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Colloquium 2013-2014


Monday, June 30th, 11AM:
Advection-diffusion-réaction de scalaires passifs et grandes déviations

J. Vanneste (University of Edinburgh)

La dispersion de scalaires passifs dans un écoulement fluide résulte de l’interaction entre l'advection par l’écoulement et la diffusion moléculaire. Leur effet combiné est souvent décrit par une diffusivité effective, bien plus grande que la diffusivité moléculaire, qui permet de représenter le profil de concentration aux temps longs par une Gaussiennne. Cette approximation n’est cependant pas valable à grande distance du centre du profil ; il faut alors lui substituer une approximation de type grandes deviations, caractérisée par une fonction de taux (ou de Cramér). Nous discuterons du calcul de cette fonction — en particulier de sa forme asymptotique lorsque la diffusivité moléculaire est faible — dans des écoulements cisaillés classiques et dans des écoulements cellulaires périodiques. Les résultats seront aussi appliqués à des problèmes de propagation de fronts pour des scalaires réactifs : pour des réactions de type Fisher-Kolmogorov, la vitesse de propagation des fronts est directement reliée à la fonction de taux du problème de dispersion correspondant. Dans ce cas, les grandes déviations gouvernent un phénomène macroscopique dominant plutôt que des événements rares.

Monday, June 23th, 11AM:
Order from Chaos - The Solar Cycle

Steve Tobias (University of Leeds)

In this talk I will discuss the observations of and the mechanisms responsible for the generation of magnetic field in our nearest star, the Sun. The systematic eleven year solar cycle is generated by the interaction of turbulence with rotation and magnetic fields. Though a mechanism describing the generation was presented by E.N. Parker over 50 years ago, it is only within the last couple of years that this has been made to work in the astrophysically relevant limit of high magnetic Reynolds number. I shall conclude by speculating on whether statistical approaches may prove useful in describing the systematic evolution of the solar cycle.

Monday, June 16th, 11AM:
Self-assembled towers, arches, and ordered particle structures

Arshad Kudrolli (Clark University)

We will discuss experiments on creating designed structures with particles using capillarity and geometry. First, I will demonstrate an unprecedented variety of towers and arches observed by simply dripping a mixture of sand and water on a porous substrate. The various shapes are in contrast with the liquid drops which can splash, spread or bounce upon hitting a surface. Successive drops are observed to freeze rapidly upon impact due to the drainage of a small fraction of liquid, literally stacking on top of each other into surprisingly slender structures. Next, we will discuss experiments investigating the packing structure of spheres, rods, and the Platonic solids with various packing protocols. A rich array of structures can be demonstrated to be formed using 3-D imaging techniques including X-ray tomography. Besides hexagonal close packed and face centered cubic exhibited by spheres, we show that cubes can grow in a simple cubic lattice or in a disordered packing by changing the deposition angle with respect to the substrate, and cylinders/rods form crystalline packing with axis aligned with gravitational field.

Monday, May 26th, 11AM:
L'irréversibilité en dynamique des gaz, une histoire de probabilités

Laure Saint-Raymond (LJLL-UPMC, Paris)

Le but de cet exposé est de présenter une dérivation de l'équation de Boltzmann à partir de la dynamique hamiltonienne des particules dans la limite de Boltzmann-Grad, i.e. quand le nombre de particules "N" tend vers l'infini et leur taille "a" tend vers 0 avec N a^2 =1. On expliquera en particulier l'origine de l'irréversibilité et du phénomène de retour à l'équilibre, qui sont des propriétés apparemment paradoxales de la dynamique limite.

Monday, May 19th, 11AM:
Non equilibrium via rare fluctuations

Giovanni Jona-Lasinio (La Sapienza, Rome)

Non equilibrium is ubiquitous around us and presents a variety of phenomena much richer and more complex than equilibrium. The simplest situations are probably states of a system traversed by a constant flow of matter, electric charge, heat,....which are called stationary states. Experience has shown that it is not easy to extend to these states basic thermodynamic concepts e.g. entropy or free energy. It is clear that dynamics has to play a crucial role in non equilibrium. Macroscopic evolution equations of diffusive system are provided by hydrodynamics. The study of rare hydrodynamic fluctuations has become an important tool in recent work on stationary states. It is combined, more or less explicitly, with ideas which are one century old and have their roots in certain views of Einstein on probability in equilibrium thermodynamics. An overview of the basic ideas will be presented in the seminar.

