Controlled retardation of electrochemical Rayleigh–Bénard convection during copper electrolysis

One-Directional Fluidic Flow Induced by Chemical Wave Propagation in a Microchannel

A one-directional flow induced by chemical wave propagation was investigated to understand the origin of its dynamic flow. A cylindrical injection port was connected with a straight propagation channel; the chemical wave was initiated at the injection port. Chemical waves propagated with a constant velocity irrespective of the channel width, indicating that the dynamics of the chemical waves were governed by a geometry-independent interplay between the chemical reaction and diffusion. In contrast, the velocity of the one-directional flow was dependent on the channel width. Furthermore, enlargement of the injection port volume increased the flow velocity and volume flux. These results imply that the one-directional flow in the microchannel is due to a hydrodynamic effect induced in the injection port. Spectroscopic analysis of a pH indicator revealed the simultaneous behavior between the pH increase near the injection port and the one-directional flow. Hence, we can conclude that the one-directional flow in the microchannel with chemical wave propagation was caused by a proton consumption reaction in the injection port, probably through liquid volume expansion by the reaction products and the reaction heat. It is a characteristic feature of the present system that the hydrodynamic flow started from the chemical wave initiation point and not the propagation wavefront, as observed for previous systems.

Coupling between Buoyancy Forces and Electroconvective Instability near Ion-Selective Surfaces

Recent investigations have revealed that ion transport from aqueous electrolytes to ion-selective surfaces is subject to electroconvective instability that stems from coupling of hydrodynamics with electrostatic forces. These systems inherently involve fluid density variation set by salinity gradients. However, the coupling between the buoyancy effects and electroconvective instability has not yet been investigated although a wide range of electrochemical systems are naturally prone to these interplaying effects. In this study we thoroughly examine the interplay of gravitational convection and chaotic electroconvection. Our results reveal that buoyant forces can significantly influence the transport rates, otherwise set by electroconvection, when the Rayleigh number Ra of the system exceeds a value Ra∼1000. We show that buoyancy forces can significantly alter the flow patterns in these systems. When the buoyancy acts in the stabilizing direction, it limits the extent of penetration of electroconvection, but without eliminating it. When the buoyancy destabilizes the flow, it alters the electroconvective patterns by introducing upward and downward fingers of respectively light and heavy fluids.

Chemical control of dissolution-driven convection in partially miscible systems: theoretical classification

Dissolution-driven convection occurs in the host phase of a partially miscible system when a buoyantly unstable density stratification develops upon dissolution. Reactions can impact such convection by changing the composition and thus the density of the host phase. Here we study the influence of A + B → C reactions on such convective dissolution when A is the dissolving species and B a reactant initially in solution in the host phase. We perform a linear stability analysis of related reaction-diffusion density profiles to compare the growth rate of the instability in the reactive case to its non reactive counterpart when all species diffuse at the same rate. We classify the stabilizing or destabilizing influence of reactions on the buoyancy-driven convection in a parameter space spanned by the solutal Rayleigh numbers RA,B,C of chemical species A, B, C and by the ratio β of initial concentrations of the reactants. For RA > 0, the non reactive dissolution of A in the host phase is buoyantly unstable. In that case, we show that the reaction is enhancing convection provided C is sufficiently denser than B. Increasing the ratio β of initial reactant concentrations increases the effect of chemistry but does not significantly impact the stabilizing/destabilizing classification. When the non reactive case is buoyantly stable (RA≤ 0), reactions can create in time an unstable density stratification and trigger convection if RC > RB. Our theoretical approach allows classifying previous results in a unifying picture and developing strategies for the chemical control of convective dissolution.

Control of convective dissolution by chemical reactions: general classification and application to CO(2) dissolution in reactive aqueous solutions

In partially miscible two-layer systems within a gravity field, buoyancy-driven convective motions can appear when one phase dissolves with a finite solubility into the other one. We investigate the influence of chemical reactions on such convective dissolution by a linear stability analysis of a reaction-diffusion-convection model. We show theoretically that a chemical reaction can either enhance or decrease the onset time of the convection, depending on the type of density profile building up in time in the reactive solution. We classify the stabilizing and destabilizing scenarios in a parameter space spanned by the solutal Rayleigh numbers. As an example, we experimentally demonstrate the possibility to enhance the convective dissolution of gaseous CO_{2} in aqueous solutions by a classical acid-base reaction.