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 |
| 11 avril | Wladimir Sarlin Post-doc at LadHyX Water waves generated by a granular collapse : An experimental study of landslide-induced tsunamis Tsunamis are amongst the most destructive natural disasters for coastal populations around the world. Such natural disasters may be generated from massive landslides or rockfalls, for example when a volcano becomes destabilized and falls into a lake, a fjord, or even the ocean. The generation of such waves is studied here by using a model experimental configuration, in which a column of granular material of rectangular cross-section, which mimics the landslide, collapses into a quiescent water. Systematic measurements are performed, in which the depth of liquid as well as the geometry of the grains’ column are varied, in order to obtain the outcomes of the granular avalanche as well as those of the generated wave. In the experiments, different regimes are identified, including bore, solitary or Cauchy-Poisson waves. The formation of these waves is intimately related to the local Froude number, which corresponds to the ratio of the maximum horizontal velocity of the granular front to the speed of linear gravity waves in shallow water. The generated waves are non-linear in shallow water, but tend to the linear case in the deep-water limit. From there, by focusing on the dry granular collapse dynamics, the characteristic time scales involved are revealed. Finally, a model is proposed to relate the maximum amplitude of the produced wave to the initial geometrical parameters, by combining the results obtained for the dynamics of the collapse and those about the hydrodynamics of the waves in shallow water. The approach successfully compares with the experiments and explains the influence of the different key dimensionless numbers that were proposed in the literature on the subject. This constitutes a first simplified, albeit comprehensive, modelling of the physics underlying tsunamis caused by landslides in a shallow water context. The next steps of this experimental journey include the study of the three-dimensional effects, ubiquitous in real scenarios, as well as the influence of additional parameters such as the physical properties of the granular medium. |
| 18 avril | PhD student : Bérénice DUPONT, Kexin ZHANG & Nathan VANI |
| 25 avril | Krishan Bumma PhD student at Institut d’Alembert Freezing (almost) destroys soap-water foams. The solidification of disordered complex media is a challenging problem that appears in a wide range natural and industrial applications (sea ice, ground freezing, cryopreservation, food industry...). Foams are a type of complex material of particular interest for it’s industrial uses. However solidifying a foam poses distinct challenges due to the presence of gaz bubbles in the solidifying metastable fluid matrix. By placing a model aqueous foam in contact with a cold surface, we observe that, as it freezes, the foam undergoes a drastic change in volume revealing important liquid and gas migration in the foam. We quantify the liquid and the gaz flows as the foam solidifies and show that they follow non-trivial scaling laws. |
| 2 mai | Friedrich Walzel PhD student at Institut Charles Sadron (Strasbourg university) Adhesive bubbles and drops between circular frames : Shape, force and stability analysis We exploit the theory of axisymmetric constant mean curvature surfaces (Delauney surfaces [1]) to describe the mechanical interactions between drops, bubbles or capillary bridges [2,3] held by circular frames with radius R and distance 2h (see figure). We complement the theory with experimental and computational approaches (Surface Evolver [4]). The figure shows our obtained shape diagram, which indicates under which constraints the bubbles remain axisymmetric and in contact. Due to four different instabilities, the bubbles lose contact or lose their axisymmetry. Two of these four instabilities (2 and 3) have been discovered by us. Different contact angles between the bubbles θ are due to different adhesive forces between the bubbles. The shape diagrams of the two limiting cases with θ=0° and θ=180° have been obtained additionally to the case of θ=60° (see Fig.). Using these diagrams and theory, the stability and mechanical properties of capillary bridges or bubbles in contact under tension or compression can be predicted [5]. The provided analysis holds equally for bubbles, drops or capillary bridges and gives an approach to investigate more complex interfaces with for example elastic skins. |
| 9 mai | No coffee seminar (holiday) |
| 16 mai | Haim Diamant Tel Aviv University What hydrodynamic interactions tell us about the colloidal glass transition The motions of colloidal particles slow down tremendously as the suspension approaches the glass transition. This makes an accurate study of the transition a decades-long challenge. We use an indirect approach to bypass this difficulty. Instead of examining the dynamics of the glass-forming particles directly, we learn about it from the dynamics of small tracers that freely move in the host liquid. Correlations between the thermal motions of the tracers report on the flow response of the host liquid, which in turn reports on the state of the larger particles obstructing the flow. A model-independent phenomenological theory predicts distinctive signatures of the glass transition in these hydrodynamic correlations : (a) the power of their decay with relative distance changes ; (b) a length scale related to this decay steeply increases with the increasing viscosity ; and (c) certain correlations switch sign. These predictions are confirmed by experiment. |
| 23 mai | Kevin Simpson Post-doctorant at PMMH Using synthetic biology to study pattern formation in bacteria Understanding how spatially correlated cellular states emerge from the interaction of gene network dynamics is a fundamental challenge in biology. However, the study of gene spatial correlations emerging from cell-cell coupling in natural systems is difficult as complex interactions are the norm. In this sense, synthetic biology offers the possibility of building minimal synthetic genetic networks (SGNs) that incorporate essential features of the process under study, providing a test-bed in which mechanistic processes can be isolated as much as possible from cellular processes. Here, two examples of the use of synthetic biology as a tool to study pattern formation in bacteria will be presented. First, we applied the Ising model as a theoretical framework to study the self-organization of spatially correlated gene expression in two-state SGNs that are coupled by short-range chemical signals in E. coli. Next, we will address the effects of spatially structured habitats on the self-organization of active patterns based on the communication mediated by chemical signals between motile particles. |