Monday, May 12th, 11AM:
Random Matrices for Wireless Communcations: When Wigner meets Shannon

Merouane Debbah (SUPELEC)

The asymptotic behaviour of the eigenvalues of large random matrices has been extensively studied since the fifties. One of the first related result was the work of Eugène Wigner in 1955 who remarked that the eigenvalue distribution of a standard Gaussian hermitian matrix converges to a deterministic probability distribution called the semi-circular law when the dimensions of the matrix converge to infinity. Since that time, the study of the eigenvalue distribution of random matrices has triggered numerous works, in the theoretical physics as well as probability theory communities. However, as far as communications systems are concerned, until the mid 90’s, intensive simulations were thought to be the only technique to get some insight on how communications behave with many parameters. All this changed in 1997 when large system analysis based on random matrix theory was discovered as an appropriate tool to gain intuitive insight into communication systems. In particular, the self-averaging effect of random matrices was shown to be able to capture the parameters of interest of communication schemes. Since the year 2000, the results led to very active research in many fields such as MIMO systems currently used in 4G technologies. This talk is intended to give a comprehensive overview of random matrices and their application to the analysis and design of futur green wireless communication systems (4G and 5G systems).

Monday, April 14th, 11AM:
Scaling in cities

Marc Barthelemy (CEA Saclay)

The recent availability of data about cities and urban systems opens the exciting possibility of a 'new Science of Cities'. Urban morphology and morphogenesis, activity and residence location choice, mobility, urban sprawling and the evolution of urban networks are just a few of the important processes that can be discussed now from a quantitative point of view. In this talk, I will show how the combination of data, physical arguments and modeling helps us in understanding urban systems. I will discuss socio-economical indicators that can describe various aspects of cities such as the commuting length, the total length of the street network, the total delay due to congestion, etc. The variation of these quantities with population usually reveals interesting scaling laws with non trivial exponents that call for an explanation. I will show here how a physicist approach can help in solving these puzzles.

Monday, April 7th, 11AM:
Two-dimensional Turbulence: Fluids, Superfluids, Conducting Fluids, and Polymeric Fluids

Rahul Pandit (Indian Institute of Science, Bengalore)

It is well known that two-dimensional (2D) fluid turbulence has statistical properties that are qualitatively different than those of three-dimensional 3D turbulence. In this talk, I begin with an introduction to 2D, homogeneous, isotropic turbulence, contrast it with its 3D counterpart, and then present an overview of some of the results we have obtained on 2D turbulence over the past few years. In particular, I present our studies of (a) persistence-time statistics in 2D fluid turbulence, (b) the statistical properties of the geometries of the trajectories of heavy inertial particles in 2D fluid turbulence, (c) the turbulence of coupled, 2D, normal and superfluids, (d) inverse cascades and scaling behaviours in 2D magnetohydrodynamic (MHD) turbulence, and (e) the remarkable effects of polymer additives on 2D fluid turbulence. The studies I describe have been carried out in collaboration with Anupam Gupta, Dhrubaditya Mitra, Prasad Perlekar, Samriddhi Sankar Ray, Debarghya Banerjee, Vishwanath Shukla, and Akshay Bhatnagar.

Monday, March 31th, 11AM:
Dynamique et thermodynamique des systèmes avec des interactions a longue portée.

Pierre-Henri Chavanis (LPT Toulouse)

Les systemes avec des interactions a longue portee sont nombreux dans la nature. Quelques exemples concernent les systemes gravitationnels (etoiles, amas globulaires, galaxies), les tourbillons des ecoulements geophysiques et astrophysiques (cyclones et anticyclones des bulletins meteorologiques, tache rouge de Jupiter), et la chimiotaxie des populations de bacteries (clusters, vasculature). En outre, des modeles jouets comme le modele Hamiltonian Mean Field (HMF) et le modele Brownian Mean Field (BMF) ont ete introduits en physique statistique pour etudier ces systemes dans un cadre simplifie. Je proposerai une promenade dans le monde des systemes avec des interactions a longue portee. Je parlerai des particules Browniennes auto-gravitantes et de leur analogie avec la chimiotaxie des bacteries et avec la condensation de Bose-Einstein, puis je decrirai les transitions de phase d'un gaz de fermions auto-gravitants a temperature non nulle. Enfin, je parlerai de la dynamique et de la thermodynamique tres particulieres des modeles HMF et BMF.

