It's been long known that active particles have a tendency to accumulate near walls and get trapped at corners. More recently, these features have been used to make bacteria rectify or power micro-devices. However, a general understanding of how boundary shape affects the thermo-mechanical properties of active suspensions is still missing. Using a simple model of active suspension, I will show how one can derive a general relationship between the particle density and the shape of the box. Despite its limitations (the suspension must be strongly confined and dilute), the fact that the result holds for arbitrary box shapes makes it a promising tool to better understand and design confining boxes for active suspensions, and I will discuss some ways in which it may be applied.

Integrin receptors mediate interaction between cells and extracellular matrix. These receptors do not possess any catalytic properties by themselves and must recruit additional signaling molecules upon activation to transmit signal to the cell. Using this property and starting from elementary biochemical modules governing integrin activation, we construct a reaction diffusion network based on receptor regulation. Including integrin receptor diffusion, our model generalizes previous abstract dynamical systems and aims at describing local production of a key phosphoinositide, i.e. PIP2(4,5) , which dictates the temporal and spatial activity of crucial processes such as integrin activation and clustering or actin polymerization. We describe local PIP2(4,5) production and integrin clustering as localized spike excitation waves which can be either static or dynamic. We discuss this model in the context of adhesive structures podosome type that are the hallmarks of invasive cells. In this context, we propose that static solutions correspond to cluster of podosomes and that dynamic solutions correspond to expanding rosette of podosomes. Our model allows connecting different types of integrin organization to their kinetic parameters.

The densest way to pack objects in space, also known as the packing problem, has intrigued scientists and philosophers for millenia. Today, packing comes up in various systems over many length scales from batteries and catalysts to the self-assembly of nanoparticles, colloids and biomolecules. Despite the fact that so many systems' properties depend on the packing of differently-shaped components, we still have no general understanding of how packing varies as a function of particle shape. Here we carry out an exhaustive study of how packing depends on shape by investigating the packings of over 55,000 polyhedra. By combining simulations and analytic calculations, we study families of polyhedra interpolating between Platonic and Archimedean solids such as the tetrahedron, the cube, and the octahedron, via continuous vertex and edge truncations. We find maximum packing-density surfaces that reveal surprising richness and complexity. We expect our density surface plots to guide experiments that use shape and packing in a similar way that phase diagrams are used to do chemistry. We suggest to think about a shape no longer as a static property but rather as but one point in a higher dimensional “shape space” where the neighborhood around the given shape, as achieved by small deformations, needs to be taken into account as it may reveal why we can assemble certain crystals, transition between them, or get stuck in kinetic traps.

In cells, (bio)polymers are constantly subjected to forces, either from the environment or self-generated. We discuss the response of a polymer to a sudden force within the context of standard polymer models.

ATTENTION: CHANGEMENT DE SALLE EXCEPTIONNEL **** AMPHI 23.01 **** L'existence d'un fond d'ondes gravitationnelles primordiales est une prédiction fondamentale des modèles inflationnaires. Or ces modèles semblent être à l'origine de la formation des grandes structures et de toutes les inhomogénéités observables dans notre univers. Ils ont la faveur des observations existantes. Nous expliquerons l'enjeu scientifique des mesures de la polarisation du fond diffus par les expériences BICEP2 et Planck. Tous ces enjeux seront abordés de manière compréhensible pour non-spécialistes.

Glass-forming liquids undergo a dramatic dynamical slowdown within a modest range of temperatures. To elucidate the mechanism of this slowdown, many length scales have been proposed to underly this dramatic behavior. One promising candidate for such an emergent length scale is a point-to-set correlation length. In this talk, I review what it is, show how to analytically capture its diverging behavior, and hopefully conclude with open-ended discussions.

The measurement of a small deviation of the primordial spectrum of scalar (density) pertur- bations in the Universe from the exactly flat (Harrison-Zeldovich, n_s = 1) one in the WMAP and Planck CMB experiments confirms the general prediction of the early Universe scenario with the de Sitter (inflationary) stage preceding the radiation dominated stage (the hot Big Bang) and strongly restricts the class of viable inflationary models [1]. Thus, the status of the inflationary paradigm is changing from ”proving” it in general and testing some of its simplest models to applying it for investigation of the actual history of the Universe in the remote past and particle physics at super-high energies using actual observational data. Another generic observational prediction of the inflationary scenario is the existence of primordial tensor perturbations (primordial GW back- ground) [3] which power is suppressed compared to that of scalar ones by at least the first power of |n_s − 1| and may be more. The announced discovery of primordial GW through the measure- ment of the B-mode of the CMB linear polarization in the range of multipoles = 50 − 150 in the BICEP2 experiment [2], while formally being in agreement with this prediction, still corresponds to a too large amount of GW which contribution is not seen in the WMAP and Planck results for CMB temperature anisotropy. This suggests that the BICEP2 polarization result is partly, if not mostly, due to secondary foregrounds (mainly polarized galactic dust emission) that has been recently quantitatively confirmed by the Planck team [4]. Thus, it is premature to say that primor- dial GW have already been discovered. Much more future work is needed for this. On the other hand, I argue that the value n_s ≈ 0.96 obtained in [1] leads to the conclusion that, under natural assumptions, the expected amount of primordial GW is not too small to be unmeasurable, being suppressed parametrically by no more than |n_s − 1|^2 compared to the power of scalar perturbations, like what happens in the R + R^2 inflation [5].
References
[1] P. A. R. Ade et al. [Planck Collaboration], arXiv:1303.5082.
[2] P. A. R. Ade et al. [BICEP2 Collaboration], arXiv:1403.3985.
[3] A. A. Starobinsky, JETP Lett. 30, 682 (1979).
[4] R. Adam et al. [Planck Collaboration], arXiv:1409.5738.
[5] A. A. Starobinsky, Phys. Lett. B 91, 99 (1980).

N.B. a blackboard informal discussion with the speaker will also take place on Friday oct. 3rd, 11:30 -12:30 (petite salle PTh) during the IFAC 'Journal Club' .

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

Why are plant leaves flat? How does a plant achieve a well-defined shape? More generally, what are the physical mechanisms behind morphogenesis in living systems? We are combining various experimental and theoretical approaches to address these questions, which has led us to study the coupling between differential geometry and elasticity, to build models of active elastic media, and the develop methods to measure the mechanical properties of live plant tissues. In this talk, I will give a broad overview of our results on the physics of plant development.