Finite element numerical simulations were carried out in 2D geometries to map the magnetic field and force distribution produced by rectangular permanent magnets as a function of their size and position with respect to a microchannel. A single magnet, two magnets placed in attraction and in repulsion have been considered. The goal of this work is to show where magnetic beads are preferentially captured in a microchannel. These simulations were qualitatively corroborated, in one geometrical case, by microscopic visualizations of magnetic bead plug formation in a capillary. The results show that the number of plugs is configuration dependent with: in attraction, one plug in the middle of the magnets; in repulsion, two plugs near the edges of the magnets; and with a single magnet, a plug close to the center of the magnet. The geometry of the magnets (h and l are the height and length of the magnets respectively) and their relative spacing s has a significant impact on the magnetic flux density. Its value inside a magnet increases with the h/l ratio. Consequently, bar magnets produce larger and more uniform values than flat magnets. The l/s ratio also influences the magnetic force value in the microchannel, both increasing concomitantly for all the configurations. In addition, a zero force zone in the middle appears in the attraction configuration as the l/s ratio increases, while with a single magnet, the number of maxima and minima goes from one to two, producing two focusing zones instead of only one.

Magnetism-based microsystems, as those dedicated to immunoaffinity separations or (bio)chemical reactions, take benefit of the large surface area-to-volume ratio provided by the immobilized magnetic beads, thus increasing the sensitivity of the analysis. As the sensitivity is directly linked to the efficiency of the magnetic bead capture, this paper presents a simple method to enhance the capture in a microchannel. Considering a microchannel surrounded by two rectangular permanent magnets of different length (L m =2, 5, 10 mm) placed in attraction, it is shown that the amount of trapped beads is limited by the magnetic forces mainly located at the magnet edges. To overcome this limitation, a polyethylene terephthalate (PET) microchip with an integrated magnetic track array has been prototyped by laser photo-ablation. The magnetic force is therefore distributed all along the magnet length. It results in a multi-plug bead capture, observed by microscope imaging, with a magnetic force value locally enhanced. The relative amount of beads, and so the specific binding surface for further immunoassays, presents a significant increase of 300% for the largest magnets. The influence of the track geometry and relative permeability on the magnetic force was studied by numerical simulations, for the microchip operating with 2-mm-long magnets.

Résumé -Une approche générale de la modélisation cinétique des réactions solide-gaz à l'échelle du réacteur : application à la déshydroxylation de la kaolinite -La compréhension du comportement de réacteurs industriels est difficile dans le cas de réactions solide-gaz. En effet la phase solide est un milieu granulaire dans lequel circulent des réactifs et des produits gazeux. Les propriétés d'un tel milieu sont modifiées dans l'espace et le temps en raison des réactions se produisant à une échelle microscopique. Les conditions thermodynamiques sont fixées non seulement par les conditions de fonctionnement du réacteur, mais aussi par la chaleur et les transferts de matière dans le réacteur. Nous proposons de résoudre numériquement les équations thermohydrauliques en les combinant avec les lois cinétiques qui décrivent les réactions hétérogènes. L'avantage majeur de cette approche est la grande variété des modèles cinétiques de transformation de grains disponibles (~40) comparée à l'approche habituelle, particulièrement dans le cas de germination en surface suivie de la croissance des germes. En effet, ce type de modèle doit permettre de décrire quantitativement la cinétique à l'échelle microscopique du grain, en fonction de la fréquence surfacique de germination et de la réactivité surfacique de croissance obtenues lors d'expériences isothermes et isobares. Les termes sources de chaleur et de matière entrant dans les bilans à l'échelle macroscopique dépendent de la cinétique à l'échelle microscopique. La résolution de ces équations permet d'obtenir la température et les pressions partielles dans le réacteur, qui a leur tour influencent le comportement cinétique. Oil & Gas Science and Technology -Rev. IFP Energies nouvelles, Vol. 68 (2013), No. 6, pp. 1039-1048 Copyright © 2013 Abstract -A General Approach for Kinetic Modeling of Solid-Gas Reactions at

