Chemically Driven Hydrodynamic Instabilities

Abstract

In the gravity field, density changes triggered by a kinetic scheme as simple as A þ B ! C can induce or affect buoyancy-driven instabilities at a horizontal interface between two solutions containing initially the scalars A and B. On the basis of a general reaction-diffusion-convection model, we analyze to what extent the reaction can destabilize otherwise buoyantly stable density stratifications. We furthermore show that, even if the underlying nonreactive system is buoyantly unstable, the reaction breaks the symmetry of the developing patterns. This is demonstrated both numerically and experimentally on the specific example of a simple acid-base neutralization reaction. DOI: 10.1103/PhysRevLett.104.044501 PACS numbers: 82.40.Ck, 47.20.Bp, 47.70.Fw, 82.33.Ln Convective motion due to hydrodynamic instabilities of an interface between two different fluids are known to impact the spatiotemporal distribution and dynamics of passive scalars in numerous applications. Much less studied is the active role that processes involving such scalars can have upon the flow dynamics if these scalars influence a physical property of the fluid such as its density for instance [1][2][3][4]. However, coupling between reactivetype processes and hydrodynamics is at the heart of applications in fields as diverse as earth mantle dynamics [5], geological formations [6], supernovae dynamics [7] or CO 2 sequestration [8], to name a few. Often, the specific active role of the scalars on the flow remains difficult to interpret due to difficulties of in situ experiments, as well as a lack of quantitative modeling and of simple benchmark experiments on which theories could be tested. In this respect, it is still unclear to what extent reactions involving these scalars can trigger hydrodynamic instabilities in a system that would otherwise remain stable and whether convective structures have the same symmetries in reactive and nonreactive situations.To gain insight into these issues, let us consider the generic case of a solution containing the scalar A put on top of a solution of B in the gravity field. For nonreactive systems, various hydrodynamic instabilities can impact such a stratification of miscible fluids. The RayleighTaylor instability occurs when the heavier fluid overlies the lighter fluid [9]. If the upper fluid is lighter, the system can also be destabilized either if B diffuses faster than A, because of double-diffusive fingering [10,11], or if a diffusive-layer convection (DLC) instability is triggered when A diffuses faster than B [12]. In all cases, these buoyancy-driven instabilities lead, in nonreactive miscible fluids, to convective motions which develop similarly above and below the initial contact line because of the symmetry of the underlying density gradient [9][10][11][12]. The situation can however be very different if a chemical reaction takes place between species A and B upon contact and mixing of the solutions.We demonstrate indeed both theoretically and experimentally that a reaction as simple as A þ B ! C ...

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.…”

Convective Mixing Induced by Acid–Base Reactions

“…Figure 1 (middle) shows the typical dynamics observed when putting a solution of HCl on top of an equimolar denser NaOH one in the absence of any color indicator and using water as the solvent: quite rapidly, buoyant plumes start rising above the initial contact line, as detailed in previous studies. 23 Some of the fingers merge, and a general coarsening toward larger fingers is observed. In some cases, a finger can also split into two.…”

“…To understand the origin of the fingers observed, it is useful to recall that previous results 23 suggest that buoyancy instabilities in reactive solutions can be understood in terms of the following instabilities found in the nonreactive case: 27,28 when a denser solution lies on top of a less dense one in the gravity field, a RayleighÀTaylor (RT) instability can develop and deform the miscible interface into interpenetrating "fingers". A double diffusive (DD) fingering can also appear if the solute in the lower denser solution diffuses faster than the solute in the upper solution.…”

Self-Organized Traveling Chemo-Hydrodynamic Fingers Triggered by a Chemical Oscillator

“…6,13,28 We have performed several experiments varying the composition of the solutions to test some instability scenarios. However, in all cases, ρ 2 < ρ 1 to exclude an RT instability.…”

“…1−3 At the intersection between these two fields, chemo-hydrodynamics focuses on analyzing patterns and instabilities that result from a nonlinear coupling of hydrodynamic and RD processes. 4 Related problems range from fingering instabilities of reactive interfaces 5,6 or traveling chemical fronts 4 to the spatiotemporal dynamics of chemicals advected in complex flow fields. 7,8 The reaction−diffusion−convection (RDC) interplay impacts numerous applications areas as diverse as combustion, 9 polymer processing, 10 extraction techniques 5,11 and CO 2 sequestration.…”