HAL Portal ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles)
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Collective actuation in active solids in the presence of a polarizing field: A systematic analysis of the dynamical regimes
No full textInternational audienceCollective actuation in active solids, the spontaneous condensation of the dynamics on a few elastic modes, takes place whenever the deformations of the structure reorient the forces exerted by the active units composing, or embedded in, the solid. In a companion paper, we show through a combination of model experiments, numerical simulations, and theoretical analysis that adding an external field that polarizes the active forces strongly affects the dynamical transition to collective actuation. A new oscillatory regime emerges, and a reentrance transition to collective actuation takes place. Depending on the degeneracy of the modes on which the dynamics condensates, and on the orientation of the field with respect to the stiff direction of the solid, several new dynamical regimes can be observed. The purpose of the present paper is to review these dynamical regimes in a comprehensive way, both for the single-particle dynamics and for the coarse-grained one. Whenever possible the dynamical regimes and the transition between them are described analytically, otherwise numerically
Convection in a Rapidly Rotating Spherical Shell: Newton’s Method Using Implicit Coriolis Integration
No full textInternational audienceGeophysical flows are characterized by rapid rotation. Simulating these flows requires small timesteps to achieve stability and accuracy. Numerical stability can be greatly improved by the implicit integration of the terms that are most responsible for destabilizing the numerical scheme. We have implemented an implicit treatment of the Coriolis force in a rotating spherical shell driven by a radial thermal gradient. We modified the resulting timestepping code to carry out steady-state solving via Newton’s method, which has no timestepping error. The implicit terms have the effect of preconditioning the linear systems, which can then be rapidly solved by a matrix-free Krylov method. We computed the branches of rotating waves with azimuthal wavenumbers ranging from 4 to 12. As the Ekman number (the non-dimensionalized inverse rotation rate) decreases, the flows are increasingly axially independent and localized near the inner cylinder, in keeping with well-known theoretical predictions and previous experimental and numerical results. The advantage of the implicit over the explicit treatment also increases dramatically with decreasing Ek, reducing the cost of computation by as much as a factor of 20 for Ekman numbers of order of 10−5. We carried out continuation for both the Rayleigh and Ekman numbers and obtained interesting branches in which the drift velocity remained unchanged between pairs of saddle–node bifurcations
EXPLAINING THE RELATIONSHIP BETWEEN SENSORY PROCESSING SENSITIVITY AND ALEXITHYMIA THROUGH ATTACHMENT STYLES
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Time reversal meatasurfaces
No full textMétamatériaux électroacoustiques; GTEA - Transducteurs et Électroacoustique: GAPSUS - Acoustique Physique, Sous-Marine et Ultra-SonoreNational audienceL'effet cocktail-party fait référence à la capacité de concentrer son attention auditive sur des sources sonores spécifiques tout en ignorant les autres. Nous proposons une solution à ce problème du cocktail-party en créant un environnement actif. Plus précisément, les murs sont recouverts d'une méta-surface dépendante du temps, constituée de miroirs actifs individuels. Chacun de ces miroirs fonctionne comme une cellule unitaire programmable capable de recevoir, traiter et émettre des signaux acoustiques en temps réel, agissant ainsi efficacement comme un filtre de convolution. Pour évaluer les performances de cette méta-surface reconfigurable active, une expérience utilisant des sons audibles aériens est conçue dans une cavité fermée réverbérante. L'expérience comprend une phase où un ensemble de fonctions de Green est préenregistré entre un émetteur et N cellules, ainsi qu'entre les N cellules et le récepteur. Ces fonctions de Green sont ensuite utilisées pour calculer les filtres temporels pour chaque miroir actif à l'aide de techniques de retournement temporel. La dernière étape consiste à mettre en œuvre la convolution en temps réel au sein des cellules actives. Chacune d'elles est composée d'un microphone et d'un haut-parleur connectés via un microcontrôleur capable d'appliquer le filtre prédéfini pour la convolution. Nous démontrons comment un lien audio dédié peut être établi entre un émetteur et un récepteur
Deep-lying semi-Dirac fermions in hexagonal close-packed cadmium
No full textInternational audienceAbstract Semi-Dirac fermions are massless in one direction and massive in the perpendicular directions. Such quasiparticles have been proposed in various contexts in condensed matter. Using first principles calculations, we identify a pair of semi-Dirac bands anti-crossing at −3 eV below the Fermi level in the electronic structure of hexagonal close-packed cadmium. The linear out-of-plane dispersion is kept up to the Fermi level. We demonstrate that the dichotomy between the linear and quadratic dispersions is driven by an orientation-sensitive hybridization between the s and p z orbitals. The upper semi-Dirac band produces a lens-shaped nonellipsoidal Fermi sheet whose cross-section area has a k -dependence that is in excellent agreement with the experimentally measured period of Sondheimer oscillations
Étude comparative du blanchiment réducteur et oxydant de papiers lignocellulosiques naturellement vieillis