Mass production of hydrogen is a major issue for the coming decades particularly to decrease greenhouse gas production. The development of fourth-generation high-temperature nuclear reactors has led to renewed interest for hydrogen production. In France, the CEA is investigating new processes using nuclear reactors, such as the Westinghouse hybrid cycle. A recent study was devoted to electrical modeling of the hydrogen electrolyzer, which is the key unit of this process. In this electrochemical reactor, hydrogen is reduced at the cathode and SO 2 is oxidized at the anode with the advantage of a very low voltage cell. This paper describes an improved model coupling the electrical and thermal phenomena with hydrodynamics in the electrolyzer, designed for a priori computational optimization of our future pilot cell. The hydrogen electrolyzer chosen here is a filter press design comprising a stack of identical cathode and anode compartments separated by a membrane. In a complex reactor of this type the main coupled physical phenomena involved are forced convection of the electrolyte flows, the plume of evolving hydrogen bubbles that modifies the local electrolyte conductivity, and all the irreversible processes that contribute to local overheating (Joule effect, overpotentials, etc.). The secondary current distribution was modeled with a commercial FEM code, Flux Expert 1 , which was customized with specific finite interfacial elements capable of describing the potential discontinuity associated with the electrochemical overpotential. Since the finite element method is not capable of properly describing the complex two-phase flows in the cathode compartment, the Fluent 1 CFD code was used for thermohydraulic computations. In this way each physical phenomenon was modeled using the best numerical method. The coupling implements an iterative process in which each code computes the physical data it has to transmit to the other one: the two-phase thermohydraulic problem is solved by Fluent 1 using the Flux-Expert 1 current density and heat sources; the secondary distribution and heat losses are solved by FluxExpert 1 using the Fluent 1 temperature field and flow velocities. A set of dedicated library routines was developed for process initiation, message passing, and synchronization of the two codes. The first results obtained with the two coupled commercial codes give realistic distributions for the electrical current density, gas fraction, and velocity in the electrolyzer. This approach allows us to optimize the design of a future experimental device. Notation CpHeat capacity (J kg -1 K -1 ) gGravitational acceleration (m s -2 ) J Current density (A m -2 ) Current density normal to the interface (A m -2 ) kThermal conductivity (W m -1 K -1 ) ñNormal vector NBN Number of integration points Q S Surface heat sources (W m -2 )Nanometric discontinuity thickness of potential (m) d S Dirac surface distribution e g Gas fraction qDensity (kg m -3 ) gOverpotential (V) rViscous stress tensor (Pa) rElectrical conductivity (X -1 m -...

This paper introduces the concept of ring magnets for magnetic beads (MBs) trapping in a capillary. Such magnets enable an easy insertion of a capillary simply like a pearl on a string. With this system, high magnetic forces are obtained thanks to the proximity between the magnet and the capillary, giving the opportunity to work at higher flow rates than with classical setups using two magnets with their magnetization perpendicular to the capillary. Moreover, by alternating magnets and non-magnetic spacers either in attraction or repulsion configuration, it is possible to form a chain and as a consequence to adapt the number of magnets to the desired number of plugs, thus controlling the surface available for molecule binding. Magnetic force mapping was first carried out by numerical simulations for a single ring magnet. The usefulness of this concept was then demonstrated with the achievement of an immunoassay and an online preconcentration experiment. To study the formation of multiplugs, the magnetic force was first simulated for a chain of four magnets in repulsion. This force was then introduced into a convection-diffusion model to understand the influence of the flow velocity on their size and position. The numerical simulations were qualitatively corroborated by microscopic visualizations, carried out in a capillary placed between rectangular magnets having a magnetization parallel to the capillary, and quantitatively by bead capture efficiency experiments.