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

 

Séminaire IUSTI – 13 déc. 2019 – 11h salle 250

Emerging order, from crystals of colloids to fish schools

Sophie Ramananarivo – LadHyx, Palaiseau

Living systems constantly exploit active fluctuations in their processes, to boost transport or assist assembly. Self-propelled colloids, that consume energy to move hold the same potential for man-made assembly of microparticles. We show here that introducing a small amount of microswimmers in a monolayer of passive beads massively accelerates the relaxation to an ordered crystal. Activity-induced internal agitation allows to overcome kinetic barriers and activate the annealing. The use of such active dopants offers potential to control the properties of matter in space and time. In the first part of this talk, we make use of swimmers to assist the assembly and organization of its environment. In the second part (at a much bigger scale), a surrounding fluid environment conversely mediates interaction between fish or birds, thus promoting ordering into schools and flocks. Using physical experiments that mimic the movements of fins or wings, we discover that flapping bodies not only swim or fly faster when grouped together but that the flows also spontaneously organize the group into patterns with specific spacings. These findings suggest a powerful analogy between animal groups and states of matter, in that a school might be viewed as a ‘swimming crystal’ of fish organized by flows.

Séminaire IUSTI – 29 nov. 2019 – 11h salle 250

Propagation of defects by passive and active load in systems with micro-inhomogeneities

Nikolay Gorbushin – Institut de Géophysique du Globe, Paris

Microstructure plays important role in material modelling as it brings new insights in understanding of moving defects. Examples include the Peierls stress for dislocations or lattice trapping for cracks which predict the resistance of defect movement in lattices not observed by continuum treatments. Using a bi-stable mass-spring model, it is possible to show that the presence of external vibrational loads can enhance defect propagation. Moreover, if the mechanical system is able to generate active stresses, these defects can move with supersonic speeds without dissipation.

Séminaire exceptionnel – 15 oct. 2019 – 15h salle 259

Clogging in bottlenecks

Iker Zuriguel – Univ. de Navarra, Espagne

When a group of discrete bodies pass through a narrowing, the flow might become intermittent due to the development of clogs that obstruct the constriction. Despite the importance of clogging in fields such as crowd dynamics, colloids, granular and active matter, the physical mechanisms behind this phenomenon are still under discussion. Based on experiments of granular silos, we will disclose the main variables involved in the emergence of clogs and their subsequent destabilization by means of an external input of energy. Finally, we will briefly comment on some analogies (and differences) observed with the flow of pedestrians through narrow doors.

Séminaire fédération – 4 oct. 2019 – 11h Amphithéâtre Fermi

Hydrodynamic quantum analogs: droplets walking on the impossible pilot wave

John Bush – MIT, Boston, ÉU

Yves Couder and Emmanuel Fort discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wave field, and so represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results allow us to rationalize the emergence of quantum-like behavior in this hydrodynamic pilot-wave system in a number of settings, and explore its potential and limitations as a quantum analog.

Séminaire exceptionnel – 2 oct. 2019 – 11h salle 250

Overview of Recent and Ongoing Research of Black Carbon at NRC

Fengshan Liu – Univ. of Toronto, Canada

Black carbon (BC) emitted from combustion systems have drawn increasing attention from researchers to regulating bodies due to its strong impact on human health and climate. However, there is currently no standard BC measurement method nor reference material. Different measurement methods give quite different BC mass concentrations, which pose challenges in BC regulations and evaluations of the effectiveness of BC mitigation strategies. The Black Carbon Metrology (BCM) team at the National Research Council (NRC) Canada undertakes a wide spectrum of research from fundamental study of the effect of morphology on BC mass absorption cross section (MAC) and effect of detection wavelengths on soot volume fraction measurements using auto-compensating laser-induced incandescence (AC-LII) to applied research of BC emissions from aero-engines, vehicles, and marine engines. In this presentation, in addition to provide an overview of the BCM research activities, I will discuss in some details of recent developments of the CPMA-Electrometer Reference Mass Standard (CERMS) technique as a calibration method for nvPM, the Argonaut mini-inverted soot generator, effects of non-uniform laser fluence and detection wavelengths effect on AC-LII measurements, and the scattering truncation issue of the Aerodyne cavity attenuated phase-shift PMSSA monitor.

