Anne De Wit

From Chemical Gardens to Chemobrionics

Chemical gardens in laboratory chemistries ranging from silicates to polyoxometalates, in applications ranging from corrosion products to the hydration of Portland cement, and in natural settings ranging from hydrothermal vents in the ocean depths to brinicles beneath sea ice. In many chemical-garden experiments, the structure forms as a solid seed of a soluble ionic compound dissolves in a solution containing another reactive ion. In general any alkali silicate solution can be used due to their high solubility at high pH. The cation should not precipitate with the counterion of the metal salt used as seed. A main property of seed chemical-garden experiments is that initially, when the fluid is not moving under buoyancy or osmosis, the delivery of the inner reactant is diffusion controlled. Another experimental technique that isolates one aspect of chemical-garden formation is to produce precipitation membranes between different aqueous solutions by introducing the two solutions on either side of an inert carrier matrix. Chemical gardens may be grown upon injection of solutions into a so-called Hele-Shaw cell, a quasi-two-dimensional reactor consisting in two parallel plates separated by a small gap.

Combined influences of viscous fingering and solvent effect on the distribution of adsorbed solutes in porous media

The displacement of two fluids in a porous medium can be affected by a viscous fingering instability (VF) that arises at the interface between the fluids when their viscosities are different. In parallel, one of the fluids may contain solutes that reversibly adsorb on the porous matrix at a rate that depends on the composition of the two-fluid mixture, a so-called solvent strength effect. In some systems encountered for instance in liquid chromatographic columns or in underground flows in environmental applications, both VF and solvent strength effects may combine to influence the spatio-temporal distribution of solutes. Here, a computational investigation of such dynamics is performed. The distribution of the solute in the porous medium is affected by the combined effects of VF and solvent strength. A three component system (displacing fluid, sample solvent and solute) is modeled using Darcys law for the fluid flow velocity coupled to a convection–diffusion equation for the sample solvent and a mass balance equation for the solute in the mobile and stationary phases. The sample solvent is assumed to have a larger solvent strength than the displacing fluid, in which the retention parameter due to the linear adsorption of the solute depends exponentially on the concentration of the sample solvent. A parametric study of the influences of the factors controlling the VF and solvent strength effects on the displacement velocity of the fronts of solute zone and on its width along the porous medium has been performed by direct numerical simulation of the governing equations. While each of the two effects (VF and solvent strength effects) distorts and significantly increases the broadening of the solute zone, the simulations reveal that, when they are acting in combination, these solute zone perturbations are reduced.

A + B → C reaction fronts in Hele-Shaw cells under modulated gravitational acceleration.

The dynamics of A + B → C reaction fronts is studied under modulated gravitational acceleration by means of a combination of parabolic flight experiments and numerical simulations. During modulated gravity the front position undergoes periodic modulation with an accelerated front propagation under hyper-gravity together with a slowing down under low gravity. The underlying reason for this is an amplification and a decay, respectively, of the buoyancy-driven double vortex associated with the front propagation under standard gravitational acceleration, as explained by reaction-diffusion-convection simulations of convection around an A + B → C front. Deeper insights into the correlation between grey-value changes in the experimental shadowgraph images and characteristic changes in the concentration profiles are obtained by a numerical simulation of the imaging process.

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

Pulsatile chemo-hydrodynamic patterns due to a coupling between an oscillating chemical reaction and buoyancy-driven hydrodynamic flows can develop when two solutions of separate reactants of the Belousov-Zhabotinsky reaction are put in contact in the gravity field and conditions for chemical oscillations are met in the contact zone. In regular oscillatory conditions, localized periodic changes in the concentration of intermediate species induce pulsatile density gradients, which, in turn, generate traveling convective fingers breaking the transverse symmetry. These patterns are the self-organized result of a genuine coupling between chemical and hydrodynamic modes.

