En présence d'un potentiel aléatoire, le mouvement classique d'une particule est typiquement diffusif, avec une constante de diffusion dépendant du degré de désordre. Quand les interférences quantiques entre différents chemins sont pris en compte, la situation peut être radicalement différente, conduisant en particulier à un phénomène de localisation appelé localisation d'Anderson, et à une transition diffusif-localisé, i.e. métal-isolant, quand on modifie l'amplitude du désordre. Ces effets sont très sensibles à la préservation de la cohérence de phase quantique et donc difficiles à observer expérimentalement. En utilisant des ondes de matière atomiques à très basse témpérature, manipulées par des champs lasers, on a pu observer directement la localisation d'Anderson, la transition métal-isolant et en mesurer finement les propriétés caractéristiques. Ces études ouvre des perspectives nouvelles pour l'étude du transport quantique, en présence de désordre et/ou d'interactions.

Beaucoup de protéines sont concernées par la formation
d'une interface, temporaire ou permanente, entre plusieurs sous-unités
de la dite protéine, ainsi la compréhension du processus d'assemblage
est un defi majeur de la biologie actuelle.

L'approche que nous developpons depuis 2008 interprète une interface comme
un graphe (au sens de la théorie des graphes).
La topologie du graphe revèle des informations importantes sur
la forme géométrique de l'interface et sur la fonction de chaque acide
aminé participant. En effet, il est important de différentier le rôle
des acides aminés dans la formation et le maintient des interfaces protéiques.

Par des statistiques sur un grand nombre de protéines, nous observons que
la géométrie caractérisée par la présence d'un feuillet beta intermoléculaire
montre des propriétés spécifiques: deux sous-réseaux typiques, une
certaine organisation spatiale du graphe, un emploi différentiel des
acides aminés dans les sous-graphes, et d'autres encore.

Une analyse mathématique de la minimisation symétrique, processus employé
pour interpreter la structure tridimensionnelle de l'interface et la
encoder sous la forme d'un graphe, a montré
que les interactions à l'interface peuvent être classées en niveaux
selon la géometrie locale. Ceci produit une hierarchie de graphes.

The way the dynamics of glass forming systems is modified by confinement, i.e. in systems with large interface, has been the subject of intense research in recent years. This seminar is intended to present an overview of the results obtained in our group on the glassy dynamics of glass forming polymers subjected to different kind of confinement, especially nanocomposites and thin films [1-6]. The segmental dynamics is probed by broadband dielectric spectroscopy (BDS), a technique able to probe the spontaneous fluctuations at equilibrium. The temperature marking the equilibrium to out-of equilibrium transition of the glass former, namely the glass transition temperature (Tg), and the physical aging, namely the slow evolution of disordered glassy material towards thermodynamic equilibrium occurring below Tg, are probed by differential scanning calorimetry (DSC). As a general important conclusion, we show that the out-of-equilibrium dynamics, in terms of Tg and physical aging, is decoupled from the equilibrium dynamics. This result markedly contrasts with the general view that the out-of-equilibrium dynamics is solely related to the molecular mobility of the glass former. This means that, apart from the intrinsic molecular mobility, geometrical factors are also important in determining the Tg and physical aging behavior of the glass former under confinement. Possible explanations to these results are discussed. [1] V. M. Boucher, D. Cangialosi, A. Alegría, J. Colmenero, J. Gonzalez-Irun, L. M. Liz-Marzan, Soft Matter 6, 3306 (2010). [2] V. M. Boucher, D. Cangialosi, A. Alegría, J. Colmenero, Macromolecules 43, 7594-7603 (2010). [3] V. M. Boucher, D. Cangialosi, A. Alegría, J. Colmenero, J. Gonzalez-Irun, L. M. Liz-Marzan, Soft Matter 7, 3607 (2011). [4] D. Cangialosi, V. M. Boucher, A. Alegría, J. Colmenero, J. Chem. Phys. 135, 014901 (2011). [5] D. Cangialosi, V. M. Boucher, A. Alegría, J. Colmenero, Polymer 53, 1362 (2012). [6] V. M. Boucher, D. Cangialosi, H. Yin, A. Schönhals A. Alegría, J. Colmenero, Soft Matter DOI: 10.1039/C2SM25419K.

Si le mot "matière molle" est largement utilisé dans beaucoup de recherches récentes, il ne semble pas avoir reçu de définition précise, sinon en opposition implicite avec la physique du solide. Un des aspects concrets de cette "matière molle" est l’absence d’angles, d’arêtes dans les matériaux concernés. Ceci renvoie à des effets physiques très négligés, ceux de la tension superficielle en mécanique des solides. Ils sont caractérisés par une échelle de longueur qui, dans les solides habituels, est d’ordre moléculaire. En revanche dans les matériaux mous, cette échelle de longueur peut dépasser largement ces dimensions moléculaires et même devenir macroscopique. Ainsi nous avons mis en évidence l’existence d’une instabilité de Rayleigh-Plateau dans un cylindre élastique de diamètre suffisamment petit, avec un bon accord théorie-expérience. La tension superficielle permet aussi de régulariser à petite échelle la croissance de l’instabilité de Biot d’un solide comprimé.

