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Séminaires Internes

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Séminaires généraux donnés par les membres du LIPhy. L’objectif est que les membres du LIPhy connaissent les recherches menées par leurs collègues.

Les séminaires se déroulent en principe tous les premier jeudi du mois, à 11h.



  • Jeudi 5 février 2015 11:00-12:00 - Eric Bertin - LIPhy

    Statistical characterization of non-equilibrium states

    Résumé : The statistical characterization of non-equilibrium systems like granular matter, foams, turbulent flows, or even complex systems outside physics like bird flocks or interacting social agents, is an important and still partly open issue.
    Some of the open questions include for instance the identification of relevant "thermodynamic" parameters to characterize the collective state of these systems, the origin of the often-observed non-Gaussian fluctuations of global observables like energy or magnetization, or the characterization of pattern formation in terms of the microscopic parameters of the system.
    These issues will be illustrated on several simple examples.

    Lieu : LIPhy

  • Jeudi 26 février 2015 11:00-12:00 - Samir Kassi - LIPhy

    Sustainable Absorption Spectroscopy

    Résumé : After shortly introducing the absorption spectroscopy, we will show how extreme sensitivities can be achieved, thanks to some properties of resonant optical cavities. In particular, Cavity Enhanced and state of the art Cavity Ring Down setups will be described.
    We will also present our newest experimental schemes aiming at producing high precision and accuracy spectra, what we would define as sustainable data, according they are bias-less and go far beyond the most challenging demand at present.
    Through selected examples, we will illustrate what very high sensitivity spectroscopy brings to environmental, atmospheric and planetary sciences as well as fundamental physics tests.

    Lieu : Conference Room

  • Jeudi 26 mars 2015 11:00-12:00 - G. Cappello - LIPhy

    [Interne] Dynamics and thermodynamics of homologous recombination

    Résumé : Double strand damages may be very harmful for the genome integrity. As they often involve a loss of genetic information, a simple end to end joining would introduce severe deletion or mutations on the chromosome. In order to avoid such unfortunate event, the cell uses the homologous chromosome (or the sister chromatid) as a template to reconstitute the defective DNA. This mechanism is called ?Homologous Recombination ?.
    To investigate the mechano-chemical aspects of Homologous Recombination, we choose the single molecule approach. We use a new type of magnetic tweezers to hold the intact DNA molecule during the whole reaction. Our system allows us to monitor the torsion and the elongation of the DNA molecule, as well as the applied stretching force and the torque exerted by the recombinase enzyme hRad51. Is such a way, we determine the Energy Landscape during the Homologous Recombination process and how it drives the reaction to the strand exchange.

  • Jeudi 21 mai 2015 11:00-12:00 - Pierre Thibault - LIPhy

    Acoustic torque : principle and application to the rotation of biological cells and small particles

    Résumé : I will present some recent progress in the generation and control of acoustic fields that allow to exert some torque on microscopic objects and induce their rotation. I will first recall some features of the acoustic streaming at the boundary of a spherical object. Then, I will discuss the trajectories of fluid particles in a 2D acoustic field having a ?/2 phase lag and introduce the concept of acoustic vortices and acoustic angular momentum. Last, I will illustrate the capabilities of ultrasounds to move and rotate particles in 2 different cases : a single-beam acoustical vortex and a 2D acoustic cavity made of PDMS.

