Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can jam into a solid-like disordered state. In this state each particle has a certain equilibrium position and can vibrate around it.

I show how to decompose the dynamical matrix M into the form M = AA^T. This representation allows to apply the random matrix theory and estimate the vibrational density of states. The resulting vibrational density of states is approximately constant over a wide frequency range which is determined mostly by the ratio of the number of degrees of freedom and the total number of contacts in the system. The results are in a good agreement with numerical experiments performed by various authors.

Examples for single cell volume and density measurements and elastography of cellular aggregates. Cell volume is a fundamental parameter for understanding cell cycle progression and regulation. I will describe a micro-fabricated system to accurately measure the volume of individual cells, adherent or not, over several cell division cycles. By following single cells through several division cycles, we found signatures of different cell size homeostasis mechanisms in different cell lines. Surprisingly, cells in mitosis exhibit a significant volume increase associated with a drop in cell density that could generate strong pushing forces.

On a multicellular level, tumors are subjected to external stress generated by their own growth in a confined environment and their surrounding tissues. Using full field optical coherence tomography, we measure spatially and temporally the strain inside spheroids after applying an isotropic osmo-mechanical pressure. The results show that spheroids do not behave as homogeneous viscoelastic spheres as they overreact to the applied stress. We finally estimate viscoelastic parameters of spheroids at the macro-scale.

"Thé au cube" est la pause café scientifique de l'axe PTh. L'orateur y expose de manière interactive et accessible aux théoriciens non-spécialistes, ses intérêts scientifiques du moment. N'oubliez pas de ramener vos tasses et autres mugs préférés.

It is generally accepted that the plasticity of amorphous material comes from localized reorganizations, which may produce form large structures of deformation. This description is supposed to describe the plasticity and the failure in numerous experimental systems (emulsions, dense colloidal suspension, metallic glasses,..). However the experimental observations of such behavior are missing. In this seminar, we will discuss an experimental study of the plastic deformation of a granular material. The material is compressed and the deformations are measured using a dynamic light scattering set-up. A intermittent structure of the plastic flow appears. The anisotropy of the deformation are given by the mechanical stress redistributions of isolated events. This structure of the plastic flow is more intense when the failure is approached. However the final failure of the material seems to be driven by the internal solid friction of the material.

Il sera démontré que, par l’action combinée du potentiel de confinement des électrons bidimensionnels dans le plan (xy) et du champ magnétique appliqué perpendiculaire à ce plan (force de Lorentz), un accepteur ionisé, donc répulsif pour un électron, peut localiser un ou deux électrons 2D de conduction. Le confinement des électrons de conduction autour de l’ion accepteur a pour résultat la création d’états discrets au dessus des états libres de Landau. Seront alors évoquées les conséquences observées sur les propriétés physiques d’un gaz 2D d’électrons sous champ magnétique quantifiant (domaine de l’effet Hall quantique).

In this talk, I will give a modern introduction to the challenges of studying black hole physics, both form bottom-up and top-down perspectives. As a top-down example, I will discuss a simple construction of gauged supergravity theories with spontaneously broken supersymmetry in Minkowski vacua with light charged matter in the low energy spectrum; one in which the microscopic entropy of black holes can still be counted reliably. I will also point out how such a simple idea naturally provides a stringy realization of the $\rho$ algebras of Schwimmer-Seiberg. Additionally, I will point out how this leads to a generalization of the AdS_3/CFT_2 correspondence; to one with twisted boundary conditions. Finally, I will briefly mention some work in progress on a bottom-up approach to black hole horizon dynamics.

Self-propelled particles convert energy from their surrounding environment into translational motion. Collectively, they are generically able to inject local energy into the macroscopic motion of the whole system, thus sustaining an orientationally ordered stationnary state in two dimensions. In this talk, I will discuss the transition between this polar state (whose existence is forbidden for equilibrium systems) and the less exotic isotropic state. A key observation is that the interaction of self-propelled particles does not conserve momentum. I will show how this can be made more quantitative in the dilute regime, in order to obtain predictions on the transition between isotropic and polar phases. Although the theory is general, I will focus the discussion on a model of self-propelled hard discs that was already shown to describe very well a vibrated grain experiment. The quantitative predictions of the theory are checked against data from molecular dynamics simulations. I will finally discuss our recent findings about how density effects alter the mean-field transition.

Il existe plusieurs raisons pour l'augmentation de la résistance électrique dans un champ magnétique (magnétorésistance positive). Par contre, la magnétorésistance négative est rare et pas très bien comprise. Récemment, quelques études expérimentales ont démontré une très forte magnétorésistance négative des électrons deux-dimensionnels à basses températures, voir un exemple en attaché (Q. Shi et al, 2014).

L'explication qu'on propose est basée sur l'idée de considérer les électrons comme un fluide visqueux. La résistance est donc proportionnelle à la viscosité électronique et la chute de résistance est due à la diminution de la viscosité en champ magnétique.