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FACULTÉ DE PHYSIQUE
UFR 925

Année 2014

  1. Tadashi TOKIEDA - "Modèles en jouet"
  2. Marvin L. COHEN - "Condensed matter Physics: The Goldilocks Science"
  3. Frank C. ENGELHART - "Crystallization at the nanoscale: insights from nanowire growth"
  4. Peter FRATZL - "Water as a fuel: the material basis for passive plant"
  5. FranÇoise COMBES - "Un peu de lumière sur la matière noire"
  6. Frank JÜLICHER - "Active processes in cells and tissues"
  7. Nicolas BERGERON - "Poincaré et le problème de la forme de l’espace"

 

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 13 OCTOBRE 2014

Tadashi TOKIEDA - "Modèles en jouet"

 

Tadashi TOKIEDA

Directeur des études en mathématiques, Trinity Hall, Université de Cambridge.

 

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• 17 SEPTEMBRE 2014

Marvin L. COHEN - "Condensed matter Physics: The Goldilocks Science"

 

Marvin L. COHEN

Professor at the Department of Physics, University of California at Berkeley.

 

RÉSUMÉ :

Since the focus in Condensed Matter Physics (CMP) is on energies, sizes, and time scales that are not extremely big or extremely small, but somewhere we loosely call the “middle”, it is an area of science that reminds us of Goldilocks who said, "Ahhh, this porridge is just right," and she happily ate it all up. It can be argued that because of its “Goldilocks nature”, CMP has many links to other branches of physics and more generally other areas of science and engineering. Because the domain of CMP is very broad, it has both an applied and a fundamental physics component. I will focus mainly on the latter with emphasis on this field’s intellectual and conceptual contributions to science. I plan to describe some research involving semiconductors, superconductors, and nanoscience. I’ll begin by briefly discussing the development of these areas over the past hundred years, and then I’ll discuss some current achievements and discoveries. I’ll also relay a few observations about Einstein and his seminal research in this field. However I should add that Einstein suggested that we might never have a theoretical explanation of superconductivity, but Einstein was not always right.

 

 

 

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• 6 JUIN 2014

Frank C. ENGELHART - "Crystallization at the nanoscale: insights from nanowire growth"

 

Frank C. ENGELHART

Professor of Materials Science and Engineering.

 

RÉSUMÉ :

The mechanisms responsible for the growth of crystals from a liquid are commonly thought to be well understood. However, recent experiments on the growth of crystals that have dimensions on the order of nanometers have shown that much remains to be discovered. In particular, nanoscale crystals growing from a liquid in the form of wires exhibit unique growth mechanisms, or can be used to probe the importance of the structure of the solid-liquid interfaces on the crystal growth process. These nanowires grow by the well-known solid-liquid-vapor process. A review of the field will be given, highlighting the new insights provided by studying nanowire growth. Following this review, a more recent work showing the effects of facet size, facet mobility, and crystallographic defects on the nanowire growth process will be discussed.

 

 

 

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1er AVRIL 2014

Peter FRATZL - "Water as a fuel: the material basis for passive plant"

 

Peter FRATZL

Max Planck Institute of Colloids and Interfaces (director).

 

RÉSUMÉ :

Dead plant bodies such as seed capsules are able to generate movement by the cyclic absorption of water from the environment. This is based on water swelling of the (secondary) plant cell wall, which is a composite of cellulose nano-fibrils and a matrix containing hemicelluloses and lignin. Cell-wall swelling helps generating growth stresses, e.g., in conifer branches or in the tree stem when subjected to loads. A simple mechanical model for the cell wall predicts that – depending on the detailed architecture of the cellulose fibrils – swelling may lead either to significant compressive or tensile stresses or to large movements at low stresses. A similar mechanism also provides motility to various seeds. The general principle is based on the modification of the isotropic swelling of a gel by embedded oriented fibres and a suitable spatial confinement. Actuation systems in plants provide guidelines for designing material architectures suitable to convert isotropic swelling into complex movements which might be useful for many applications, including soft robotics.