Monday, March 24th, 11AM:
Mouvement à long terme dans le Système Solaire

Jacques Laskar (IMCCE, Observatoire de Paris)

Les intégrations à long terme du mouvement des planètes du système solaire ont été un défi des dernières décennies. Les progrès dans ce domaine ont suivi l’évolution du perfectionnement des ordinateurs, mais aussi l’amélioration des algorithmes d’intégration numérique, qui ont abouti au développent d’intégrateurs symplectiques d’ordre élevé qui ont une bonne stabilité à long terme. En même temps, la parallélisation des algorithmes a aussi permis une réduction des temps de calculs. L’intégration numérique des équations est seulement une partie du travail, car il faut aussi déterminer avec précision les conditions initiales et paramètres du modèle par comparaisons aux observations existantes. Une fois que toutes ces étapes sont satisfaites, la principale limitation dans l’obtention d’une solution précise pour le mouvement des planètes réside dans la nature chaotique du système qui limite la validité des solutions à environ 60 millions d’années.

Monday, March 17th, 11AM:
Dynamique de l’adaptation chez les bactéries.

Jean Baudry (LCMD, ESPCI)

Les bactéries ont développé beaucoup de stratégies pour s’adapter au stress et à la pression de sélection de leur environnement. Pour comprendre ces mécanismes, une première approche, la plus développée aujourd'hui, est d'étudier à l'échelle moléculaire les processus mis en jeu. La deuxième approche, détaillée ici, est de s'intéresser à l'échelle intégrée du microorganisme. Nous avons développé une approche basée sur la manipulation de goutte d’eau dans de l’huile qui permet de caractériser simplement les propriétés observables de bactéries, c'est-à-dire leur phénotype. Ces mesures quantitatives donnent accès à la dynamique de l’adaptation des bactéries face à un stress. Je détaillerai ici l’exemple de la variation de phase, mécanisme qui permet aux bactéries de s’adapter plus rapidement à un changement d'environnement, puis décrirai la diversification mesurée dans un système en l’absence de nourriture (phase stationnaire prolongée).

Monday, February 24th, 11AM:
Strings, Integrable Models and Biology - A New Frontier?

Antti Niemi (LMPT, Universite de Tours)

The biological function of a protein depends critically on its three dimensional geometry. But at the moment we do not know how the shape of a protein could be deduced from the DNA sequence alone. As a consequence the protein folding problem endures as one of the most important unresolved problems in science, it addresses the origin of life itself. In this talk we shall argue that the shape of a protein can actually be determined from very general principles, that are also utilized in the context of string theory and integrable models. We shall argue, that there is a universal energy function which relates to the discrete nonlinear Schrodinger equation, the paradigm integrable lattice model, that describes all known folded protein structures. We show how to derive this energy function from fundamental geometrical concepts. We show that it supports soliton solutions, that describe folded proteins with a precision where the root-mean-square distance between an experimental crystallographic structure in Protein Data Bank and its theoretical description is less than the radius of a carbon atom. We present a number of examples of numerical simulations that show how a protein folds. The simulations are performed with a laptop computer, and the simulation proceeds practically as fast as the folding does in vivo.

Monday, February 17th, 11AM:
Fluctuations à proximité d’un point critique

Caroline Crauste (Laboratoire de Physique, ENS de Lyon)

La description des systèmes hors équilibre est encore une question ouverte, de même que le retour à l’équilibre d’un système après perturbation. Si un système retourne à l’équilibre avec un temps de relaxation plus long que l’échelle de temps de l’expérience, que ce soit un système vitreux ou un système ralenti par la proximité d’un point critique, les fonctions de réponses ou de corrélation ne dépendent plus seulement du temps ou de la fréquence d’acquisition mais aussi du temps écoulé depuis la trempe, le temps d’attente. A l’approche d’un point critique comme le point critique de démixtion d’un mélange binaire, le temps de relaxation et la longueur de corrélation divergent, les fluctuations de concentration, de densité, ou de réponse diélectrique deviennent prépondérantes. Je présenterai ici tout d’abord un bref aperçu de mes travaux de thèse et de mon premier post- doctorat, sur la transition vitreuse, le vieillissement et l’auto-organisation dans les liquides. Je détaillerai ensuite les travaux effectués en post-doctorat dans le cadre de l’ERC de Sergio Ciliberto et mes projets à plus long terme.

