HAL Portal ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles)
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Direct assembly of micrometer-long polymeric cylinders in water via supramolecular sticker engineering
No full textInternational audienceWe report a direct, solvent-free method to produce micrometer-length, well-organized polymer nanocylinders in water. To achieve this, a hydrophilic poly(N,N-dimethylacrylamide) (PMDAc) was functionalized at one chain-end with a perylene diimide (PDI) sticker using RAFT polymerization. Two PDI RAFT agents were prepared and studied: one featuring two tri(ethylene glycol) (TEG) units at the PDI bay-positions and one without. The corresponding PDI-PDMAc conjugates spontaneously self-assemble in water, driven by π-π interactions made of H-aggregates, and show a morphological evolution from cylinders to spheres when increasing the polymer chain length. The introduction of TEG units was found to be important to avoid the clustering of nanocylinders or the formation of ill-defined assemblies, which were observed in the TEG-free system. Moreover, we found that the PDI-TEG 2 -PDMAc with degrees of polymerization (DP n ) below 24 self-assembled into micrometer-long nanocylinders. By heating the aqueous polymer solution, this process can be accelerated and is accompanied by a large increase in viscosity. Fluorescence spectroscopy revealed an excimer emission signal for the PDI polymers in water, with a higher emission for cylinders, suggesting better organization within the PDI H-aggregates. This strategy provides a sustainable approach for developing functional nanomaterials with precise morphological control, eliminating organic solvents and complex processing
Hemiwicking : Revisiting the Dynamic of Viscous Liquid in Elastic Pillar Arrays
No full textInternational audienceHemiwicking is an old phenomenon observed and characterized by many studies along the years [1,2].The spreading of the liquid is described macroscopically by the Lucas-Washburn’s law where thedisplacement of the liquid front scales with the square root of the time [3]. The shape of the “porous”substrate can directly organize the liquid diffusion, mimicking the material pattern [4]. We explored asimple experiment where a droplet of wetting fluid settles on a PDMS pillar array and spreads untila liquid film is reached [6]. The liquid infuses with respect to the droplet reservoir, in the height of thepillars (their top surfaces are not wetted). Two important information has been described in theliterature, without having been connected yet. First, the critical contact angle of the droplet reservoirhas not been described quantitatively, just that it “does not vanish” [6]. Second, on the liquid advancinginside a pillar array, few publications observed the spreading of the fluids in between two pillar lines,perpendicular to the spreading direction [5]. We asked ourselves how the dynamic of the advancingfront is connected to the liquid reservoir dynamics (fig. 1). We demonstrated i) that the reservoircontact angles adopt the advancing and receding contact angle on flat PDMS; ii) the spreading of themineral oil (h = 170 mPa.s) is a succession of rapid spreading of pillar line perpendicular (with aconstant velocity proportional to the pillar high) to the imbibition direction; iii) the liquid film itselfplays the role of an intermediate reservoir between the droplet and the rapid spreading inhemiwicking, as shown by its flickering. These experiments shed a new light on imbibition processes
Quantifying the impact of fiber-to-fiber thermal contact resistance on conduction in fibrous insulation materials
No full textInternational audienceWe theoretically studied and optimized the thermal rectification of spherical and cylindrical conductive thermal diodes operating with two phase-change materials (PCMs), whose thermal conductivities significantly changes in a narrow interval of temperatures. This is done by deriving simple analytical expressions for the heat flows, temperature profiles and rectification factors of both diodes. It is shown that diode geometry has a significant impact on the heat flows and temperature profiles, but not so much on the thermal diode rectification factor. Optimal rectification factors of 63.5 and 63.2% are obtained for the spherical and cylindrical thermal diodes operating between the terminals of VO 2 and polyethylene with a temperature difference of 150 K spanning the metal-insulator transition of both PCMs. These similar rectification factors could be enhanced even more with a phase-change material exhibiting higher contrast thermal conductivity than the ones in the present study. The obtained results can thus be useful to guide the development of PCMs capable of optimizing the rectification of conductive heat flows with different geometries.Nous décrivons une nouvelle approche de simulation nodale, ainsi qu'un modèle théorique correspondant, permettant de quantifier la conduction thermique à travers la phase solide dans des réseaux fibreux tridimensionnels, en considérant l'impact des résistances de contact entre fibres. Nous démontrons que la conductivité solide peut être calculée par une courbe maîtresse, uniquement à partir de paramètres