In this seminar, I will present two of our current research topics: the motion of soap films in tubes, and the response of a foam to air injection. The two subjects are strongly related to viscous dissipation in thin films, in relation with surfactant dynamics. We devised a simple experiment in which a single soap film is pushed at constant velocity through a tube. In the case of SDS soap films, expected to be in the mobile regime, we show how viscous friction in the transition zones between the Plateau borders and the wetting film controls the departure from the equilibrium shapes. We propose a simple theoretical criterion for the highest possible velocity of a soap film through a tube, for different tube geometries, in excellent agreement with our measurements [Dollet & Cantat 2010]. As a model system for immobile interfaces, SLES/CAPB/MAc [Golemanov et al. 2008] displays a wealth of behaviors very distinct from SDS. We show that the moving soap film influences the wetting film at long distances in front of him. The thickness profile of the wetting film varies continuously along this "influence" zone. The soap film can show an intermittent regime, strikingly reminiscent of stick-slip. We attempt to rationalize these observations, by coupling hydrodynamics in the films and surfactant properties. The response of a two-dimensional liquid foam to a localised air injection is investigated experimentally and theoretically. The experiments show a rich phenomenology (figure 1). At low flux, the injected air forms a central bubbles that grows inside the foam and induces plastic rearrangements, without film rupture. This "pure swelling" regime is reminiscent of ductile fracture. In this regime, the central bubble shows fingering patterns beyond a certain velocity. The dependences between the swelling rate, the injection overpressure and the other control parameters: cell gap, bubble size and foam area, is captured by a simple balance between the pressure drop and bubble/wall friction, within a radial assumption. Fingering is successfully modelled by the linear stability analysis of an azimuthal perturbation of the radial model; yield stress becomes an important parameter to determine the finger width. At high injection rate, films are broken and narrow cracks form rapidly through the foam, which reminds brittle fracture. Criteria on the transition between ductile and brittle behaviours are investigated, both at the local and global scales.

Living cells rely heavily on cytoskeletal transport as an efficient way of delivering vesicles and biochemically active cargos within the cytoplasm for cell viability and function regulation. Non-equilibrium stochastic motion of molecular machines such as motor proteins on cytoskeletal filaments plays a fundamental role in intracellular and intercellular traffic and cytoplasm dynamical organization, whereas perturbations of these functions are known to lead to diseases.
Recent progresses in experimental techniques are giving access to a wealth of data down to the single-molecule level in-cellulo, and models are becoming increasingly important in order to interpret these data and understand general mechanisms.
One approach to obtain fundamental insights into collective mechanisms and provide predictions for experimental situations is to use non-equilibrium statistical mechanics transport models.
Building on our recent work on one of the most paradigmatic lattice gas models, the “Totally Asymmetric Simple Exclusion Process” (TASEP), we generalize TASEP on networks of increasing complexity: from complex junctions up to large random networks that better mimic the cytoskeleton complexity in cells.
We provide here a general theoretical framework supported by simulations, numerical and analytical results, to acquire a throughout understanding of collective stochastic transport of motor proteins on simple and complex topologies, with emergent properties, dependent on the network connectivity, which could be observed in experiments.
Last but not least, the understanding of non-linear transport processes on complex networks inspired by this study can provide hints of great interest also for fundamental and applied sciences not necessarily related to the biological realm.

There exist various approximation methods (CFT, Luttinger liquid, non-linear Luttinger liquid) allowing one to study certain limiting regimes of the behavior of various correlation functions in one dimensional gapless quantum Hamiltonians. In this talk, I will overview the recent developments in respect to setting up yet another approach to characterizing certain properties of the correlatior in one dimensional gapless models. The approach that I will discuss is based on the study of form factor expansions for two-point functions. Under a specific assumption on the structure of the large-volume asymptotic behavior of the form factors and of the spectrum, I will argue that, starting from a gapless model in large but finite volume, it is possible to sum-up the form factor expansion in the large-distance and/or long-time regime. In such a way, one reporduces the CFT and Luttinger-liquid based predictions for the long-distance asymptotics of two-point functions. One is furthermore able to access to the
large-distance and long-time asymptotics. Finally, I will also breifly discuss the progress in the computation, starting from the first principles, of the edge exponents characterizing the power-law behavior of the density structure factor and the spectral functions near the edges of the particle or hole excitation tresholds. I will illustrate the talk with examples arizing in integrable models such as the XXZ spin-1/2 chain or the non-linear Schr\"odinger model. In particular, I will show that one reproduces the predictions of the amplitudes and the edge exponents stemming from the use of the non-linear Luttinger liquid theory. These results stem from a joint work with Kitanine, Maillet, Slavnov and Terras.

What determines the mechanical properties of dense collections of active
particles? The answer to this
question is highly relevant to a wide range of physical and biological
phenomena from tissue formation
to the dynamics of vibrated granular layers. We present a numerical study of
the phases and dynamics
of a dense collection of self-propelled particles with soft repulsive
interactions with and without polar alignment.

The phase diagram of the confined aligning system consists of a polar liquid
phase at low
packing fraction and high self-propulsion speed, and an active jammed phase
at high density and low
self-propulsion speed. The liquid phase exhibits local alignment and giant
number fluctuations typical
of the Vicsek class of models. The dynamics of the jammed phase is dominated
by oscillations along
the low frequency modes of the underlying packing. We show analytically that
at long times the energy
is carried entirely by the lowest available excitations of the system. We
measure and compare the local stresses in our active system, with added
attraction, to both granular materials and the tissue experiments.

Les lasers à cascade quantique sont des émetteurs unipolaires dont la fréquence d'émission est ajustable depuis la gamme THz jusqu'à l'infrarouge moyen.  Mais il est de plus en plus difficile de réaliser l'inversion de population lorsque la fréquence laser diminue. Après un rappel des propriétés et du fonctionnement de ces hétérostructures nous examinerons les raisons des pertes non radiatives de porteurs d'une part ainsi que les mécanismes de réabsorption des photons émis par les électrons de la structure.