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Juillet 2018 à Marseille : 16th International Workshop on the Physics of Compressible Turbulent Mixing (IWPCTM)

Site web : http://www.iwpctm16.fr/
Evènement organisé par les membres de l’axe de Recherche ECOCI

 

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Séminaire IUSTI – 25 juin 2021 – 11h amphi

Active matter for self assembly

Jérémie Palacci – UC San Diego, ÉU

Biological systems show remarkable and robust self-assembly: bacteria form colonies, cells reshape and muscle fibers collectively contract… Those phenomena stem from the non-equilibrium nature of living matter, a prototypical example of active matter in which self-driven units convert an energy source into useful motion and work. Inspired by the success of the biological world, we will show how we can build and control man-made materials powered from within.
As a first example, we will discuss the effect of an active bacterial bath on the aggregation of attractive microbeads. We will notably show that we can control the morphology of the aggregates, possibly programming the mechanical response of a soft material.
In a second part, we will show how active particles self-assemble or can be assembled into autonomous and programmable metamachines. Because active particles can differentiate to provide multiple functions, machines are readily reconfigurable, merged and annealed, allowing for more sophisticated machinery. Our work shows the potential of active matter for self-assembly and the development of dynamical and reconfigurable materials.

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Séminaire IUSTI – 11 juin 2021 – 11h amphi

Elastically limited liquid-liquid phase separation inside cells

Pierre Ronceray – CENTURI, Marseille

Many intracellular bodies have been shown to be membrane-less liquid droplets that form through liquid-liquid phase separation (LLPS), both in the cytoplasm and in the nucleoplasm. In contrast to the archetypal oil-in-water demixing, the intracellular environment puts mechanical constraints to the formation of large droplets. In the cell nucleus, in particular, the elastic response of the chromatin network has been shown to oppose LLPS. Here we theoretically consider three scenarios by which LLPS can occur in such an elastic network: (i) by cavitation of large droplets that exclude the network, (ii) by forming many mesh-size-scale microdroplets in the pores of the network, and (iii) by permeating through the network and including it in large droplets. We propose simple criteria for which scenario is preferred, introducing a phase diagram controlled by the trade-off between elastic modulus, liquid-liquid surface tension, and liquid-network wetting properties. Our theory predicts the possibility of yet-unobserved mesh-size-limited liquid droplets in the cytoplasm and nucleoplasm.

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The CoPerMix project on mixing, led by E. Villermaux and involving H. Lhuissier and B. Metzger at IUSTI and many other partners across Europe, has just started.

Please check out our new website (https://www.copermix-itn.eu/) and also the PhD offers (https://www.copermix-itn.eu/vacancies/) including ours at IUSTI (https://bloenmetzger.wordpress.com/2021/02/01/marie-curie-phd-position-available/).

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Mathieu Souzy is the winner of the visualization challenge from the Digital Rock Portal (https://www.digitalrocksportal.org/) of the National Science Foundation. Watch this video to experience the flow within a 3D porous media (https://www.youtube.com/watch?v=NkHmAcAheAQ&feature=youtu.be). Note that this video was generated from 3D experimental measurements of the fluid velocity field within a random stack of spherical particles [data are available here (https://www.digitalrocksportal.org/projects/175) and corresponding paper here (https://bloenmetzger.files.wordpress.com/2020/03/2020_souzy_jfm.pdf)]. This data has been used to develop Pore Aventura, a 3D velocity field explorer applicable to any 3D velocity field, see for more on GitHub (https://github.com/Nico04/Pore-Aventura).

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Sanjeev Kumar, Marc Medale, Paolo Di Marco and David Brutin

The evaporation of sessile drops of various volatile and non-volatile liquids, and their internalflow patterns with or withoutinstabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experimentplanned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate dropevaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation ofhydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 SoundingRocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravityfor six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the de-pinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and themeasured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions onthe sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process.

npj Microgravity (2020) 6:37 ; https://doi.org/10.1038/s41526-020-00128-2

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Séminaire IUSTI – 05 fév. 2021 – 11h amphi

Bulles inertielles confinées : une drôle d’hydrodynamique et un transfert de masse fascinant

Karine Loubière – LGC, Toulouse
Nicolas Dietrich – INSA, Toulouse
Véronique Roig, IMFT, Toulouse

