Séminaires café 2018

Archive pour les séminaires café de 2018.

–  6 décembre 2018 - Lisa Maillard
Flow-induced bending of disks settling in water Interactions between flexible bodies and the surrounding fluid medium are ubiquitous in nature. Examples range from the great trash island of plastic bags in the Pacific Ocean to the swimming of jellyfish. To study such interactions, a widely used model system is the settling of a body under the influence of gravity. Here, we investigate the bending of a thin, freely-falling elastic disk in water as a function of its relative flexibility. The parameters of the disk are fixed to produce a zig-zag, fluttering motion. Non-monotonous changes in the fluttering amplitude are observed as a function of increasing flexibility. We show that, as the disk flutters, it experiences out-of-plane bending that dynamically switches between the leading edge and the sides of the disk. These observations differ from those of rectangular fluttering wings in which only the leading edge deflects. These findings suggest that flexibility alters the nature of the fluid-disk interactions in a non-trivial matter, that could be potentially exploited for various applications in engineering and bio-related problems.

–  29 novembre 2018 - Ryan Ewing (Dept. of Geology and Geophysics, Texas A&M University)
New thresholds predict frequent aeolian transport on Mars

–  22 novembre 2018 - Zeke Berman (Artiste-photographe et professeur au City College of New York)
Measuring Cup with Necker Cube Spill
Zeke Berman will show his still life photographs and offer a discussion about both the similarities and differences between working in a scientist’s laboratory and an artists’s studio. *Zeke Berman* has been making singular, studio-based photographs since the late 1970’s, when, while recording his sculptures, his attention turned from objects to images. The formal range of his work, and his inventive use of materials, such as string, clay and ice is varied, original and idiosyncratic. His photographs reflect a long-standing interest in the evolution of visual cognition, the mystery and paradox of optics, as well as finding an intersection between sculpture, photography and drawing. If an artist can be said to possess esthetic values, his would best be
described as epistemological.

–  15 novembre 2018 - Roberta Baggio (Doctorante au PMMH) Homogeneous Nucleation of Dislocations

–  8 novembre 2018 - Elena Marensi (Postdoc à l’université de Sheffield)
Stabilisation and drag reduction of pipe flows by flattening the base
profile
It is well known that turbulent wall flows exert a much higher friction drag than the laminar ones and thus, in many industrial applications, such as oil and gas pipelines, are not desirable. Recent experiments (Kuehnen et al., 2018) showed that a full collapse of turbulence in pipe flows can be achieved by placing an obstacle in the core of the flow that flattens the streamwise velocity profile. My focus is on theoretically capturing the phenomenon observed experimentally so that the process can then be optimised. Of fundamental interest from this viewpoint is the so-called minimal seed, i.e. the initial perturbation of lowest energy capable to trigger transition to turbulence. In this talk, I will first address the question of how representative the minimal seed is of typical ambient disturbances by comparing its transition behaviour with those of different randomised perturbations. A set of suitable initial conditions are thus generated to investigate the stabilising effect of a simple model for the presence of the experimental baffle in the core of the flow. Enhanced nonlinear stability of the laminar flow and significant drag reductions are obtained, which are encouraging first results in modelling the experiments of Kuehnen et al. and motivate us to further investigate this promising direction for flow control.
–  25 octobre 2018 - Alexander Morozov (Institute for Condensed Matter and Complex Systems)
The role of correlations in the collective behaviour of microswimmer suspensions
Recent years witnessed a significant interest in physical, biological and engineering properties of self-propelled particles, such as bacteria or synthetic microswimmers. The main distinction of this ’active matter’ from its passive counterpart is the ability to extract energy from the environment (consume food) and convert it into directed motion. One of the most striking consequences of this distinction is the appearance of collective motion in self-propelled particles suspended in a fluid observed in recent experiments and simulations : at low densities particles move around in an uncorrelated fashion, while at higher densities they organise into jets and vortices comprising many individual swimmers. Although this problem recieved significant attention in recent years, the precise origin of the transition is poorly understood.

