Activity-controlled annealing of colloidal monolayers.

Molecular motors are essential to the living, generating fluctuations that boost transport and assist assembly. Active colloids, that consume energy to move, hold similar potential for man-made materials controlled by forces generated from within. Yet, their use as a powerhouse in materials science lacks. Here we show a massive acceleration of the annealing of a monolayer of passive beads by moderate addition of self-propelled microparticles. We rationalize our observations with a model of collisions that drive active fluctuations and activate the annealing. The experiment is quantitatively compared with Brownian dynamic simulations that further unveil a dynamical transition in the mechanism of annealing. Active dopants travel uniformly in the system or co-localize at the grain boundaries as a result of the persistence of their motion. Our findings uncover the potential of internal activity to control materials and lay the groundwork for the rise of materials science beyond equilibrium

Propagating waves in bounded elastic media: a transition from standing wave motion to anguilliform kinematics

Waves propagating in confined geometries usually evolve into spatially stationary patterns, built from the interference between the waves that have been reflected upon hitting the boundaries. However, a recent study on bio-locomotion [1] has reported that traveling wave kinematics can naturally emerge in a forced elastic rod, even with boundary conditions involving significant reflections. It has been shown that this particular behavior is observed only in the presence of strong damping. Based on those observations, we aim at giving a quantitative description of the mechanism involved to prevent the built-up of standing waves and establish traveling fish-like kinematics (that optimizes the global swimming efficiency). The question is discussed here in the framework of hand-made artificial swimmers as an example of practical application. REFERENCE [1] Ramananarivo, S., Godoy-Diana, R., Thiria, B. Passive elastic mechanism to mimic fish-muscle action in anguilliform swimming. Journal of the Royal Society Interface. 2013, 10(88), 20130667

Propulsion biomimétique de structures élastiques

Birds and aquatic animals exploit the surrounding fluid to propel themselves in air or water. In inertial regimes, the mechanisms of propulsion are based on momentum transfer; by flapping wings or fins, animals accelerate fluid in their wake, creating a jet that propels them forward. The structures used to move can be flexible, and are thus likely to experiment large bending. Literature showed that those passive deformations can improve propulsive performance, when exploited in a constructive way. The mechanisms at play however remain poorly understood. In the present thesis, we aim at studying how a flapping elastic structure generates thrust, using two experimental biomimetic models. The first setup is a simplified mechanical insect with flexible wings, and the second one is a swimmer whose elastic body mimics the undulating motion of an eel. We show that propulsive performance is significantly influenced by the way the systems passively bend, and that their elastic response can be described by simplified theoretical models of forced oscillators. Those models also bring forward the crucial role of the quadratic fluid damping that resists the flapping motion. This result introduces the counter-intuitive idea that it is sometimes desirable to dissipate part of the energy in the fluid, in order to improve performance.Les oiseaux et poissons se déplacent dans leur environnement fluide en interagissant avec l'air/eau qui les entoure. Pour des régimes inertiels, les mécanismes de propulsion se basent sur un transfert de quantité de mouvement au fluide; les battements d'ailes ou de nageoires générant un jet dans le sillage de l'animal qui le propulse vers l'avant. Pour les oiseaux comme pour les poissons, les structures utilisées possèdent une certaine flexibilité, et sont donc susceptibles de plier de façon importante. La littérature montre que ces déformations passives peuvent améliorer les performances de propulsion lorsqu'elles sont exploitées de façon constructive. Le détail des mécanismes en jeu reste cependant mal compris. L'objectif de cette thèse est d'étudier, à travers deux modèles biomimétiques, la façon dont une structure battante déformable génère des forces de propulsion. Le premier modèle est une version mécanique simplifiée d'insecte dotée d'ailes flexibles, tandis que le deuxième est un nageur dont le corps élastique reproduit le mouvement d'ondulation d'une anguille. Nous montrons que la façon dont ces systèmes se déforment passivement est déterminante pour leurs performances, et que leur réponse élastique peut être décrite par des modèles théoriques simplifiés d'oscillateurs forcés. Ces modélisations mettent par ailleurs en avant le rôle crucial joué par le frottement fluide quadratique qui s'oppose aux mouvements de battements de la structure. Ce résultat introduit l'idée, un peu contre-intuitive, qu'il peut s'avérer avantageux de dissiper une part de son énergie dans le fluide pour améliorer ses performances

Propulsion biomimétique de structures élastiques

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Activity-controlled annealing of colloidal monolayers.

Molecular motors are essential to the living, generating fluctuations that boost transport and assist assembly. Active colloids, that consume energy to move, hold similar potential for man-made materials controlled by forces generated from within. Yet, their use as a powerhouse in materials science lacks. Here we show a massive acceleration of the annealing of a monolayer of passive beads by moderate addition of self-propelled microparticles. We rationalize our observations with a model of collisions that drive active fluctuations and activate the annealing. The experiment is quantitatively compared with Brownian dynamic simulations that further unveil a dynamical transition in the mechanism of annealing. Active dopants travel uniformly in the system or co-localize at the grain boundaries as a result of the persistence of their motion. Our findings uncover the potential of internal activity to control materials and lay the groundwork for the rise of materials science beyond equilibrium

Propagating waves in bounded elastic media: Transition from standing waves to anguilliform kinematics

Confined geometries usually involve reflected waves interacting together to form a spatially stationary pattern. A recent study on bio-locomotion, however, has reported that propagating wave kinematics can naturally emerge in a forced elastic rod, even with boundary conditions involving significant reflections. It has been shown that this particular behavior is observed only in the presence of strong damping. Based on those observations, this study aims at giving a quantitative description of the mechanism involved to prevent the build-up of standing waves and generate traveling solutions. The question is discussed here in the framework of handmade artificial swimmers as an example of practical application but we believe that its potential is beyond this scope

Paragliders' Launch Trajectory is Universal

International audienceWe design, build and run a reduced-scale model experiment to study the paragliding inflation and launching phase at given traction force. We show that the launch trajectory of the glider is universal, that is, independent of the strength of the exerted force. As a consequence, the length of the takeoff run required for the glider to reach its "ready to launch" vertical position is also universal. We successfully confront our results to real scale experiments, and show that such universality can be understood through a simple theoretical model