Most bacteria contain a single circular chromosome a thousand-fold longer than the bacteria itself. This chromosome is highly condensed and is organized in part by a large amount of associated proteins. Single-molecule experiments revealed that these proteins impact DNA structure by inducing bridging, bending, twisting, looping, or coiling. However, it remains unclear how such DNA-binding proteins collectively organize the bacterial chromosome. I will focus on what is arguably the most demanding organizational task for the bacterial chromosome: DNA replication and segregation, which is mediated by the ParABS system in many bacteria. At its heart, this segregation machinery includes a large protein-DNA complex consisting of roughly 1000 ParB proteins. However, the physical organization of ParB proteins on the DNA remains unclear. What controls the formation, localization, and stability of the ParB complex? We developed a simple model for interacting proteins on DNA. This model shows that a combination of 1D spreading bonds and a 3D bridging bond between ParB proteins constitutes a minimal model for condensation of ParB proteins into a 3D ParB-DNA complex.

Segregation in granular materials plays a major role in a large number of industrial and geological processes. However, still as of today, many of the segregation phenomena are not yet fully understood. In the current study we addressed both experimentally and numerically the question of shear-induced segregation of dry granular media. With the help of numerical simulations we can remove the effects of gravity, and draw further conclusions about the dynamics of the system. In my talk I would like to introduce our most recent results on the topic.

Les polaritons excitoniques sont des quasi-particules qui ont révélé ces dernières années de très riches propriétés telles que la possibilité de condenser dans leur état fondamental. Elles sont le fruit d'un couplage fort entre les transitions excitoniques présentes dans les semiconducteurs et un champ électromagnétique confiné au sein d'une microcavité.
Je présenterai deux aspects complémentaires de la physique de ces particules : (1) un aspect dispositif d’optique quantique et non-linéaire. Plus particulièrement, j’illustrerai cet aspect grâce à des résultats obtenus et corroborés par des expériences sur des OPO (optical parametric oscillator) à polariton dans des microcavités de basse dimensionnalité, de type pilier. (2) l’aspect fluide de polariton, traitant de la dynamique interne aux microcavités semiconductrices. Ces quasi-particules mi-lumière mi-matière, ont la particularité de présenter un régime superfluide. Je donnerai quelques éléments de la théorie associée à ce phénomène et présenterai sa démonstration expérimentale, pour ensuite considérer les régimes d’écoulement turbulent dans ces fluides quantiques de polaritons.

Ce séminaire a pour ambition de vous présenter le mouvement international du libre accès aux publications scientifiques (Open Access). Né au début des années 1990, ce mouvement est aujourd'hui largement répandu et connu mais parfois mal compris par la communauté scientifique, notamment en raison des amalgames et idées reçues qui lui sont associés. Ces derniers sont le fruit de l'intense travail de lobbying des éditeurs traditionnels, qui s'élèvent autant que possible contre ce nouveau modèle d'édition qui, outre sa vocation à diffuser librement la production scientifique, menace de faire largement diminuer leurs bénéfices...
Afin de décrypter les tenants et aboutissants de l'Open Access, nous en survolerons donc à la fois la théorie et les pratiques, les freins à son développement ainsi que les avantages indiscutables qu'il induit.

We use emulsions as a model system for a wide range of applications : from the synthesis of biomimetic tissues to the production of patchy particles for self assembly, or the study of granular matter. In particular we design bio-inspired emulsions that are stabilized with phospholipids and functionalized with proteins in order to mimic specific cellular functions, such as lipid domain formation, cell-cell adhesion or even hemifusion. The assembly of the droplets in 3D is controlled through their adhesion energy, which can be tuned through the binders grafted on the surface. For example quasi-permanent bonds are created through biotin-streptavidin links, while reversible bonds are created with complementary single DNA strands. We use this versatile system to quantify cellular interactions in a simplified in vitro framework. Moreover we use the emulsion system to control the interactions between the droplets in order to drive their directed self-assembly into 3D structures. For example, immiscible lipids on the surface of emulsion droplets are used to create stable domain patterns, thus providing a route for patchy particle synthesis. Functionalizing these patches with binders allows the droplets to aggregate with a given valency. The mobile adhesion patches can then self-assemble linear chains of thermal droplets into well-defined compact structures. Bio-inspired emulsions are therefore a useful system for problems in biophysics, but also provide a new class of liquid patchy particles for self-assembly.

In recent years thermally or chemically active Janus particles have become a toy model for artificial microswimmers, which move in their self-generated temperature or concentration gradients. The driving mechanisms are poorly understood, and often even the direction of the motion lacks a rationale so far. Here we discuss the thermal forces acting on charged particles with non-uniform surface temperature.
Comparison with experiment suggests the importance of the thermoelectric effect in the electric double layer.