In this talk I will present a numerical method based on a Lattice-Boltzmann algorithm to simulate hydrodynamic interactions between a large number of model swimmers (order 10^5), represented by extended force dipoles. Using this method we simulate the transition to large-scale structures in dilute suspensions of self-propelled particles and show that, even well below the transition, swimmers move in a correlated fashion that cannot be described by a mean-field approach. We develop a novel kinetic theory that captures these correlations and is non-perturbative in the swimmer density. To provide an experimentally accessible measure of correlations, we calculate the diffusivity of passive tracers and reveal its non-trivial density dependence. The theory is in quantitative agreement with the Lattice Boltzmann simulations and captures the asymmetry between pusher and puller swimmers below the transition to turbulence.
Reference : J. Stenhammar et al. Phys. Rev. Lett. 119, 028005 (2017)

–  18 octobre 2018 - Soutenance de thèse de Salomé Gutierrez-Ramos : Pas de séminaire café

–  11 octobre 2018 - Gaspard Junot (Doctorant au PMMH)
Swimming of a bacteria in a Poiseuille flow : the quest for active Bretherton-Jeffery trajectory
Using a 3D Lagrangian tracking technique, we determine the swimming trajectories of a non-tumbling E.coli mutant in a Poiseuille flow. We identify many trajectories displaying the qualitative features of the “Bretherton-Jeffery” model featuring an axi-symmetric self-propelled ellipsoid swimming in a flow. By deriving the close-forms for two phase-portraits involving one spatial and two angular coordinates as well as the mean angular shift displayed by families of cycloid trajectories, we show that a mapping with the actual experimental trajectories can be made and values for the effective aspect ratio of the bacteria can be extracted. This analysis also allows to discuss the crucial influence of the Brownian rotation noise, on the swimmer trajectories.

–  4 octobre 2018 - Rhoda Hawkins (Université de Sheffield)
Modélisation de mécanique et mouvement des cellules
The mechanics of biological cells is dominated by the polymer protein network known as the cytoskeleton. In this talk I will outline ways in which we model the cytoskeleton using both analytical theory and simulations in combination with experimental data. My goal is to obtain a more profound understanding of the physics of this fascinating, complex, active matter. Furthermore we seek to comprehend how this out of equilibrium material not only affects cell mechanics but also drives cell shape change and movement. I will focus on two examples, both involving the cytoskeletal protein actin. The first involves stochastic simulations of force generation by actin polymerisation in phagocytosis, which is a biological process important in the immune system. The second involves continuum hydrodynamic models of contractile actomyosin and its role in cell migration in complex confining environments.

–  20 septembre 2018 - Léo-Paul Euvé (Postdoc au PMMH)
Sillage de bateau en milieu anisotropique

–  13 septembre 2018 - Sergio Chibbaro (chercheur à L’IJLRDA)
Fluctuations et événements extrêmes : le cas de la turbulence d’onde
Après avoir présenté brièvement mes travaux de recherche, je me concentrerai sur l’analyse des fluctuations dans la turbulence d’onde dans des plaques. La présence des fluctuations anomales, aussi appelée intermittence, est une propriété importante des écoulements turbulents. Dans la turbulence d’onde, c’est à dire lorsque beaucoup d’ondes dispersives interagissent de façon faiblement non-linéaire, l’intermittence a été moins étudiée et les résultats ne sont peu concluants, l’intermittence semblant a priori moins forte que pour la turbulence des fluides. Nous avons étudié l’intermittence en turbulence d’onde pour des plaques en vibration à l’aide de simulations numériques. Nous obtenons une faible intermittence lorsque l’amplitude de forçage est faible, qui devient plus forte lorsque les effets non-linéaires augmentent. Les résultats peuvent être interprétés dans le cadre d’un modèle multifractale, ce qui montre que des grandes déviations caractérisent le système et que la théorie de champ moyen ne peut pas capturer tous les détails.

