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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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Le réseau des mécaniciens (RDM) de la MITI organise avec le soutient de la Formation Permanente de la DR12 une Action Nationale de Formation intitulée “LA MECANIQUE ET LE MONDE DU VIVANT”. Informations et inscriptions sur notre site rdm.prod.lamp.cnrs.fr. Un évènement CNRS, avec le soutien de la DR12 CNRS pour les agents des EPST. A suivre en présentiel du 19 au 22 octobre 2021 à Carqueiranne (VAR) ! Télécharger le descriptif de la formation.

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Une visite insolite à l’IUSTI le 6 octobre pour découvrir : Comment voir dans le noir ? Pourquoi peut-on courir sur une piscine de maïzena ? Comment les plantes carnivores attrapent les insectes ? Peut-on faire cuire des aliments avec la chaleur du soleil ?https://www.fetedelascience.fr/les-visites-insolites-du-cnrs-tromper-ses-sens-l-institut-universitaire-des-systemes-thermiques

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Séminaire IUSTI – 1er octobre 2021 – 11h salle 250

Two new jamming scenarios – oscillatory and directional

Martin Trulsson – Lund Univ, Suède

Recent studies have shown that cross-flows oscillatory shear flows can reduce a suspension’s viscosity. Furthermore, these oscillations can push the shear-jamming transition to higher packing fractions. In this talk, we will see that these oscillations lead to a cross-over from steady-state rheology to an oscillatory, distinct from the steady-state one for small oscillatory strains. Moving to elliptical particles, we will see yet another jamming scenario where a suspension flows in one direction but jams in the opposite direction. Applying oscillations to try to unblock these jammed states turns out harder than predicted, leading to protocol-dependent oscillatory shear rheology.

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Séminaire IUSTI – 17 septembre 2021 – 11h salle 250

Surface tension and the strain-dependent topography of soft solids

Nicolas Bain – ETH Zurich, Suisse

Despite its importance in any adhesion and wetting phenomena, there is a fundamental property that is not yet understood in soft solids: surface elasticity. Also called the Shuttleworth effect, surface elasticity can be boiled down to one question. Does stretching the surface of a soft solid change its surface tension? In 2017, Xu et. al designed an experiment in which the opening angle of a wetting ridge was a proxy to evidence a dramatic increase of surface tension with stretch. In 2020, however, Dervaux et al. claimed that the coupling between nonlinear mechanics and the singular nature of the wetting ridge suffice to explain the behavior of the opening angle observed by Xu et al, without invoking the Shuttleworth effect. The question therefore remains open. This presentation will focus on an experimental setup with no geometric singularity, that leaves no doubt on the existence or absence of surface elasticity in soft solids, hopefully closing this long-lasting controversy.

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Séminaire IUSTI – 03 septembre 2021 – 11h salle 259

Building with fluids, lazy design of functional materials

Pierre-Thomas Brun – Princeton Univ, ÉU

From acoustics to optics, electronics and mechanics, the need for structured materials is well established. Examples include lightweight structural materials, photonics and phononic materials, super-hydrophobic materials and mechanical metamaterials. Despite the recent progress of 3D printing, the fabrication of such structures spanning a wide range of sizes remains difficult or impossible, prompting the development of new fabrication pathways. The work I will present is concerned with the directed control of mechanical instabilities to program shapes. While instabilities are traditionally regarded as a route towards failure in engineering, I aim to follow a different path; taming instabilities and harnessing the patterns and structures they naturally form to fabricate functional materials. This methodology capitalizes on the inherent periodicity, scalability, versatility and robustness of instabilities. This new design paradigm – building with instabilities – calls for an improved understanding of instabilities and pattern formation in complex media. While stability analysis is a classic topic in mechanics, little is known on the so called inverse problem: finding the optimal set of initial conditions and interactions that will be transmuted into a target shape without direct external intervention. Three examples will be presented: (1) a fluid-instability based approach for digitally fabricating geometrically complex uniformly sized structures, (2) the rapid fabrication of nearly uniform hemispherical elastic shells by drainage and (3) their pneumatic actuation towards shape morphing applications.

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Séminaire exceptionnel – 09 juillet 2021 – 11h amphi

Point-particle simulations of complex turbulent dispersed flows

Cristian Marchioli – Univ. Udine, Italie

Particle transport and mixing in turbulent flows are fundamental to science as well as to technology. The simplest numerical framework to study the dynamical and statistical features of turbulent particle dispersion is based on the assumption that particles can be modeled as point-like spheres brought about by the flow. In spite of its simplicity, this framework has led to significant advancements in the study of particles-turbulence interactions, allowing the precise identification of the coherent structures responsible for particle sedimentation and re-entrainment in turbulent boundary layers. In this talk we examine two possible sources of bias in particle dispersion, which arise when particles are non-spherical (elongated) and may actively move within the fluid (motile). In particular, we present results relevant for particles suspended in environmental and wall-bounded turbulence, showing how particle motion, preferential concentration and accumulation in turbulent boundary layer can be modulated by elongation and by motility.

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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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