No full textInternational audienceMit zunehmendem Alter versauern lignozellulosehaltige Papiere – die auf Holzfasern basieren – sie vergilben und werden sehr brüchig, was eine große Herausforderung für die Konservierung darstellt. Das Bleichen ist eine kurative Behandlung, die hauptsächlich dazu dient, die Bräunung des Papiers zu mildern und so den Zugang zu seinem historischen oder künstlerischen Inhalt zu ermöglichen. Die Literatur zum Thema Papierbleiche ist umfangreich, und das Fehlen standardisierter Protokolle erschwert den Vergleich der Ergebnisse. Darüber hinaus konzentrieren sich diese Studien häufig auf Zellulosepapiere, die kein oder nur wenig Lignin enthalten. Um diese Probleme zu beheben, haben wir die Auswirkungen der reduktiven und oxidativen Bleichung auf die Farbe, den pH-Wert, die Wasseraufnahme und die mechanische Festigkeit von zwei natürlich gealterten Lignocellulosepapieren verglichen.With natural ageing, wood pulp papers acidify, yellow and become very fragile, all of which pose a major preservation challenge. Bleaching is a remediation treatment primarily used to increase the brightness of paper, thereby enabling access to its historical or artistic content. Literature on paper bleaching is vast and the lack of standardised protocols makes it difficult to compare the results. Moreover, these studies often focus on papers containing no or little lignin. To tackle these issues, we compared the impact of reductive and oxidative bleaching on two naturally aged lignocellulosic papers' color, pH, water absorption and mechanical strength.Avec le vieillissement naturel, les papiers lignocellulosiques - à base de pâte de bois- s'acidifient, jaunissent et deviennent très fragiles, ce qui constitue un défi majeur de conservation. Le blanchiment est un traitement curatif principalement mis en oeuvre pour atténuer le brunissement du papier, permettant ainsi d'accéder à son contenu historique ou artistique. La littérature sur le blanchiment du papier est vaste et l'absence de protocoles standardisés rend difficile la comparaison des résultats. De plus, ces études se concentrent souvent sur des papiers cellulosiques, ne contenant pas ou peu de lignine. Pour remédier à ces problèmes, nous avons comparé l'impact du blanchiment réducteur et oxydatif sur la couleur, le pH, l'absorption d'eau et la résistance mécanique de deux papiers lignocellulosiques vieillis naturellement
Ultraviolet in swallowtail butterflies: contrasted dorso-ventral evolution highlights a trade-off between natural and sexual selection on visual cues
No full textInternational audienceAbstract The evolutionary dynamics of color pattern diversification in animals is strongly influenced by visual interactions within and among species. While much attention has been given to color pattern variation in the human-visible range, perception outside this range is observed in a wide array of species and is poised to influence color pattern evolution. Butterfly species often show sensitivity to ultraviolet (UV) light, impacting wing color pattern diversification as their evolution is influenced by both predator vision and sexual selection. Here, we explore UV color pattern diversification in Papilionidae within a comparative phylogenetic framework by quantifying variation from UV photographs of museum specimens using a machine-learning-based method. We find decoupled dorsal and ventral UV color pattern evolution, with brighter and more rapidly evolving ventral sides, especially in males. Conversely, we find a smaller dorso-ventral difference in visible-light color patterns. Moreover, we find divergence in male ventral UV patterns in closely related sympatric species, even after accounting for variation due to visible-light patterns. These results suggest an influence of sexual selection on UV ventral pattern diversification. These findings highlight how the trade-off between sexual and natural selection may lead to contrasted evolution of ventral vs. dorsal sides of the same organ
High throughput measurement of bubble coalescence times using digital millifluidics: Coalescence of N2 bubbles in decane−cyclopentanol mixture
No full textFoams may form in oil mixtures, such as lubricants, as a result of air entrainment. The long lifetimes of those foams significantlyimpair the thermal properties of lubricants and increase power losses by engines [Zhan et al. (2022)]. In order to improve theefficiency of lubricants, we offer here to gain insights in the stability of bubbles in binary mixtures of miscible oils as a function ofbubble size and liquid composition. To do so, using a micro-millifluidic set-up, we control the formation of bubbles in oil mixturesand study variations in their coalescence time. The set-up allows to easily vary the curvature of the bubbles over one decade,perform statistics over a large number of coalescence events and measure coalescence times that span more than three orders ofmagnitude
CO-to-sugars conversion from one-pot two-step electroorganocatalytic process
No full textInternational audienceThe conversion of C1 molecules (single-carbon species) into C n products (carbon chains) is a key challenge for developing sustainable chemical feedstocks to replace fossil resources. Carbohydrates, a vital class of complex polycarbon molecules, are mainly extracted from biomass, but de novo synthesis provides a complementary route to access rare and non-natural carbohydrates. Here, we report a fully integrated, one-pot two-step system converting carbon monoxide (CO) into carbohydrates. This process couples the electroreduction of CO to formaldehyde with the organocatalytic oligomerization of formaldehyde into C5-6 carbohydrates selectively. This work establishes a novel pathway to utilize CO as a building block for synthesizing complex carbon chains
pH-dependent interfacial rheology of polymer membranes assembled at liquid-liquid interfaces using hydrogen bonds
No full textInternational audienceSelf-assembly of polymers at liquid interfaces using non-covalent interactions has emerged as a promising technique to reversibly produce self-healing membranes. Besides the assembly process, it is also crucial to control the mechanical properties of these membranes. Here, we measure the interfacial rheological properties of PMAA-PPO (polymethacrylic acid - polypropylene oxide) polymer membranes assembled using hydrogen bonds at the interface between water and a polar oil, Mygliol. Varying the pH enables us to modify the degree of ionization of the PMAA chains, and hence their ability to establish hydrogen interactions with PPO. Frequency sweeps of the interfacial layers show a crossover between a viscous regime at low frequencies and an elastic regime at high frequencies. The crossover elastic modulus, measured one hour after the two phases were put into contact, decreases by a half over the pH range investigated, which can be accounted for by a decrease of the layer thickness as pH increases. Furthermore, we find that the crossover frequency varies exponentially with the degree of ionization of PMAA. To account for these observations, we propose a simple picture where the short PPO chains behave as non-covalent cross-linkers that bridge several PMAA chains. The dissociation rate and hence the crossover frequency are controlled by the number of PO units per PPO chain involved in the hydrogen bonds