 

Retour au sommaire (année 2014)

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• 4 MARS 2014

FranÇoise COMBES - "Un peu de lumière sur la matière noire"

 

Francoise COMBES

Astronome à l'Observatoire de Paris.

 

RÉSUMÉ :

La plupart de la matière dans l'Univers (80%) est de nature mystérieuse, faite de particules exotiques dont la masse reste inconnue. Ces particules ont échappé à toute détection aussi bien astrophysique que dans les accélérateurs de particules. Elles ne se manifestent que par leur gravité. Le modele "standard" de matière noire froide (CDM) est celui qui représente le mieux aujourd'hui la formation des grandes structures de l'Univers. Pourtant il subsiste des problèmes pour expliquer la formation des galaxies, telles qu'on les observe aujourd'hui. Notamment le modèle prévoit des galaxies dominées par la matière noire, et possédant une myriade de satellites, qui ne sont pas observés. Nous passerons en revue les solutions possibles, en termes de physique des galaxies, de nature de la matière noire, ou de gravité modifiée.

 

 

Retour au sommaire (année 2014)

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• 4 FEVRIER 2014

Frank JÜLICHER - "Active processes in cells and tissues"

 

Frank JÜLICHER

Professeur au Max Planck Institute for the Physics of Complex Systems (Dresde, Allemagne).

 

RÉSUMÉ :

Living cells are extraordinarily dynamic and have the ability to generate movements and forces. This is particularly striking in the case of swimming microorganisms or the process of cell division. A key example for force generating processes in cells is the operation of molecular motors that interact with filaments of the cytoskeleton. In the cell, cytoskeletal networks form gel-like materials with unconventional active material properties that are the consequence of the action of such motors. Active cellular processes have also interesting effects on larger scales. Tissues are collections of many cells which can also be considered as active media. Active processes in tissues result e.g. from cellular dynamics, cellular force generation and cell division. These processes introduce mechanical stresses and permit active rearrangements and flows in tissues. I will discuss theoretical approaches that capture general principles that govern the dynamics of active media such as the cytoskeleton or tissues. In biological systems, active processes are regulated by cellular signaling systems. Such coupling of active mechanical and chemical processes, including feedbacks that influence chemical cues via flow patterns leads to novel and original mechanisms of Pattern formation in biology.

 

 

 Retour au sommaire (année 2014)

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7 JANVIER 2014

Nicolas BERGERON - "Poincaré et le problème de la forme de l’espace"

 

Nicolas BERGERON

Professeur à l’UPMC et membre junior de l’Institut Universitaire de France.

 

RÉSUMÉ :

Un classique des récits de science-fiction nous représente emprisonnés dans une pièce enchantée : vous essayez de vous échapper par une porte mais, comme vous sortez par la porte, vous rentrez par la fenêtre ! Est-ce si étrange ? On verra plusieurs exemples d'espaces tridimensionnels de cette nature, tous tirés d'un long article et de ses cinq compléments publiés par Henri Poincaré entre 1895 et 1905. En étudiant ces exemples on sera naturellement amené à discuter du groupe de Poincaré, de la conjecture de Poincaré et plus généralement à parler de «topologie algébrique». Cette branche de la mathématique est une création de Poincaré --- il l'appelait « Analysis Situs ». Son objet d'étude est la forme des espaces indépendamment de leur géométrie. Les concepts tirés de la topologie algébrique ont bouleversés tout le 20ème siècle mathématiques et continuent de jouer un rôle fondamental dans la recherche moderne notamment en physique mathématique.
On décrira quelques uns de ces concepts sur des exemples jusqu'à s'interroger : l’univers est-il une pièce enchantée ? et si oui comment cette pièce est-elle agencée ?

 

 

Retour au sommaire (année 2014)

 

 

02/06/17

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