Monday, February 10th, 11AM:
Ondes et sillages à la surface de l'eau

Elie Raphael (Gulliver, ESPCI)

Les ondes de propageant à la surface de l’eau sont à la fois fascinantes et d’une grande importance pratique. Elles peuvent être générées par le vent à la surface de l’océan, par un bateau se déplaçant sur les eaux calmes d'un lac, ou par un caillou lancé dans une mare. Ces ondes résultent d'un bilan entre l'inertie du liquide (qui tend à maintenir le mouvement) et sa tendance, sous l'action des forces de gravité et des forces de tension de surface, à retourner vers un état d’équilibre horizontal. Nous nous intéressons ici aux ondes crées par une perturbation se déplaçant à la surface de l’eau et analysons à la fois le sillage et la résistance de vague. Les applications concernent aussi bien le mouvement des navires que la locomotion de petits insectes vivant à la surface de l’eau (tels les gyrins).

Monday, January 27h, 11AM:
Hyperelastic fingering in soft solids and viscoelastic liquids

Elisabeth Bouchaud (CEA Saclay, ESPCI Paris)

While fingering instabilities in confined viscous fluids have been extensively studied, purely elastic instabilities arising in confined elastomers have not been often reported. Our experiments show that if a fluid penetrates into a polyacrylamide gel which is confined in a Hele-Shaw cell, the interface destabilizes as soon as the strain overpasses a critical value which is independent of the shear modulus of the gel. This instability manifests itself through the formation of fingers which grow within the bulk of the material. It is sudden and hysteretic, which suggests a sub-critical character. Key elements of the physical mechanism are the adhesion of the gel to the cell plates, its incompressibility, its confinement, and its ability to sustain huge strains before breaking. We also study a perfect Maxwell fluid (characterized by a single relevant frequency) made of a microemulsion of oil droplets linked by telechelic polymers in water. When the flow rate is high enough, we observe the same elastic instability as for the gel. At low flow rates, we show that the nature of the fingering instability is of a Saffman-Taylor type.

Monday, January 20th, 11AM:
Laboratory experiments of turbulence-particle interactions: preferential concentration, modified gravitational settling and collision/coalescence

Alberto Aliseda (University of Washington, Seattle)

Turbulent multiphase flows are of great relevance in many industrial and environmental applications, and still present many challenges to the scientific and engineering communities. Injection of fuel droplets in a combustion chamber, droplet collisions in atmospheric clouds or drug delivery particles in the human circulation are some examples of the relevance of this problem and the lack of understanding that hinders progress in the accurate modeling and prediction of these processes. In this talk, I will focus on inertial effects in the dynamics of droplets injected into a turbulent flow. The interaction of the droplets with the turbulent vortical structures results in accumulation of droplets in regions of high strain and the modification of the drift velocity of droplets due to gravity. Both of these effects lead to a higher probability of collisions due to smaller inter-droplet distance and higher relative velocities. I will describe an experimental-numerical collaboration aimed at solving this long standing problem. The dynamics of bubbles in turbulent flows are also affected by these phenomena. I will discuss experiments and modeling of the dynamics of bubbles in both homogeneous isotropic turbulence and the human circulation.

Monday, January 13th, 11AM:
Exact solution of integrable quantum models by separation of variables.

Giuliano Nicolli (Laboratoire de Physique, ENS de Lyon)

Quantum integrability and classical exact solvability are subjects of great relevance not only in the mathematical physics area (quantum groups etc.) but also in research areas like statistical mechanics, condensed matter theory, string theory and gauge theory. Prototypical models in this class are the Heisenberg (XY, XXX, XXZ) chains, for the quantum description of magnetism and the ice-type (six-vertex) models for the statistical mechanics description of crystal lattices with hydrogen bonds. The exact solution of several fundamental models, by the complete characterization of their spectrum and dynamics (correlation functions) remains still one fundamental open problem in mathematical physics, of great interest as it should lead to non-perturbative results in several areas of physics. Here, we present an approach aimed to solve this problem by quantum separation of variables. This is the natural quantum analogue of the classical method of separation of variables and allows a more symmetric description of classical and quantum integrability w.r.t. traditional Bethe ansatz methods. Moreover, it has the advantage to be applicable to a large class of models for which its implementation gives a characterization of the spectrum complete by construction. In the seminar the Hydrogen atom will be used to recall the main ideas about classical and quantum separation of variables as it defines one of the simplest and still nontrivial examples of integrable models. Then, my approach will be presented for a paradigmatic model of relativistic integrable QFT, the sine-Gordon model. The results for both the spectrum and the dynamics of its lattice regularization will be described and the beautiful feature of universality, emerging from the comparison with the results of other key integrable quantum models, will be pointed out.

