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A Lemarchand

Publications and source records attributed to A Lemarchand.

11 recordsLinked to original sources
PubMed2006

Microscopic simulations of an exothermic chemical wave front: departure from the continuity equations.

We perform microscopic simulations of a reactive dilute gas and study the propagation of an exothermic chemical wave front in an infinite, one-dimensional medium. The simulation results concerning front propagation speed and concentrations, temperature and stream velocity profiles are compared with the results of the integration of the macroscopic continuity equations in the case of fast reactions. The discrepancies between the two approaches are related to a chemically induced departure from local equilibrium.

Journal Article↗
PubMed2006

Molecular-dynamics simulations and master-equation description of a chemical wave front: effects of density and size of reaction zone on propagation speed.

We compare the master-equation description and molecular-dynamics simulations of a chemical wave front. We find that the front propagation speed depends on the number of particles in the reaction zone. For the master equation the dependence follows the well-known logarithmic prediction obtained when introducing a cutoff into the macroscopic reaction-diffusion equation. The molecular-dynamics simulations reveal that the logarithmic law is compromised for dense fluids.

Journal Article↗
PubMed2005

Symmetry-broken reactant motion upon phase-related symmetrically modulated excitations: application to highly selective molecular sorting.

This paper introduces a separation protocol relying on affinity chromatography that exhibits unprecedented selectivities. We submit the mixture contained in the separative medium to the simultaneous action of two symmetrically modulated excitations. The first is a uniform periodic field (e.g., electric field) with zero mean value, whereas the second is the periodic modulation of a thermodynamic parameter such as the temperature. Under appropriate tuning of the modulations with the dynamics of the discriminating chemical reaction, we predict a symmetry breaking of molecular motion: the mixture components that are addressed by their rate constants exhibit an oriented motion for a particular phase relation between the modulations of the field and the thermodynamic parameter. The resulting velocity of the mixture components depends on the rate constants and on a conjugated thermodynamic value such as the standard enthalpy of the discrimination process in the case of a temperature modulation. In particular, it may be possible to separate mixture components with identical rate constants. We use the present approach to design a protocol to sort nucleic acids by their sequence.

Algorithms↗
PubMed2003

Sensitivity of explosion to departure from partial equilibrium.

We study a thermochemical gaseous system in the vicinity of the bifurcation related to the emergence of bistability. Corrections to the standard deterministic dynamics induced by the perturbation of the particle velocity distribution are obtained from the solution of the Boltzmann equation. Using these results, analytical expressions including the nonequilibrium effects are derived for the ignition time in the explosive regime and mean first passage time in the bistable regime. It is demonstrated that a departure from partial equilibrium can shift the bifurcation point. The system which was bistable according to the standard deterministic approach, can become monostable and explosive in the presence of nonequilibrium effects. Even when the system remains in the bistable regime, the mean first passage time can be changed by several orders of magnitude. In the monostable domain, the ignition time can be about ten times smaller than the unperturbed value. These analytical predictions agree well with the results of the microscopic simulations of the dilute gas system.

Journal Article↗
PubMed2001

Stochastic effects in a thermochemical system with Newtonian heat exchange.

We develop a mesoscopic description of stochastic effects in the Newtonian heat exchange between a diluted gas system and a thermostat. We explicitly study the homogeneous Semenov model involving a thermochemical reaction and neglecting consumption of reactants. The master equation includes a transition rate for the thermal transfer process, which is derived on the basis of the statistics for inelastic collisions between gas particles and walls of the thermostat. The main assumption is that the perturbation of the Maxwellian particle velocity distribution can be neglected. The transition function for the thermal process admits a continuous spectrum of temperature changes, and consequently, the master equation has a complicated integro-differential form. We perform Monte Carlo simulations based on this equation to study the stochastic effects in the Semenov system in the explosive regime. The dispersion of ignition times is calculated as a function of system size. For sufficiently small systems, the probability distribution of temperature displays transient bimodality during the ignition period. The results of the stochastic description are successfully compared with those of direct simulations of microscopic particle dynamics.

Journal Article↗
PubMed2000

Macroscopic effects of the perturbation of the particle velocity distribution in a trigger wave

Departure from the equilibrium particle velocity distribution induced by a chemical reaction is studied in an inhomogeneous chemical bistable system in which a trigger wave can propagate. These nonequilibrium effects influence the speed and shape of the trigger wave propagating between the two stable stationary states. In contrast to the Fisher front, the discretization of the variables and the internal fluctuations do not sensitively modify the macroscopic properties of the trigger wave. Analytical results deduced from the Boltzmann equation agree well with microscopic simulations using Bird's method. Both approaches lead to large relative corrections to the front speed, in particular for parameter values close to that corresponding to a stationary interface between the two stable states.

Journal Article↗
PubMed1985

Modified oligonucleotides as alternatives to the synthesis of mixed probes for the screening of cDNA libraries.

Two simple alternatives to the synthesis of mixed oligodeoxyribonucleotide probes are described: mixed deblocking of triazolodeoxynucleosides for the A/G and C/T degeneracies or incorporation of 2-amino-2'-deoxyadenosines into a determined sequence for a higher stability of the hybridization duplexes. The synthetic oligodeoxyribonucleotides obtained were successfully tested in comparison with a classical mixed probe for their capacity to identify specific cDNA clones of human antithrombin III. The results are discussed with respect to the utilization of these synthetic oligonucleotides as hybridization probes for the isolation of cloned cDNA sequences.

Antithrombin III↗