    Lieu : Conference Room

  • Jeudi 19 novembre 2015 12:00-17:38 - Irène Wang - Liphy

    Soutenance HDR Irène Wang

    Résumé : Développements en microscopie optique pour le vivant : mesures de mobilité et de forces
    La microscopie optique est largement utilisée pour visualiser les structures qui composent le vivant. Je présenterai un ensemble de techniques de microscopie de fluorescence qui visent à obtenir, non des images, mais des informations quantitatives (densité absolue) et fonctionnelles (interaction, agrégation ?), essentielles pour comprendre les processus cellulaires. Ces méthodes sont basées sur l’analyse des fluctuations temporelles causées par les molécules qui diffusent à travers un petit volume d’observation.
    Je montrerai quelques pistes pour donner à ces mesures à une dimension spatiale : le façonnage de front d’onde pour créer de multiples points d’excitation couplé à une matrice de détecteurs et les méthodes de corrélation d’images. Nous avons appliqué ces techniques à l’étude de la mécanosensibilité cellulaire. En contrôlant la géométrie des cellules, nous montrons qu’il est possible de localiser les contraintes qu’elles exercent sur leur substrat et que ce niveau de contrainte est corrélé à la mobilité des protéines impliquées dans l’adhérence cellulaire.
    Finalement, je discuterai de l’une des limites actuelles des méthode de fluctuations de fluorescence qui est leur extrême sensibilité aux aberrations optiques induites par l’échantillon lui-même, ces dernières pouvant créer des biais importants dans les mesures. Une approche d’optique adaptative utilisant un miroir déformable a été mise en ?uvre pour corriger ces aberrations et fiabiliser les mesures de fluctuations de fluorescence dans les échantillons épais.
    Jury :
    - M. François Amblard (Institut Curie)
    - M. Claude Boccara (Institut Langevin)
    - M. Hervé Rigneault (Institut Fresnel)
    - Mme Cendrine Moskalenko (Laboratoire de Physique ENS Lyon)
    - M. Patrick Ferrand (Institut Fresnel)
    - M. Dominique Bourgeois (Institut de Biologie Structurale)

  • Jeudi 26 novembre 2015 11:00-12:00 - Lionel Bureau

    [INTERNE] The (A ?) physics of solid friction

    Résumé : Friction is the force that resists the relative motion of objects in contact. While scientific studies of friction date back to the 15th century, the phenomenon received its first sound physical interpretation only five centuries later, and is still at the heart of a whole research field named « tribology ». This shows how the simple phenomenological laws established by Amontons and Coulomb, as they are taught in highschool, actually hide a highly complex phenomenon.
    This seminar will be for me the opportunity to come back on a topic on which I used to work before joining the LIPhy… I will first give a historical overview of friction studies, from which the textbook laws of friction have been derived. I will then discuss the main underlying physical mechanisms and illustrate how to go beyond the classical Amontons-Coulomb’s law in order to describe properly the behavior of frictional interfaces.

  • Jeudi 17 décembre 2015 11:00-12:00 - Hugues Guillet de Chatellus

    [INTERNE] Application of self-­imaging to the amplification of periodic images and generation of arbitrary optical frequency combs

    Résumé : Talbot effect is an elementary manifestation of self-imaging : when a periodic pattern (or grating) is shined with a plane wave, the exact image of the pattern reappears at periodic distances from the grating (integer Talbot effect) while images with a reduced periodicity are observed at intermediate positions (fractional Talbot effect). The concept of self-imaging can be generalized to other variables such as spatial frequency, time or frequency. 
    In this talk, I will explain how a proper combination of self-imaging in dual domains (e.g. space - spatial frequency, or time - frequency) enables fascinating effects. For instance I will show how this concept can be applied to the passive amplification of periodic images. Then I will present an equivalent demonstration in the domain of optical frequencies, by showing how the spacing of an input optical frequency comb can be controlled over 6 orders of magnitude using joint time and frequency self-imaging.

  • Jeudi 4 février 2016 11:00-12:00 - Sigolène Lecuyer

    [INTERNE] Surface-sensing in bacteria : how do cells transition from motile to sessile ?

    Résumé : Bacteria can exist in a motile planktonic state, or alternatively enter into a sessile multicellular lifestyle where they form organized communities called biofilms. I am interested in understanding the biophysical basis of how and why this transition occurs. More specifically, in this seminar I will illustrate the importance of mechanical forces and surface-sensing during adhesion of biofilm-forming bacteria.
    In my talk, I will first give a broad introduction of the problem and show how this field, which used to be the playground of biologists has now been invested by physicists. I will then show how we use a combination of biological, bioengineering and physical approaches to learn more about bacterial adhesion and biofilm formation. In particular, I will discuss results obtained using microfluidic tools and traction force microscopy.

  • Jeudi 11 février 2016 11:00-12:00 - Kirsten Martens

    [INTERNE] Mesoscopic modeling for the rheology of yield stress fluids

    Résumé : In this talk I will discuss several mesoscopic approaches, such as lattice models and mean-field descriptions for the yielding transition and the non-linear rheology of driven yield-stress materials. Despite the fact that this type of models require some phenomenological ingredients, notably the detailed local yielding rules, they have been shown to match several aspects of the mechanical response very well, such as avalanche statistics, mechanical noise descriptions and rheological features.
    Further the mesoscopic approach provides a unique tool to test basic assumptions for different flow phenomena and can serve as a bridge to large scale descriptions of the complex yielding dynamics.

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  • Lundi 8 janvier 2018 15:00-17:00 -

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