Monday, January 6th, 11AM:
Contrôle des interactions entre chromophores et nanoparticules inorganiques. Applications à l'imagerie bi-photons et la protection contre les lasers.

Stephane Parola (Laboratoire de Chimie, ENS de Lyon)

Le domaine des nanomatériaux est en plein essor en raison des innombrables possibilités d’architectures proposées et des propriétés générées par les effets de taille (optique, mécaniques, électroniques..). La possibilité d’associer un nanoobjet inorganique à un système organique moléculaire ouvre des perspectives importantes pour le contrôle et la combinaison d’un certain nombre de propriétés, soit de la partie organique soit de la partie inorganique, telles que les réponses optiques. Nous nous sommes intéressés aux interactions dans des architectures nanohybrides organominérales et en particulier à l’impact de l’interface ou de phénomènes plasmoniques sur les propriétés optiques (luminescence, absorption non linéaire). Les résultats obtenus seront illustrés par des exemples d’applications en bio-imagerie et pour la protection optique.

Monday, December 16th, 11AM:
La dualité onde-particule à l’œil nu

Emmanuel Fort (Institut Langevin ESPCI ParisTech)

We have recently discovered a macroscopic object composed of a material particle dynamically coupled to a wave packet. The particle is a droplet bouncing on the surface of a vertically vibrated liquid bath; its pilot-wave is the result of the superposition of the surface waves it excites. Above an excitation threshold, this symbiotic object, designated as a "walker" becomes self-propelled. Such a walker exhibits several features previously thought to be specific to the microscopic realm. The unexpected appearance of both uncertainty and quantization behaviors at the macroscopic scale lies in the essence of its “classical” duality. The dynamics of the droplet depends on previously visited spots along its trajectory through the surface waves emitted during each bounce. Although based on fundamental concepts, commonly found in living systems, this path-memory driven dynamics is still unexplored in physics elementary objects. This new class of memory-encoded systems which possess a spatio-temporal non-locality shakes the frontiers between macroscopic and microscopic world. In this talk, I will first briefly present the dynamics of this object in experiments analog to the ones in quantum physics (diffraction and interference through slits, Landau quantization). Then, I will focus on the recent results we obtained showing the emergence of self-organized eigenstates for walkers submitted to a central force.

Monday, December 2nd, 11AM:
What can impurities tell us on the iron based superconductors? Mn and Ru, the evil and the good.

Pietro Carretta (Universite de Pavia, Italie)

I shall review on the effect of diamagnetic and paramagnetic impurities on the microscopic properties of optimally doped LnFeAsO1−yFy iron-based superconductors (Ln a lanthanide ion), in the light of recent µSR and NMR studies. Two limiting cases will be considered: the effect of the isovalent diamagnetic substitution of Fe by Ru and the effect of the doping with Mn paramagnetic impurities. In the first case it will be shown that Ru doping causes the appearance of static magnetism, nanoscopically coexisting with superconductivity. The magnetic ordering temperature TN and the order parameter display a dome-like dependence on the Ru content x, peaked around x = 1/4, with higher TN values for those materials characterized by a larger z cell coordinate of As. Remarkably, those are also the ones with the highest superconducting transition temperatures (Tc) in the impurity free compound. The reduction of Tc is found to be significant in the region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only for an impurity content x 0.6. On the other hand, Mn doping yields a significant reduction of the superconducting transition temperature which is particularly pronounced for Ln=La. Indeed, in LaFeAsO1−yFy Tc drops from its optimal value to zero for a Mn content around 0.2%, more than two orders of magnitude less than the Ru content requiered to suppress it! At larger Mn doping levels electron localization and a magnetic order are recovered. The implication of these results on the possible pairing mechanisms underlying those materials will be discussed. A fruitful comparison with the effect of impurities on the undoped magnetic pnictides will also be presented .

