Design of a 2-D Cementation Experiment in Porous Medium Using Numerical Simulation

International audienceThe validation and qualification of reactive transport simulation tools has motivated the design and simulation, at a laboratory scale, of an experiment in which flow, advective/dispersive transport of solutes and physicochemical transformation affecting a porous medium are strongly coupled. Several possible experimental setups (or designs) have been evaluated using numerical simulation. The selected experimental design involves the successive precipitation and perforation of a clogging obstacle composed of calcium oxalate. Before an experiment can actually be conducted, this problem is proposed as a numerical benchmark for reactive transport codes and simulated using the coupled reactive transport code Hytec (CIG-École des mines, France)

Scroll waves in isotropic excitable media : linear instabilities, bifurcations and restabilized states

Scroll waves are three-dimensional analogs of spiral waves. The linear stability spectrum of untwisted and twisted scroll waves is computed for a two-variable reaction-diffusion model of an excitable medium. Different bands of modes are seen to be unstable in different regions of parameter space. The corresponding bifurcations and bifurcated states are characterized by performing direct numerical simulations. In addition, computations of the adjoint linear stability operator eigenmodes are also performed and serve to obtain a number of matrix elements characterizing the long-wavelength deformations of scroll waves.Comment: 30 pages 16 figures, submitted to Phys. Rev.

Évaluation des effets de mélanges de contaminants stables et radioactifs sur le nématode C. elegans à l’aide d’outils d’écotoxicologie prédictive.

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Etude expérimentale des effets combinés du mélange uranium – cadmium sur la croissance et la fécondité du nématode C. elegans

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Etude expérimentale des effets combinés du mélange uranium – cadmium sur la croissance et la fécondité du nématode C. elegans

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Nested interactions in the uranium and cadmium combined toxicity to the C. elegans.

International audienceUranium (U) is a natural ubiquitous radioelement whose occurrence may be magnified in the vicinity of some nuclear fuel cycle facilities orintensive farming areas. As mixtures are common in environment, U may be found associated with other contaminants such as cadmium (Cd).The exposure of organisms to these mixtures may result in unexpected synergism or antagonism that may be explained by interactions occurringat the exposure, toxicokinetics and/or toxicodynamic levels. The identification of these interactions and their underlying mechanisms may be ofinterest for a better risk characterization of multi-metallic polluted sites. In the present study, toxicity of binary mixture of U and Cd was assessedover time on physiological parameters, maximal length and brood size, in a partial lifespan experiment with the soil nematode Caenorhabditiselegans. A 49-condition fractional factorial design (extended design) was used with U and Cd concentrations, representative of some highlypolluted soils, ranging from 0.95 to 1.30 mM and 0.006 to 0.040 mM, respectively. Combined effects were analyzed using MixTox tool (Jonkeret al., 2005) to identify possible synergistic or antagonistic interactions.The best description of the toxic effects was met with the response addition concept and considering a dose-level dependent interaction model.An early significant antagonism was found at 1.2 d. Then, a continuous antagonism was identified for mixture levels upper than the EC36 or atlow U concentrations. As diet is suspected to be the main route of exposure of C. elegans to contaminants, an investigation of the diffusion of Uand Cd from agar to bacteria and their bioaccumulation by nematodes was achieved in a reduced design (8 conditions). A significant reductionof Cd internalization by C. elegans was observed in case of co-exposure with U and these results were correlated with the concentration of contaminants in bacteria. The observed antagonism of toxicity may thus be the resultant of an interaction at the exposure level where the diffusion oradsorption of Cd to bacteria is decreased by U. These findings underline the importance of studying chemical mixtures at various levels to fullyunderstand the underlying mechanisms and thus, avoid misinterpretations in the prediction of combined toxicity. Measurements of U and Cdconcentrations in bacteria are currently performed with the extended design in order to reinterpret the combined toxic effects on the basis ofthese concentrations

Combined effects of Uranium and Cadmium on Physiological Parameters of the Nematode Caenorhabditis elegans

International audienceUranium (U) is a natural ubiquitous radioelement whose occurrence may be magnified in the vicinity of some nuclear fuel cycle facilities orintensive farming areas. As mixtures are common in environment, U may be found associated with other contaminants such as cadmium (Cd).The exposure of organisms to these mixtures may result in unexpected synergism or antagonism that may be explained by interactions occurringat the exposure, toxicokinetics and/or toxicodynamic levels. The identification of these interactions and their underlying mechanisms may be ofinterest for a better risk characterization of multi-metallic polluted sites. In the present study, toxicity of binary mixture of U and Cd was assessedover time on physiological parameters, maximal length and brood size, in a partial lifespan experiment with the soil nematode Caenorhabditiselegans. A 49-condition fractional factorial design (extended design) was used with U and Cd concentrations, representative of some highlypolluted soils, ranging from 0.95 to 1.30 mM and 0.006 to 0.040 mM, respectively. Combined effects were analyzed using MixTox tool (Jonkeret al., 2005) to identify possible synergistic or antagonistic interactions.The best description of the toxic effects was met with the response addition concept and considering a dose-level dependent interaction model.An early significant antagonism was found at 1.2 d. Then, a continuous antagonism was identified for mixture levels upper than the EC36 or atlow U concentrations. As diet is suspected to be the main route of exposure of C. elegans to contaminants, an investigation of the diffusion of Uand Cd from agar to bacteria and their bioaccumulation by nematodes was achieved in a reduced design (8 conditions). A significant reductionof Cd internalization by C. elegans was observed in case of co-exposure with U and these results were correlated with the concentration of contaminants in bacteria. The observed antagonism of toxicity may thus be the resultant of an interaction at the exposure level where the diffusion oradsorption of Cd to bacteria is decreased by U. These findings underline the importance of studying chemical mixtures at various levels to fullyunderstand the underlying mechanisms and thus, avoid misinterpretations in the prediction of combined toxicity. Measurements of U and Cdconcentrations in bacteria are currently performed with the extended design in order to reinterpret the combined toxic effects on the basis ofthese concentrations

Bénard instabilities in a binary-liquid layer evaporating into an inert gas: stability of quasi-stationary and time-dependent reference profiles

This study treats an evaporating horizontal binary-liquid layer in contact with the air with an imposed transfer distance. The liquid is an aqueous solution of ethanol (10 % wt). Due to evaporation, the ethanol mass fraction can change and a cooling occurs at the liquid-gas interface. This can trigger solutal and thermal Rayleigh-B´enard-Marangoni instabilities in the system, the modes of which corresponding to an undeformable interface form the subject of the present work. The decrease of the liquid-layer thickness is assumed to be slow on the diffusive time scales (quasi-stationarity). First we analyse the stability of quasistationary reference profiles for a model case within which the mass fraction of ethanol is assumed to be fixed at the bottom of the liquid. Then this consideration is generalized by letting the diffusive reference profile for the mass fraction in the liquid be transient (starting from a uniform state), while following the frozen-time approach for perturbations. The critical liquid thickness below which the system is stable at all times quite expectedly corresponds to the one obtained for the quasi-stationary profile. As a next step, a more realistic, zero-flux condition is used at the bottom in lieu of the fixed-concentration one. The critical thickness is found not to change much between these two cases. At larger thicknesses, the critical time at which the instability first appears proves, as can be expected, to be independent of the type of the concentration condition at the bottom. It is shown that solvent (water) evaporation plays a stabilizing role as compared to the case of a non-volatile solvent. At last, an effective approximate Pearson-like model is invoked making use in particular of the fact that the solutal Marangoni is by far the strongest as an instability mechanism here