Les écoulements à bulles, le plus souvent réactifs, sont utilisés dans de nombreux procédés industriels, notamment en pétrochimie, traitement des eaux ou industrie pharmaceutique. L’analyse des phénomènes de transport en leur sein, et de leur couplage avec les cinétiques chimiques, est fondamentale pour optimiser les procédés actuels (rendement, sélectivité, efficacité énergétique, etc), mais également pour proposer de nouvelles méthodes et/ou technologies en rupture. Récemment, des réacteurs microstructurés continus (i.e. monolithes, réacteurs-échangeurs) mettant en jeu des géométries extrêmement confinées se sont développés comme alternatives aux procédés batch existants, permettant notamment d’intensifier de manière significative le transfert de masse aux interfaces gaz-liquide.
Ce séminaire se propose de présenter plusieurs techniques optiques de mesures, novatrices et prometteuses, pour analyser le transfert de masse dans des écoulements à bulles. Ces techniques ont été développées dans différents laboratoires toulousains, notamment au sein de la fédération de rechercher FERMaT. Elles s‘appuient sur des réactions chimiques mettant en jeu des espèces colorées, ou sur l’inhibition de la fluorescence du réactif gazeux transféré.
Dans un premier temps, les potentialités de ces techniques seront illustrées au travers de résultats expérimentaux récents sur le transfert de masse réactif, obtenus dans des configurations d’écoulements à bulles confinées ou non. Un focus sera ensuite porté sur une géométrie de réacteur à bulles de taille intermédiaire, dans lequel des bulles de diamètre d sont confinées entre deux plaques planes séparées par une distance inférieure à d ; l’idée étant qu’un tel réacteur profiterait ainsi des bonnes propriétés de mélange des colonnes et des bonnes propriétés de transfert de masse des monolithes.
Nous nous intéresserons ici au problème générique du mouvement d’une bulle inertielle en ascension dans un fluide au repos confiné entre deux plaques planes, à la perturbation qu’elle induit dans le fluide et au transfert de masse qui peut être activé dans cette configuration. Nous montrerons que, pour de larges gammes de nombres d’Archimède, des lois d’échelle génériques ont pu être obtenues concernant la cinématique des bulles, et que, dans cette configuration, un couplage fort existe entre instabilité de trajectoire et instabilité de sillage.
Enfin, via les techniques optiques de mesure précédemment décrites, le transfert de masse autour d’une bulle d’oxygène pur en ascension libre dans ce réacteur sera étudié. Les champs de concentrations en oxygène dissous mesurés dans le sillage de la bulle serviront de base pour introduire les lois d’échelle associées et discuter les mécanismes de transport du constituant dissout en phase liquide.

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Séminaire IUSTI – 22 janv. 2021 – 11h amphi

Mechanical properties of fiber networks

Mehdi Bouzid – UGA, Grenoble

Gels of fibrous biopolymers are ubiquitous within cells and their rigidity is crucial for their function [1]. Our current understanding of their elastic response is usually understood as an interplay between the bending and stretching of their filaments [2]. This point of view however fails when applied to the weakly coordinated branched actin networks found throughout the cell [3, 4]. Through experiments and theory, we show that their elasticity crucially involves reversible entanglements between their filaments. Additional entanglements may get locked in during network growth, setting the final properties of the network [5]. These properties could be key to understanding how moving cells dynamically adapt their cytoskeleton to their environment.

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Baptiste Darbois Texier, Henri Lhuissier, Yoël Forterre & Bloen Metzger

Summary

The New Wave’

Oobleck continues to fascinate us. Researchers from Aix-Marseille University in France showed that, as it flows down an inclined slope, Oobleck forms very regular and prominent surface waves. Similar waves can be observed on gutters and windows on rainy days. However, scientists have shown that here, for Oobleck, the origin of wave formation is completely different: they do not arise from the effect of inertia as for water, but from Oobleck’s specific flowing properties.

Under impact, as shown by recent studies, Oobleck suddenly changes from liquid to solid because of the activation of frictional contacts between the starch particles. When flowing down a slope, this proliferation of frictional contacts leads to a very curious behavior: the flow velocity of the suspension decreases when the imposed stress is increased (As if stepping on the gas pedal would suddenly make your car decelerate !). Researchers have shown that this effect couples to the flow free-surface and can spontaneously generate a regular wave pattern.