Monday, November 25th, 11AM:
Les Oxides de Pyrochlore Magnétiques: une Ecole Permanente en Magnétisme & Mécanique Statistique

Michel Gingras (Department of Physics & Astronomy, University of Waterloo and Canadian Institute for Advanced Research/Quantum Materials ProgramWaterloo University, Canada)

Le magnétisme, aussi bien du coté expérimental que théorique, s’avère depuis longtemps un contexte unique pour explorer, découvrir et comprendre les principes généraux qui régissent les phénomènes collectifs de la Nature. Depuis une vingtaine d’années, les physiciens de l’état condense s’attaquent avec enthousiasme au problème du magnétisme hautement frustré, que l’on rencontre quand les diverses interactions magnétiques microscopiques sont en forte compétition. Une classe de composés hautement frustrés particulièrement intéressants, et qui font couler beaucoup d’encre, sont les oxides de pyrochlores magnétiques, de formule chimique A2B2O7 (A=Y, Gd, Tb, Dy, Ho, Yb ; B=Mo, Ti, Sn). Dans ces systèmes, les ions A3+ et B4+ occupent chacun deux réseaux indépendants de tétrahèdres connectés par leurs sommets – une architecture apte en engendrer une haute frustration quand les interactions magnétiques entre ions sont effectivement antiferromagnétiques. Dans cette présentation, je ferai un survol de la physique de ces composés, mettant emphase sur les leçons demécanique statistique qu’ils nous ont appris et les devoirs de magnétisme quantique qui restent à faire.

Monday, November 18th, 11AM:
Information processing in gene regulatory circuits

Aleksandra Walczak (LPT, ENS)

Many of the biological networks inside cells can be thought of as transmitting information from the inputs (e.g., the concentrations of transcription factors or other signaling molecules) to their outputs (e.g., the expression levels of various genes). Given the molecular limits (small concentrations, intrinsic randomness), not all networks perform equally well, and maximizing information transmission provides a optimization principle from which we might hope to derive the properties of real regulatory networks. Inspired by the precision of transmission of positional information in the early development of the fly embryo, I will discuss the properties of specific small networks that can optimally transmit information. Concretely, I will show how the form of molecular noise drives predictions not just of the qualitative network topology but also how the quantitative parameters for the input/output relations at the nodes of the network depend on the molecular regulator elements. I will then show how we can consider time dependent information transmission.

Monday, November 4th, 11AM:
Kronecker covariance decompositions for high dimensional data

Alfred Hero (University of Michigan, US)

Kronecker covariance decompositions can be interpreted as a generalization of the matrix singular value decomposition (SVD) where the components are Kronecker product matrices. While these components are not orthogonal, the number of Kronecker components, called the Kronecker separation rank, of the decomposition plays a similar role as the rank in low rank covariance matrix approximation. We obtain high dimensional convergence rates on the approximation error of the Kronecker decomposition under a Wishart sample covariance model as a function of the number of free variables $p$ and the number of independent samples $n$. We illustrate the power of Kronecker covariance decompositions for spatio-temporal data applications in sensor networks and video processing.

Monday, October 21th, 11AM:
Non equilibrium thermodynamic of space-time

Laurent Freidel (Perimeter Institute, Canada)

In this talk I will review the evidence for a mysterious and deep relationship between gravitational dynamics and thermodynamics. I will show how we can extend this connection to non equilibrium thermodynamics. Using the fact that the gravitational equations are fundamentally holographic we show how they can be recasted in a way that shows a deep connection with the non-equilibrium dynamics of viscous bubbles. We will explore some aspects of this correspondence

Monday, October 14th, 11AM:
Higgs et Brisure de Symetrie

Fawzi Boudjema (LAPTH, Annecy)


Monday, October 7th, 11AM:
Le mouvement Brownien de Liouville

Christophe Garban (UMPA, ENS de Lyon)

L'exposé portera sur la gravité quantique de Liouville (en dimension deux), vue du point de vue mathématique. Je commencerai par un aperçu des travaux récents initiés par Duplantier-Sheffield qui donnent un nouvel éclairage à la célèbre formule KPZ. En quelque sorte, l'idée de cette formule est d'établir une correspondance entre des modèles de physique statistique définis sur des réseaux euclidiens du plan et ces mêmes modèles définis sur des "cartes aléatoires". L'origine de cette correspondance reste aujourd'hui encore en grande partie mystérieuse. Les travaux de Duplantier-Sheffield lèvent en partie le voile sur KPZ en conjecturant que la "métrique fluctuante" est donnée par l'exponentielle d'un champ libre Gaussien. Dans la seconde partie de l'exposé, j'introduirai une diffusion naturelle, le "mouvement Brownien de Liouville", qui vivra sur la limite d'échelle de ces cartes aléatoires (i.e. dans un environnement aléatoire donné par l'exponentielle d'un champ libre Gaussien). Travail en collaboration avec Rémi Rhodes (Dauphine) et Vincent Vargas (Dauphine)