Convective dynamics of traveling autocatalytic fronts in a modulated gravity field

When traveling in thin solution layers, autocatalytic chemical fronts may be deformed and accelerated by convective currents that develop because of density and surface tension gradients related to concentration and thermal gradients across the front. On earth, both buoyancy and Marangoni related flows can act in solution layers open to the air while only buoyancy effects operate in covered liquid layers. The respective effects of density and surface tension induced convective motions are analysed here by studying experimentally the propagation of autocatalytic fronts in uncovered and covered liquid layers during parabolic flights in which the gravity field is modulated periodically. We find that the velocity and deformation of the front are increased during hyper-gravity phases and reduced in the micro-gravity phase. The experimental results compare well with numerical simulations of the evolution of the concentration of the autocatalytic product coupled to the flow field dynamics described by Navier-Stokes equations.

Flow control of A + B → C fronts by radial injection

The dynamics of A+B→C fronts is analyzed theoretically in the presence of passive advection when A is injected radially into B at a constant inlet flow rate Q. We compute the long-time evolution of the front position, r_{f}, of its width, w, and of the local production rate R of the product C at r_{f}. We show that, while advection does not change the well-known scaling exponents of the evolution of corresponding reaction-diffusion fronts, their dynamics is however significantly influenced by the injection. In particular, the total amount of product varies as Q^{-1/2} for a given volume of injected reactant and the front position as Q^{1/2} for a given time, paving the way to a flow control of the amount and spatial distribution of the reaction front product. This control strategy compares well with calcium carbonate precipitation experiments for which the amount of solid product generated in flow conditions at fixed concentrations of reactants and the front position can be tuned by varying the flow rate.

Localized stationary and traveling reaction-diffusion patterns in a two-layer A + B -> oscillator system

When two solutions containing separate reactants A and B of an oscillating reaction are put in contact in a gel, localized spatiotemporal patterns can develop around the contact zone thanks to the interplay of reaction and diffusion processes. Using the Brusselator model, we explore analytically the deployment in space and time of the bifurcation diagram of such an A+B→ oscillator system. We provide a parametric classification of possible instabilities as a function of the ratio of the initial reactant concentrations and of the reaction intermediate species diffusion coefficients. Related one-dimensional reaction-diffusion dynamics are studied numerically. We find that the system can spatially localize waves and Turing patterns as well as induce more complex dynamics such as zigzag spatiotemporal waves when Hopf and Turing modes interact.

From microscopic compartmentalization to hydrodynamic patterns: New pathways for information transport

Can we exploit hydrodynamic instabilities to trigger an efficient, selective and spontaneous flow of encapsulated chemical information? One possible answer to this question is presented in this paper where cross-diffusion, which commonly characterizes compartmentalized dispersed systems, is shown to initiate buoyancy-driven hydrodynamic instabilities. A general theoretical framework allows us to predict and classify cross-diffusion-induced convection in two-layer stratifications under the action of the gravitational field. The related nonlinear dynamics is described by a cross-diffusion-convection (CDC) model where fickian diffusion is coupled to the Stokes equations. We identify two types of hydrodynamic modes (the negative cross-diffusion-driven convection, NCC, and the positive cross-diffusion-driven convection, PCC) corresponding to the sign of the cross-diffusion term dominating the system dynamics. We finally show how AOT water-in-oil reverse microemulsions are an ideal model system to confirm the general theory and to approach experimentally cross-diffusion-induced hydrodynamic scenarios.

The broken idiotypic mirror hypothesis

This chapter presents the broken idiotypic mirror hypothesis. The immune response Ars-KLH in mice from the A/J strain is characterized by a major recurrent idiotype whose code name is CRIA. The molecular basis for this recurrent idiotype has been unraveled. During the primary response, the anti-arsonate antibodies are characterized by the expression of a Y-gene segment, VH id CRI 1, which can be associated with different D segments. As the response proceeds, the selection of a major canonical combination is observed: the recurrent idiotype, which is mainly associated with the heavy chain, is made up of the VH id CRI 1, the DF1 16.1, and the JH2 segments. Most other strains of mice, which belong to other IgH haplotypes, do not express this idiotype when immunized with the same antigen. Using a panel of monoclonal anti-idiotypic antibodies, and a panel of hybridoma, which are somatic variants of the germline combination, one can distinguish at least five idiotopes in the CRIA molecules.