| 30 mai | Corentin Bisot ENS Lyon & AMOLF, Amsterdam Carbon supply for growth drives bidirectional flows in mycorrhizal networks The plant-mycorrhizal symbiosis is fundamental to terrestrial ecosystems and is responsible large flows of nutrients at the planetary scale. In this symbiosis, two distinct organisms are trading Phosphorus against energy in the form of Carbon. Yet, the mechanism behind the bidirectional transport of ressources within their microscopic hyphae has not been entirely elucidated. Our findings suggest a tight coupling between network growth, internal carbon (lipid) transport dynamics, and bidirectional fluid movement within the network. The movement of lipid in the closed pipe system inherently produces a backflow of cytoplasmic fluid which may be responsible for the movement of Phosphorus. It has the advantage of providing a simple yet elegant solution to the observed proportionality in ressource transactions between the two organism. |
| 6 juin | No coffee seminar ESPCI new building inauguration |
| 13 juin | General assembly of the lab |
| 20 juin | Lorenzo Betti Post-doc at SIMM laboratory (ESPCI) |
| 27 juin | Katie Wu PhD student (MAE, Princeton University) Title : Capillary Rise in Sharp Corners Abstract : I will discuss the capillary rise of viscous liquids into sharp corners formed by two surfaces whose geometry is described by power laws such that the gap between the increases with the distance from the line of contact according to h(x) = cx^n for n ≥ 1. Prior investigations of capillary rise in sharp corners have shown that the meniscus height increases with time as t^1/3, a result which is universal, i.e., applies to all corner geometries. The universality of the phenomenon of capillary rise in sharp corners is revisited in this work through the analysis of a PDE for the evolution of a liquid column rising into power-law-shaped corners, which is derived using lubrication theory. Despite the lack of geometric similarity of the liquid column cross-section for n>1, there exists a scaling and a similarity transformation that are independent of c and n, which gives rise to the universal t^1/3 power-law for capillary rise. However, the prefactor, which corresponds to the tip altitude of the self-similar solution, is a function of n, and it is shown to be bounded and monotonically decreasing as n goes to infinity. Similarly, the profile of the interface radius as a function of altitude is independent of c and exhibits slight variations with n. Theoretical results are compared against experimental measurements of the time evolution of the tip altitude and of profiles of the interface radius as a function of altitude. |
| 4 juillet | Alejandro Ibarra Post-doc in Mecawet team, PMMH Pneumatic surface morphing plates fabricated through a direct ink writing. We present how to use the direct ink wirting printing technique to create channels within a silicone plate, which when pressurized produce stress in the plane. Depending on the architecture of the channels, it is possible to change the shape of the plate to a three-dimensional structure. |
| 11 juillet | Antoine Bouvier A true coffee seminar : coffee stats in the lab & mechanics of injuries |
| 5 septembre | Sébastien Gomé Title : Near-resonant inverse cascade in rotating turbulence. Inverse cascades occur in flows conserving two sign-definite quantities. This is the rule in 2D turbulence (and in many geophysical models) : large scales are energized because of the constraints of conservation of energy and enstrophy (squared vorticity). 3D turbulence notoriously fails to follow such a rule, because the 3D Navier-Stokes equations conserve energy and sign-indefinite helicity (the correlation between velocity and vorticity), which can be both transferred to small scales. However, 3D flows under rotation are known to exhibit an inverse energy cascade, which generates a large-scale 2D flow orthogonal to the rotation axis. In this talk I will explain why such an inverse cascade occurs : 2D modes interact via near resonances with 3D inertial waves of the same helicity sign. I will also show why the inverse cascade can be blocked if rotation is too strong, and present, to my knowledge, the first numerical evidence of the breaking of the Taylor-Proudman theorem. |
| 12 septembre | Paul Desmarchelier Post-doc at PHENIX Lab, Sorbonne University Topological characterization of rearrangements in amorphous solids In amorphous materials, plasticity is localized and occurs as shear transformations. It was recently shown by Wu et al. that these shear transformations can be predicted by applying topological defect concepts developed for liquid crystals to an analysis of vibrational eigenmodes [Wu et al. ; Nat. Com.,2023]. This study relates the -1 topological defects to the displacement fields expected of an Eshelby inclusion, which are characterized by an orientation and the magnitude of the eigenstrain. A corresponding orientation and magnitude can be defined for each defect using the local displacement field around each defect. These parameters characterize the plastic stress relaxation associated with the local structural rearrangement and can be extracted using the fit to either the global displacement field or the local field. Both methods provide a reasonable estimation of the MD-measured stress drop, confirming the localized nature of the displacements that control both long-range deformation and stress relaxation. If I have the time I will briefly present other work on the propagation of waves in amorphous media. |
| 19 septembre | No coffee seminar : international conference |
| 26 septembre | Giovanni Volpe University of Gothenburg Deep Learning for Microscopy Video microscopy has a long history of providing insights and breakthroughs for a broad range of disciplines, from physics to biology. Image analysis to extract quantitative information from video microscopy data has traditionally relied on algorithmic approaches, which are often difficult to implement, time consuming, and computationally expensive. Recently, alternative data-driven approaches using deep learning have greatly improved quantitative digital microscopy, potentially offering automatized, accurate, and fast image analysis. However, the combination of deep learning and video microscopy remains underutilized primarily due to the steep learning curve involved in developing custom deep-learning solutions. To overcome this issue, we have introduced a software, currently at version DeepTrack 2.2, to design, train and validate deep-learning solutions for digital microscopy. |
| 3 octobre | Jishen Zhang |
| 10 octobre | PhD defense : Camille Aracheloff |
| 17 octobre | Phd defense : Joseph Vermeil |
| 24 octobre | Phd defense : Gauthier Bertrand |
| 31 octobre | Autumn holidays : no coffee seminar |
| 7 novembre | PhD student : Diane Komaroff & Auriane Huyghues-Despointes |