Monday, September 30th, 11AM:
Collective motion in populations of colloidal robots

Denis Bartolo (Laboratoire de Physique, ENS de Lyon)

Could the behavior of bacteria swarms, fish schools, and bird flocks be understood within a unified framework? Can one ignore the very details of the interaction mechanisms at the individual level to elucidate how strikingly similar collective motion emerges at the group level in this broad range of motile systems? These seemingly provocative questions have triggered significant advance in the physics and the biology, communities over the last decade. In the physics language these systems, made of motile individuals, can all be though as different realizations of ‘’active matter’’. In this talk, I will show how to gain more insight into this vivid field using self-propelled colloids as a proxy for motile organism. -Firstly, I will show how to motorize colloidal particles capable of sensing the orientation of their neighbors. The simplicity to make, to handle and to visualize this system offers new opportunities to make accurate statistical measurements on motile populations at the lab scale. -Secondly, I will demonstrate that these archetypal populations display spontaneous transitions to swarming motion, and to global directed motion with very few density and orientation fluctuations. - Finally, I will attempt to rationalize our experimental findings by introducing a quantitative theory that accounts for the large-scale behavior of colloidal populations

Monday, September 23th, 11AM:
Magnetic fluids and aggregation in dilute suspensions

Yuri Sobral (Universidade de Brasilia)

Magnetic fluids, also known as ferrofluids, are man-made colloidal suspensions of magnetic nano particles in a liquid carrier. The influence exerted by an applied magnetic field on a magnetic fluid opens a vast array of possibilities for controlling fluid systems with a magnetic component. In particular, suspensions of magnetic nano and micro-particles, as well as emulsions consisting of suspended magnetic fluid droplets surrounded by a Newtonian continuous phase, offer a high potential for technologically important applications such as magnetic clean up of oil spills and drug targeting. In recent years, there has been a major effort in characterising such complex materials from micro/nano scale hydrodynamic simulations. In this seminar, I will give a brief presentation of magnetic fluids and then I will present some results of a numerical study of the relative trajectories of two magnetic particles interacting in a dilute suspension.

Monday, September 16th, 11AM:
Transition laminaire-turbulente dans les sillages stratifiés

Patrice Meunier (IRPHE, Marseille)

Les écoulements géophysiques tels que les sillages atmosphériques ou océaniques sont fortement influencés par la présence d'une stratification continue de la densité. Le but de cette étude est d'analyser expérimentalement et théoriquement comment cette stratification modifie le sillage d'un objet simple tel que le cylindre, dont les caractéristiques sont bien connues dans un fluide homogène. Je me focaliserai d'abord sur la transition d'un sillage stationnaire vers l'allée périodique de von Karman. Le mode instable de von Karman est stabilisé par des stratifications modérées. Mais étrangement, il peut aussi être déstabilisé pour de fortes stratifications si le cylindre n'est pas horizontal. Je m'intéresserai ensuite aux instabilités tri-dimensionnelles, qui révèlent notamment le mode A caractéristique des fluides homogènes. Un autre mode instable peut apparaître pour des cylindres fortement inclinés, et qui semble être relié à l'instabilité des tourbillons de von Karman inclinés par rapport à la stratification. Enfin, je décrirai brièvement le régime turbulent, dont l'expansion verticale est fortement diminuée comparée à l'expansion horizontale, permettant ainsi de construire un modèle semi-empirique de l'intensité du sillage lointain.

Monday, September 9th, 11AM:
Rotating Rayleigh-Benard Convection

Robert Ecke (CNLS, Los Alamos)

I will discuss experiments on Rayleigh Benard convection over a broad range of parameters including traveling wall mode states, pattern dynamics, rotation modified turbulent convection and rotation dominated convection. I will review existing work on this problem and present new results on low-Prandtl number convection at high Ra and Ta number.