Laboratoire de Physique
Theorique d'Orsay

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Séminaire commun LPT/LPTMS 2012-2013

Physique Statistique des Systèmes Complexes

Les séminaires de Physique Statistique des Systèmes Complexes se tiennent le jeudi à 14 heures (sauf indication contraire), soit au LPT (bâtiment 210) en salle 114, soit au LPTMS (bâtiment 100A) en salle 201.

Seminars of Statistical Physics of Complex Systems are held either at LPT (Bdg 210) in room 114, either LPTMS (Bdg 100A) room 201, generally on Thursdays at 2 PM, unless stated otherwise.

Contact : Cecile Appert-Rolland et Silvio Franz.

-  Jeudi 29 Novembre 2012 à 14h, Salle 114, bât. 210 (LPT)


Lane formation in a lattice model for oppositely driven binary particles

I will introduce a two-dimensional lattice model for oppositely driven binary particles with purely repulsive interactions where some classes of steady states have been constructed in systematic ways. Using such constructions, I will discuss nonequilibrium phase transitions in this lattice model [1]. Also, I would like to discuss those relevance to lane formations in colloidal suspensions [2] [3].

References :

[1] H. Ohta, Europhys. Lett. 99, 40006 (2012).

[2] J. Dzubiella, G. P. Hoffmann, H. Löwen, Phys. Rev. E 65, 021402 (2002).

[3] T. Vissers, A. Wysocki, M. Rex, H. Löwen, C. P. Royall, A. Imhof, A. van Blaaderen, Soft Matter 7, 2352 (2011).

-  Jeudi 25 Octobre 2012 à 14h, Salle 114, bât. 210 (LPT)

Joel STAVANS (Dpt Physics of Complex Systems, Weizmann Institute of Science, Israel)

Effects of Post-Transcriptional Regulation on Phenotypic Noise in Bacteria

Cell-to-cell variations in protein abundance or noise give rise to phenotypic variability between isogenic cells. Studies of noise have focused on stochasticity introduced at transcription, yet the effects of post-transcriptional regulatory processes on noise remain unknown. We study the effects of RyhB, a small-RNA of E. coli produced upon iron stress, on the phenotypic variability of two of its down-regulated target proteins, using dual chromosomal fusions to fluorescent reporters and measurements in live individual cells. Remarkably, the total noise of each of the target proteins is robust over a wide range of RyhB production rates despite cells being in stress, and that coordinate down-regulation of the two target proteins by RyhB reduces the mutual correlation between their levels. Hence an increase in phenotypic variability under stress is achieved by decoupling the expression of different target protein in the same cell, rather than by an increase in the total noise of each. Extrinsic noise provides the dominant contribution to the total protein noise over the same range of RyhB production rates. No evidence of a theoretically-predicted peak in noise at finite RyhB production rates is found. Stochastic simulations incorporating extrinsic and intrinsic noise sources reproduce qualitatively key features of our observations.

-  Jeudi 18 Octobre 2012 à 14h, Salle 201, bât. 100A (LPTMS)


Interplay of anisotropy and interactions in charged colloidal disks

Charged plate-like colloidal suspensions such as gibbsite and some clays exhibit interesting and rich phase behavior including liquid-crystalline and arrested gel-like states. The anisotropic shape of such particles suggests that these materials could form liquid crystalline phases due to the competition between positional and orientational entropy. In some cases, however, isotropic-nematic transition is hindered by formation of a gel-like viscoelastic phase. The question that arises is how the electrostatic interactions influence the liquid crystal formation. Motivated by these experimental findings, we have investigated the structure and dynamics of charged disks by means of Monte-Carlo simulations. In this talk, I will clarify the the role of anisotropic nature of screened electrostatic interactions on the formation of liquid-crystalline phases and arrested states. Finally, I will compare the results of our simulations with experiments.

-  Jeudi 27 Septembre 2012 à 14h, Salle 114, bât. 210 (LPT)

Ellen SAADA (Laboratoire MAP5, Université Paris Descartes)

Comportement hydrodynamique de systèmes de particules attractifs en milieu aléatoire ; applications à des modèles de misanthropes et de trafic.

Travaux en collaboration avec C. Bahadoran (Clermont-Ferrand), H. Guiol (Grenoble), T. Mountford (Lausanne), K. Ravishankar (SUNY, New Paltz).

Nous avons obtenu la limite hydrodynamique trempee (’quenched’) sous un changement d’echelle hyperbolique pour des systemes de particules attractifs sur Z dans un environnement aleatoire ergodique. La limite est donnee par la solution entropique d’une loi de conservation scalaire avec une fonction de flux macroscopique lipschitzienne. Notre resultat est une loi forte des grands nombres, nous le demontrons par une methode constructive.

Nous illustrons notre resultat sur plusieurs exemples, comme des generalisations du processus de misanthropes, et de processus de K-exclusion a k etapes, avec differents types d’environnement aleatoire. Nous detaillerons un modele de trafic et un modele de file d’attente.