–  06 septembre 2018 - Véronique Lazarus (chercheuse au FAST)
Motifs de fissures complexes

Que ce soit dans le vivant (gerçures, fractures,...), nos aliments (comté, caramel...), nos logements (murs, sols, peintures...), la Nature (failles sismiques, colonnes basaltiques, écorces...), ou encore dans des composants industriels (moyens de transport, écran de téléphone, turbines...), les fissures constituent des motifs de rupture qui souvent nous inquiètent et dont on cherche à évaluer la dangerosité, mais qui sont parfois utiles ou provoqués à dessein, et dont la richesse toujours nous fascine. Dans le contexte de la mécanique de la rupture fragile, je présenterai certains résultats récents ayant comme trait commun des formes complexes : propagation le long d’une interface hétérogène, déstabilisation d’une fissure plane en segments, multi-fissuration de rétraction.

–  05 juillet 2018 - Cyprien Gay (chercheur au MSC)
Peut-on inférer la mécanique d’un épithélium ? / Can one infer the mechanics of an epithelium ?

–  21 juin 2018 - Juan F Marin (doctorant à l’Université de Chile en visite au PMMH)
Generating and controlling soliton bubbles in two-dimensional Josephson
junctions

–  14 juin 2018 - Reda Belbahri (doctorant au PMMH)
Rôle des protéines régulatrices de l’actine pour la génération de forces des réseaux branchés

–  07 juin 2018 - Julien Heuvingh (maître de conférences au PMMH)
Séminaire d’ouverture sur l’archéo-génétique

–  31 mai 2018 - Lucie Ducloué (post-doctorante au PMMH)
Secondary flows of viscoelastic fluids in curved microchannels

–  24 mai 2018 - Séminaire spécial : 50 ans de mai 68 (chercheurs au PMMH)
{}

–  17 mai 2018 - Francesca Tesser (post-doctorante au PMMH)
Chiral micro-particles interacting with viscous shear flows

–  10 mai 2018 - Pas de séminaire : jeudi de l’ascension

–  03 mai 2018 - Andreas Zöttl (post-doctorant au PMMH)
Modeling surfaces rheotaxis of E. coli bacteria

–  19 avril 2018 - Joachim Trosseille (doctorant au PMMH)
Écoulement gravitaire de gouttes de condensation sur substrat rugueux

–  12 avril 2018 - Jean Cappello (doctorant au PMMH)
Dynamics of flexible fibers in confined viscous flow

–  5 avril 2018 - Charlotte De Blois (doctorante au laboratoire Gulliver )
Water swimming droplets : Effect of confinement
2D and 1D geometries

–  29 mars 2018 - Benoit Vincenti (doctorant au PMMH)
Vortex generation in magnetotactic bacteria droplets

Magnetotactic bacteria are micro-organisms that are able to swim along magnetic field lines. The swimming hydrodynamics of these bacteria at low Re is currently the focus of a tremendous research activity due to their potentiality to control flows at the microfluidic scale. We recently showed that the rheology of magnetotactic bacteria suspensions can be tuned by the application of a constant magnetic field such that a macroscopic shear flow can be blocked or enhanced by the bacteria swimming.

During the seminar, I will present an experiment that shows the emergence of macroscopic spontaneous flows for such suspensions. We study collective motion of magnetotactic bacteria confined in aqueous droplets (diameters from 50 to 300micro) surrounded by oil. The application of a magnetic field leads, above a critical threshold in bacteria density (dense limit), to the generation of a vortex flow at the center of the droplet. I will summarize the different results I got so far, in particular the influence of the different parameters of the problem (bacteria density, magnetic field amplitude ...) on the vortex generation.

–  15 février 2018 - Pas de séminaire café : PC Focus à Chimie Paris

–  8 février 2018 - Ambre Bouillant (doctorante au PMMH)
The life of a Leidenfrost drop

–  01 février 2018 - Présentation des sujets de thèse : 1ère année

–  25 janvier 2018 - Pas de séminaire café : Journées de la Physique Statistique

–  18 janvier 2018 - Lucie Domino (doctorante au PMMH)
Peut-on faire de la physique du solide avec des vagues ?

–  11 janvier 2018 - Olivier Mesdjian (post-doctorant au PMMH)

–  04 janvier 2018 - Pas de séminaire café : vacances de Noël