelength for the developed fingering instability. Our linear stability analysis is in excellent agreement with two-dimensional numerical simulations of the fully nonlinear system.

Buoyancy-driven hydrodynamic instabilities of acid-base fronts are studied both experimentally and theoretically in the case where an aqueous solution of a strong acid is put above a denser aqueous solution of a color indicator in the gravity field. The neutralization reaction between the acid and the color indicator as well as their differential diffusion modifies the initially stable density profile in the system and can trigger convective motions both above and below the initial contact line. The type of patterns observed as well as their wavelength and the speed of the reaction front are shown to depend on the value of the initial concentrations of the acid and of the color indicator and on their ratio. A reaction-diffusion model based on charge balances and ion pair mobility explains how the instability scenarios change when the concentration of the reactants are varied.

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