The proposed mechanism is very generic. These findings could thus provide new grounds to understand other flow instabilities observed in various configurations, particularly in industrial processes facing problematic flow instabilities when conveying Oobleck like materials, such as concrete, chocolate or vinyl materials.
ref : Baptiste Darbois Texier, Henri Lhuissier, Yoël Forterre & Bloen Metzger, « Surface instability without inertia in shear-thickening suspensions », Communications Physics 3:232 (2020) https://doi.org/10.1038/s42005-020-00500-4

see the Highlight in Nature Reviews Physics https://www.nature.com/articles/s42254-020-00277-z, (shareable link at https://rdcu.be/cc9BE) and in the newspaper Le Monde (in french) https://www.lemonde.fr/sciences/article/2021/01/13/la-maizena-fait-des-vagues_6066135_1650684.html.

Contact : Bloen Metzger, Laboratoire IUSTI, CNRS UMR 7343,Aix-Marseille Université

Projet financé par l’ERC PLANTMOVE, l’ANR Science-Friction et le LABEX MEC

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Article dans la provence sur l’analyse de fumigènes par  la plateforme Feux de l’IUSTI: 
https://www.laprovence.com/article/om/6147102/lom-etudie-les-fumis.html
Quand le monde universitaire rencontre le milieu de l’OM, il faut se méfier des étincelles. Qui plus est quand il s’agit de fumigènes. Pas de quoi affoler pour autant Bernard Porterie, professeur à l’Institut Universitaire des Système Thermiques Industriels (IUSTI), tout comme Yannick Pizzo, responsable de la plateforme Feux d’Aix-Marseille Université. Il faut dire que leur spécialité est la recherche sur les incendies (feux de forêts, de navires, industriels…), à base d’expérimentations et de modélisations. “L’idée était de faire une étude comparative entre les fumigènes standards, dits chauds, et les nouveaux, dits froids. Cette dénomination ne nous convient pas trop car, d’après nos essais, il y a quand même de la chaleur. On est à 200-300 degrés de moins avec le modèle danois (1 000 degrés à 20 cm de la flamme) par rapport aux fumigènes classiques (1 300 degrés)”, nuance Bernard Porterie. “On a fait l’essai avec un banc expérimental, sur lequel on retrouvait un support avec les deux sortes de fumigènes dans des attitudes identiques, développe Yannick Pizzo. On voulait mesurer la température, les différentes élévations le long de l’axe de la flamme, l’opacité et la toxicité des fumées générées, la luminosité, mais aussi la température sur le manche, où la main est posée, ainsi que sur le corps du fumigène. Avec une cible placée au-dessus des fumigènes, on a également mesuré le flux de chaleur. »
*credit photo: Photos nicolas vallauri

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Best-Paper-_ALLAM_6_11_20

Présentation. – Proceedings : https://phdsymp2020.sciencesconf.org/data/pages/Proceedings_phdsymp_2021.pdf
– Youtube presentation : https://www.youtube.com/watch?v=TosfqmLU9oo

Self-sensing concrete, also known as “Smart concrete”, is obtained by including electrically conductive fibers in cement-based materials. These fibers may allow to reduce electrical resistivity and develop a piezoresitive behaviour. Smart Concrete could therefore be simultaneously both a structural and a sensing material, which eliminates the need for external instrumentation in Structural Health Monitoring.
By increasing the fiber volume fraction within the cement matrix, the electrical resistivity  of the material is reduced once percolation threshold is reached. Above this percolation threshold, the fiber content is high enough to allow conductive particles to be in contact or very close to each other, thus creating a continuous conductive network within the insulative matrix. Sand’s presence was stated to have an influence on resistivity in case of fibred mortar: a high sand content may prevent the network of conductive fibers from percolating. This phenomenon is referred to as “double percolation”: the cement paste needs to be a continuous phase between sand aggregates in order to allow fibers to maintain their efficiency in reducing the electrical resistivity of composites.
However, little attention has been given to the impact of the size of sand grains on the electrical percolation. This work intends to study the effect of the grain size distribution and volume fraction of sand within mortars containing various fiber volume fractions. The results confirm. the “double percolation” phenomena: when the volume fraction of sand is close to its maximum packing density, the addition of fibres was not as effective in reducing the electrical impedance of mortar samples. In addition, sand’s grain size distribution proved an influence on impedance of mortar: fine sand showed higher impedance compared to standard sand, especially in case of high sand volume fraction. This could be related to the smaller maximum packing density in case of fine sand, where distance between particles would be in average reduced. This effect, combined with the higher number of insulative particles, could probably disrupt the continuity of the conductive network of fibres within mortar.

 

 

 

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