Des polyanions, de formule chimique PW12O403-, ont pu être insérés au sein de la phase lamellaire de polymères non-ioniques C12EO4 (Brij-30). L’étude du diagramme de phases de ce système montre que le milieu aqueux peut accueillir une assez importante proportion de polyanions, jusqu’à atteindre une fraction volumique de l’ordre de 5 % dans le milieu aqueux. La diffusion des rayons X aux petits angles suggère que les polyanions sont en fait localisés, voire adsorbés, près des chaînes EO4. La production d’échantillons orientés de phase lamellaire nous a permis d’obtenir le facteur de structure des polyanions dans le plan des couches. Son étude montre que de telles nanoparticules chargées situées sur des bicouches de tensioactifs subissent une interaction répulsive. Enfin, des expériences de RMN à gradient de champ prouvent le caractère bidimensionnel de la diffusion des polyanions et confirment leur confinement près des chaînes EO4.

Glasses play an important role in modern technology. The large number of local packing arrangements in a glass underlies their important properties, including macroscopic homogeneity (e.g., the clarity of window glass) and the ability to be tuned by composition changes. A problem with glasses, also associated with their local disorder, is that they are unstable with respect to lower energy glasses and crystalline states. We have used physical vapor deposition and the mobility of glassy surfaces to prepare what are likely the most stable glasses on the planet. Our materials have the properties expected for “million-year-old” glasses, including high density, low enthalpy, and high mechanical moduli. Computer simulations verify and extend our understanding of these unusual materials, and allow a detailed investigation of structure/property relationships. These developments present major opportunities to learn about the lower portions of the potential energy landscape and to design new classes of anisotropic solids.

I would like to report my recent research regarding dynamic heterogeneities employing multi-point and multi-time correlations. My presentation will consist of two parts. (a) A three-point correlation function is calculated following the recent inhomogeneous mode-coupling theory (IMCT). We utilize a non-equilibrium molecular dynamics simulation, where the system is perturbed by the external potential coupled with the spontaneous density field. The three-point correlation is evaluated as the response of the two-point correlation due to the external field. Various scaling properties regarding the extracted length are investigated in comparison with the IMCT. (b) A multi-time extension of the usual four-point correlation is introduced to reveal the characteristic time scales of dynamic heterogeneities in glass-forming liquids. Molecular dynamics simulations are carried out for various models such as Kob-Andersen, Wahnstrom, soft-sphere, and Coslovich-Pastore network-forming liquids. From the comprehensive numerical calculations of the three-time correlation function, the temporal structures of the dynamic heterogeneities are characterized. We determine the associated lifetimes of above models and examine its role in the fragility.

ATTENTION: horaire et salle inhabituels ! Le séminaire est à 15h30 dans la salle Andromede. Cependant, à 15h, Gilbert rappellera les notions introduites dans son premier séminaire pour les personnes n'ayant pas pu venir au premier. Résumé: Il est généralement admis que les phénomènes quantiques reflètent intrinsèquement un indéterminisme fondamental. La mécanique quantique Bohmienne fournit un antidote à ce consensus! Je présenterai les postulats de cette mécanique et la manière dont son contenu déterministe cohabite avec l'élément statistique, menant, dans le cas non-relativiste, à des prédictions équivalentes à celles de la mécanique quantique orthodoxe. Cette dernière est un algorithme effectif dont les habituelles difficultés conceptuelles disparaissent complètement dans l'approche Bohmienne. J'aborderai enfin, brièvement, ce que je crois le plus pertinent dans cette approche: la possibilité d'une physique s'écartant des phénomènes quantiques standard, et l'enjeu de la construction d'une théorie relativiste des champs.

Epithelial cells can migrate collectively on free surface while maintaining strong cell-cell adhesions. To experimentally study this collective motility, we culture epithelial (MDCK) cells to confluence within the apertures of micro-stencils previously put on the substrate. The peeling-off of the stencil triggers the migration without damaging the border cells. This collective motility can be characterized by using Particle Image Velocimetry. It involves long-range coordinated displacements of large groups of cells well within the monolayer that are well described by a simple model of self-propelled interacting particles. In a second stage, the edges of these epithelia roughen drastically and exhibit a strong directional fingering led by a cell of different phenotype - a “leader cell” - initially not discernible from the others. Interestingly, similarly looking leader cells are found in a large number of different situations in morphogenesis or local invasion from tumors. In this talk I’ll question the nature of these cells and the properties of the migration fingers by using a variety of physical techniques (image analysis, force measurements, laser photoablation) together with the mapping of the biochemical activity of migration-involved small GTPases.

We consider the possibility that along the thermal history of the Universe, dark matter (DM) would have been created through a portal interaction between the visible sector (where the SM particles lie) and a hidden sector (where the DM particles lie). Depending solely on the DM particle mass, on the portal and on the DM hidden sector interaction, we show how the observed DM relic density can be obtained. There are four possible freeze-in/reannihilation/freeze-out regimes, which together result in a simple characteristic relic density phase diagram, with the shape of a "Mesa". In the case of a kinetic mixing portal, we show that, unlike other freeze-in scenarios discussed in the literature, the freeze-in regime can be probed by forthcoming DM direct detection experiments. This portal predicts furthermore an energy dependence of the signal which differs from the one predicted by usual WIMP DM models.

We are interested in proving the possibility and, more importantly, the impossibility of behaviours within large-scale models of biological networks. The presented results apply in the scope of qualitative models of biological networks, where each interacting component (gene, protein, ...) is represented by a variable varying within a finite (small) set of qualitative levels. These sets typically result from a quantization of the population of the species. We first introduce the Process Hitting formal language for the qualitative modelling of biological networks. The Process Hitting allows to specify the local interactions between components with various degrees of precision for the involved cooperations. Thanks to the particular structure of Process Hitting models, we show how to extract events that are necessary for activating a component of interest. Such a causality analysis actually results in a very compact abstract representation of the model dynamics, and can be used to prove the existence or absence of some reachability properties. Finally, we discuss how our method can be used to extract sets of components playing a crucial role for achieving a given reachability property, suggesting potential therapeutic targets. We illustrate our results with applications to boolean models of signalling and regulatory pathways, ranging from 100 to more than 9000 interacting components.

Partant d’une extension de l’approche d’Esptein-Glaser, on montre comment une formulation de la TCQ basée sur les distributions à valeur d’opérateurs (OPVD), avec des fonctions test appropriées,
traite de façon rigoureuse les singularités des propagateurs sur la diagonale. Ainsi les divergences IR et UV sont évitées à chaque étape, seules des renormalisations finies interviennent, reliées à
l’échelle arbitraire présente dans les fonctions test. Quelques cas bien connus de divergences UV de la TCQ standard sont pris en exemples. La puissance du traitement IR est démontrée pour la théorie du champ scalaire massif traité en tant que développement pertubatif en masse habituellement sans espoir. La conservation, obtenue en TLRS, de symétries fondamentales telles que l’invariance de jauge en QED et QCD, et l’invariance par rapport au choix de référentiel en dynamique du front de lumière, est exposée.