Séminaire IUSTI – 20 sept. 2019 – 11h salle 250

Colloids and liquids from suspensions to superhydrophobicity

Philippe Bourrianne – MIT, Boston, ÉU

Colloidal suspensions are ubiquitous in our daily life. Micrometric particles dispersed in a solvent are indeed present in common liquids such as paints, inks or even food products. We will discuss the properties of those colloidal suspensions from their liquid phase to solid deposits after drying. First, colloidal suspensions exhibit a wide range of rheological behaviors from shear-thinning to yield stress fluids. We will focus on the shear-thickening transition as dense suspensions experience a dramatic increase in viscosity above a critical shear-stress. By modifying the physico-chemistry of the particles, we can tune this rheological transition and thus understand the interactions involved in this behavior. Increasing concentration can also be noticed during drying when solvent evaporates: particles finally form a solid deposit. After drying, a drop of a colloidal suspension leads to a variety of patterns from coffee-stain to more homogeneous coatings in paintings. We will discuss the effect of the initial concentration of particles on the drying pattern and on the subsequent mechanical instabilities such as cracks propagation. Finally, after the whole drying of the colloidal suspension, coatings are achieved. Depending of the nature of the particles, we can tune the wettability of the substrate up to superhydrophobic solid. We will briefly discuss how such a water-repellent substrate can allow levitation of liquids.

Séminaire IUSTI – 6 sept. 2019 – 11h salle 250

Hydrodynamics of motile and sinking microorganisms

Raphaël Jeanneret – LPS, Paris

In this talk I will present the results of two independent projects regarding the physics of phytoplankton. The first project deals with a very large class of non-motile phytoplankton called diatoms. Despite lacking motility we’ll see how these elongated unicellular organisms manage to encounter each other when sinking in quiescent fluids thanks to a hydrodynamically-driven density instability (instability well-studied at the IUSTI lab a few years back!). Our results pave the way for further investigating the yet poorly understood sexual reproduction that these globally-dominant organisms must occasionally go through to ensure the survival of the population. The second project is about the self-generated flow-fields that swimming microorganisms produce when swimming near surfaces. As opposed to previous theoretical predictions, we’ll show how the presence of no-slip boundaries enhances the diversity of microbial flow-fields compared to the bulk situation. Such results should have a large impact in e.g. our understanding of the emergence of collective behaviour of active suspensions in confined situations.

Séminaire exceptionnel – 2 sept. 2019 – 11h salle 252

AEvaporation-induced stabilization of non-aqueous foams

Gerry Fuller – Stanford Univ, ÉU



Depuis 2004 le dispositif Apprentis Chercheurs permet à des collégiens et lycéens associés en binômes d’être accueillis régulièrement tout au long de l’année scolaire dans des laboratoires de recherche de leur quartier. Ils y mènent un projet scientifique, s’initient à la recherche et découvrent l’écosystème de la recherche, en immersion.

Au mois de juin ils présentent leurs travaux lors d’un congrès regroupant à Marseille tous les Apprentis Chercheurs Marseillais.

Cette année, le congrès apprentis Chercheurs s’est tenu à l’Institut de Microbiologie de la Méditerranée (IMM) en présence de Monsieur Nicolas CLAIRE, Vice Président Délégué à la Culture Scientifique et à la Recherche d’Aix-Marseille Université. Voir le programme ici

Le public était au rendez-vous (près de 120 personnes), les présentations de haute volée, les parents fiers de leurs enfants. L’intervention de deux anciennes apprenties (promotion 2017-2018) était pleine de maturité et de recul. Et tout cela dans l’ambiance chaleureuse et conviviale qui caractérise ce congrès.

L’IUSTI, participant depuis la première année, a présenté des travaux intitulés : “Les ondes de choc peuvent-elles êtres les antibiotiques de demain ? “

Outre le quotidien La Provence, un journaliste de Science et Avenir était également présent en “repérage” : il souhaite faire l’année prochaine un reportage, avec visite en labo, pour leur future revue Science et Avenir Junior…

Séminaire exceptionnel – 4 juil. 2019 – 11h salle 250

A minimal-length approach unifies rigidity in under-constrained materials

Matthias Merkel – CPT, Marseille

What do a guitar string and a balloon have in common? They are both floppy unless rigidified by geometric incompatibility. The same kind of rigidity transition in under-constrained materials has more recently been discussed in the context of disordered biopolymer networks like collagen and models for biological tissues. Here we show that these materials exhibit generic elastic behavior close to the rigidity transition, which is independent of the microscopic structure and the disorder in the system. Phrasing the condition of geometric incompatibility in terms of a minimal length function, we obtain analytic expressions for the elastic stresses and moduli. We numerically verify our findings by simulations of under-constrained spring networks as well as 2D and 3D vertex models for dense biological tissues. For instance, we analytically show that the ratio of the excess shear modulus to the shear stress is inversely proportional to the critical shear strain with a prefactor of three, which we expect to be a general hallmark of rigidity in under-constrained materials induced by geometric incompatibility. This could also be used in experiments to distinguish whether strain-stiffening as observed for instance in biopolymer networks arises from nonlinear characteristics of the microscopic material components or from effects of geometric incompatibility.

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