Convective Mixing Induced by Acid–Base Reactions

Abstract

' 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...

Nonmonotonic Rayleigh-Taylor Instabilities Driven by Gas–Liquid CO2 Chemisorption

“…Note in this respect that the present analysis goes beyond recent works concerned with miscible reactive two-liquid systems. [11][12][13] Indeed, these authors do not evidence any oscillatory dynamics in the nonlinear regime, as we do here both experimentally and on the basis of a simplified model of the CO 2 chemisorption process.…”

Mixed-mode instability of a miscible interface due to coupling between Rayleigh-Taylor and double-diffusive convective modes

“…Search for analogous mixed modes in dynamics of reactive systems 15,29 or in other hydrodynamic instabilities 30 where differential diffusion effects have been evidenced would also be of particular interest.…”

Asymmetric Rayleigh-Taylor and double-diffusive fingers in reactive systems

“…25,26 In absence of a color indicator, visualization can be made by interferometry 24 or schlieren techniques. 27 In that case, the DLC convective plumes are observed only above the contact line. This is different from the non reactive situation 34 where convection is also obtained below the miscible interface where the fast diffusing species is accumulating.…”

“…12,19 In parallel, the coupling between convective motions and a simple A + B → C reaction has been addressed both experimentally [20][21][22][23][24][25][26][27][28][29] and theoretically. 24,27,[29][30][31][32] In the specific a) L. Lemaigre and M. A. Budroni contributed equally to this work. case of acid-base reactions, buoyancy-driven destabilization of the miscible interface between two solutions containing the separate reactants A and B has been studied in Hele-Shaw cells (two glass plates separated by a thin gap and oriented vertically in the gravitational field).…”

“…33 In vertical systems, if an aqueous solution of HCl is put on top of a denser aqueous solution of NaOH, convective motions rapidly develop above the miscible contact line. 24,27 These are due to a diffusive layer convection (DLC) mechanism 2,34 arising when a less dense solution of a fast diffusing component overlies a denser solution of a slow diffusing one. As the acid diffuses faster than the base, a depletion zone is created dynamically in time above the contact line where the acid has diffused out without being replaced yet by the slow diffusing base.…”