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D'Hernoncourt J.,Nonlinear Physical Chemistry Unit | De Wit A.,Nonlinear Physical Chemistry Unit
Physica D: Nonlinear Phenomena | Year: 2010

Across traveling exothermic autocatalytic fronts, a density jump can be observed due to changes in composition and temperature. These density changes are prone to induce buoyancy-driven convection around the front when the propagation takes place in absence of gel within the gravity field. Most recent experiments devoted to studying such reaction-diffusion-convection dynamics are performed in Hele-Shaw cells, two glass plates separated by a thin gap width and filled by the chemical solutions. We investigate here the influence of heat losses through the walls of such cells on the nonlinear fingering dynamics of exothermic autocatalytic fronts propagating in vertical Hele-Shaw cells. We show that these heat losses increase tip splittings and modify the properties of the flow field. A comparison of the differences between the dynamics in reactors with respectively insulating and conducting walls is performed as a function of the Lewis number L e, the Newton cooling coefficient α quantifying the amplitude of heat losses and the width of the system. We find that tip splitting is enhanced for intermediate values of α while coarsening towards one single finger dominates for insulated systems or large values of α leading to situations equivalent to isothermal ones. © 2009 Elsevier B.V. All rights reserved.

Almarcha C.,Nonlinear Physical Chemistry Unit | Trevelyan P.M.J.,Nonlinear Physical Chemistry Unit | Grosfils P.,Roosevelt University | De Wit A.,Nonlinear Physical Chemistry Unit
Physical Review Letters | Year: 2010

In the gravity field, density changes triggered by a kinetic scheme as simple as A+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. © 2010 The American Physical Society.

Rongy L.,Nonlinear Physical Chemistry Unit | Trevelyan P.M.J.,Nonlinear Physical Chemistry Unit | De Wit A.,Nonlinear Physical Chemistry Unit
Chemical Engineering Science | Year: 2010

The dynamics of initially vertical A + B → C reaction fronts propagating in covered horizontal solution layers can be influenced by buoyancy-driven convection. Experiments have provided evidence that a much faster propagation of the front occurs in solutions than that predicted by reaction-diffusion (RD) theories, thereby suggesting the influence of convective effects arising if A, B, and C have different densities. Here we analyze numerically and theoretically the dynamics resulting from the coupling of a simple A + B → C chemical reaction with diffusion and convection induced by density differences across the reaction front. The important parameters of the related reaction-diffusion-convection (RDC) model are the three dimensionless Rayleigh numbers, quantifying the contribution of each species concentration to the density of the solution, the layer thickness, and the initial reactant concentration ratio. The presence of buoyancy-driven convection at the front induces a propagation of this front even in the case of equal diffusion coefficients and equal initial reactant concentrations for which RD theories predict a non-moving front. In the case of equal initial concentrations, even in the presence of convection, the classification of the various possible dynamics and the prediction of the direction of front propagation can be obtained from simple criteria on the Rayleigh numbers. In the case of different initial reactant concentrations for which, in the absence of convection, the RD front propagates towards the side of the less concentrated reactant, the introduction of buoyancy convection not only invalidates the long time RD scalings but can lead to a double reversal in the direction of propagation of the reaction front for intermediate times. The influence of the different parameters on the RDC dynamics is presented. © 2009 Elsevier Ltd. All rights reserved.

Mishra M.,Nonlinear Physical Chemistry Unit | Martin M.,Paris West University Nanterre La Défense | De Wit A.,Nonlinear Physical Chemistry Unit
Chemical Engineering Science | Year: 2010

Viscous fingering (VF) between miscible fluids of different viscosities can affect the dispersion of localized samples in porous media. This is the case when a given fluid displaces a finite width sample consisting in a solvent of different viscosity and containing a dissolved analyte undergoing adsorption on the porous matrix. We investigate here numerically the influence of VF between the carrier fluid and the sample solvent on the spreading of a retained analyte concentration peak in a three-component system (displacing fluid, sample solvent, analyte). We compare the cases where the sample solvent is, respectively, more or less viscous than the displacing fluid or eluent by varying the log-mobility ratio R = ln (μ2 / μ1) where μ1 and μ2 are the viscosities of the eluent and sample solvent, respectively. We show that disentanglement of the analyte concentration peak from the fingering zone occurs earlier for less viscous samples, i.e. for R < 0 than for R > 0. Qualitative agreement with experimental evidences obtained in reversed phase liquid chromatography are shown. © 2009 Elsevier Ltd. All rights reserved.

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