ESPCI web site
Séminaire café 2024
| 11 janvier | Gaspard Junot Zigzag pattern of active shakers emerging from viscoelasticity Viscoelasticity is commonly observed in a broad range of systems, (polymer solutions, micelles, cells). The viscoelastic nature of those fluids affects the dynamics and the collective organization of active or driven particles embedded in it. Here we demonstrate that a collection of driven magnetic microrotors suspended in a viscoelastic solution can self organize into large scale dynamic bands displaying a zig-zag shape. We show that the shaker-like flow field created by a microrotor is at the origin of the pattern formation. These results suggest that zig-zag patterns is a general effect which could arise in a broad range of systems. |
| 18 janvier | Tom Shimizu AMOLF, Amsterdam Exploring the wave nature of life across scales : from active matter to ecology Viewed at sufficiently large scales, the dynamics of any biological population in space and time can be meaningfully described both as particles - individual agents executing random walks in space - and as waves - migrating or expanding fronts of population density. This talk will explore the provocative proposition that confronting this ’biological wave-particle duality’ head on provides a surprising and deep connection between three seemingly disparate fields : the physics of behavior, biological active matter, and ecology / evolutionary biology. I will motivate with specific examples the notion that the study of small organisms whose collective population-level migrations can be studied within the laboratory represents a rich arena for unearthing exciting new questions that cut across these fields. What aspects of the underlying mechanistic dynamics - whether they be active matter processes controling microbial transport and locomotion, or neural networks that govern animal behavior - quantitatively define population waves of ecological consequence ? Can an understanding of the evolutionary selection pressures on such collective phenotypes in turn drive deeper understanding of mechanistic design principles ? Could building up a taxonomy of such population waves provide a new coarse-grained basis for understanding, predicting, and managing spatial ecology in real-world habitats ? My talk will attempt to place these questions in a concrete context by highlighting our recent work combining experiments and mathematical modeling to address mechanistic, functional, as well as ecological aspects of population migration waves in three contrasting model systems : the bactirum E. coli, the nematode C. elegans, and the arbuscular mycorrhizal fungi (AMF). |
| 25 janvier | Pas de séminaire café (Journées de physique statistique) |
| 1er février | Pierre Bauër Former PhD student of the lab, CEO Plantibodies |
| 8 février | PhD student : Jeanne MOSCATELLI & Antoine BOUVIER |
| 14 février | Tali Khain PhD student in Vincenzo Vitelli’s group, Univeristy of Chicago Simple complex fluids Familiar fluids, such as air or water, can be described by the Navier-Stokes equations. The description of complex fluids typically requires additional fields, like a nematic field in liquid crystals or a rotation field in spinning colloids. In this talk, we focus on situations in which the dynamics of these complex fluids can be captured by a minimal extension of the Navier-Stokes equations because the additional fields can be neglected. To do so, we account for the broken symmetries in these anisotropic and chiral fluids through the viscosity tensor. Using a combination of analytical and numerical methods, we show how the additional viscosities that arise modify how the fluids flow across a range of Reynolds numbers. In the case of anisotropic fluids, we find that we can control the orientation of a sinking pushpin-like object by modulating the axis of anisotropy. In chiral fluids, the new "odd" viscosity coefficients lead to counterintuitive phenomena across scales, from the motion of particles transverse to applied forces in Stokes flow to the formation of patterns with a tunable wavelength in fully developed turbulence. |
| 22 février | PhD student : Joo-Won HONG & Magdalini KOUKOURAKI |
| 29 février | Vincent Gourmandie PhD Student at MSC Drop impact, effects of surfactants and acoustic of resonating Bubbles |
| 7 mars | PhD student : Nan HE & Guillaume VIRAYE |
| 14 mars | Grégoire le Lay PhD student at MSC Surface wave interactions on rivulets in a Hele-Shaw cell leads to parametric destabilization When injecting oil between two vertical glass plates, it forms a liquid bridge, termed rivulet, which falls down under the effect of gravity. The coupling between the flow inside the liquid filet and the geometry of its free surface leads to complex behaviour. In particular, the high shear in the menisci near the walls strongly influences the mobility of the interfaces. We excite acoustically the rivulet using speakers placed on the side of the cell, creating a spatially homogeneous forcing. While usually on this system both transverse and longitudinal waves along the rivulet are linearly damped and do not interact, the exterior forcing produces a parametric cross-coupling that causes the two types of perturbation to amplify one another. |
| 21 mars | PhD student : Baptiste AUVITY & Camille RAMBERT |
| 28 mars | Andrea Plati Post-doctorant at LPS Self-assembly of crystals and quasi-crystals in vibrated granular matter Abstract : Granular materials provide an accessible and diverse playground for nonequilibrium physics. When subjected to mechanical vibrations, they undergo so-called vibrofluidisation, reaching a nonequilibrium steady state through the balance between dissipation and external forcing. It has been known for more than two decades that monodisperse vibrofluidised granular materials undergo a liquid-solid-like phase transition to a hexagonal periodic structure similar to that of hard-sphere thermal systems. In my talk I will present experimental and numerical results showing how this analogy extends to the case of granular binary mixtures, which form much more complex structures. I will report on our recent experimental observation of spontaneous quasi-crystalline self-assembly on the millimetre scale [1]. This result is obtained in a fully athermal system of spherical grains vibrating on a substrate. Starting from a liquid-like disordered phase, the grains begin to locally arrange into three types of square and triangular tiles, which eventually align to form an eightfold symmetric quasi-crystal, which has been predicted by simulation but not yet observed experimentally in non-atomic systems. I will also outline an ongoing analysis focusing on the self-assembly of a simpler structure (i.e. a periodic square binary crystal), which provides a particularly suitable testing ground for better understanding the physical mechanisms underlying ordering phenomena in athermal systems. [1] : A. Plati, R. Maire, E. Fayen, F. Boulogne, F. Restagno, F. Smallenburg, and G. Foffi, Nature Physics 20, 465–471 (2024) |
| 4 avril | CO2 footprint of the lab |