géométriques du milieu, en particulier dans le cas de milieux peu denses. Cela constitue un outil de prédiction puissant à appliquer à l'étude d'isolants réels comme la laine de verre, notamment pour rechercher des stratégies d'optimisation basées sur les paramètres structuraux des matériaux. ABSTRACT. We describe a new nodal simulation approach, and a corresponding theoretical model, to quantify thermal conduction through the solid phase in three-dimensional fibrous networks, with the impact of fiber-to-fiber thermal contact resistances. We demonstrate that the solid thermal conductivity can be calculated by a master curve, based solely on geometric parameters of the medium, particularly in the case of low-connectivity. This provides a powerful predictive tool to be applied to the study of real insulation materials, to develop in particular optimization strategies based on structural parameters
Quantifying the Form‐Flow‐Saltation Dynamics of Aeolian Sand Ripples
No full textInternational audienceRipples are the most fundamental and ubiquitous aeolian bedforms formed on sandy surfaces, but their small size and fast response times make them inherently difficult to measure. However, these attributes also make ripples excellent flow indicators, and they have been used extensively in planetary locations for this purpose. Here, we use terrestrial laser scanning to measure ripple morphometry and celerity coincidently, as well as saltation height above rippled surfaces. We find that although ripple height and wavelength respond linearly to increased shear velocity, under strong winds ripple celerity exhibits a non‐linear increase. This relationship at high wind speeds is also reflected in the response of aerodynamic roughness and saltation dynamics, with a greater maximum saltation height present over ripple lee slopes. Importantly, when using ripple patterns as indicators of flow conditions, celerity or height should be used in preference to wavelength as their dynamics respond faster to changing wind speed. In planetary and stratigraphic settings where measuring celerity is not possible, wavelength should be considered as indicative of consistent wind conditions rather than the full range of sand transporting wind speeds
Oscillateurs critiques non linéaires à la bifurcation de Hopf comme modèle cochléaire
No full textThe cochlea, the organ responsible for the sense of hearing, is an exceptional acoustic sensor, capable of detecting sound amplitudes spanning over 12 orders of magnitude and covering three decades in frequency. Despite its remarkable sensitivity and selectivity, it is inherently a nonlinear and non-faithful detector, exhibiting distortion effects. These features suggest that the cochlea operates as an active, living system, relying on nonlinear dynamics to enhance signal processing. An analytical model has proposed that such behavior could arise from the presence of nonlinear critical oscillators operating near a Hopf bifurcation. This thesis puts this hypothesis to the test through the construction and experimental study of physical models that emulate essential nonlinear phenomena observed in the cochlear system. It aims to assess the relevance of the critical oscillator framework and contribute to a deeper understanding of the auditory mechanisms involved in perception. The work is structured into several chapters.In Chapter 1, a detailed study of the hearing process is introduced to lay the groundwork for the experimental work that follows. The chapter identifies the key characteristics of auditory processing, setting the stage for replicating these processes experimentally.Chapter 2 focuses on the fabrication and analysis of a single delayed resonator operating near a Hopf bifurcation. This resonator, using a feedback loop involving a microcontroller, a microphone, and a speaker, mimics the response of a bullfrog hair cell. The system successfully reproduces the sensitivity curve characteristic of the cochlear amplifier at low amplitudes. In addition, it demonstrates tunability and the capacity to produce nonlinear hearing effects, such as the masking effect and phantom tones.From one to many, Chapter 3 explores the nonlinear coupling of two resonators. A digital coupling method is experimentally studied to investigate nonlinear dynamics between resonators. By varying the coupling strength, the study analyzes its effect on both synchronization and system gain, establishing a framework for more complex models that can be used in future studies of auditory systems.In Chapter 4, the study is extended to an artificial cochlea comprising 38 Helmholtz resonators. Five resonators are successfully activated and coupled nonlinearly. The system’s behavior is examined, ranging from the response of individual resonators to interactions between coupled units. The chapter culminates in the analysis of synchronization and divergence phenomena among the five active resonators. Careful calibration and a sequential activation process are essential for ensuring resonance stability. Once these conditions are met, the nonlinear phenomena and active cochlear characteristics are tested, supporting the theory that the cochlea functions as a network of active nonlinear oscillators.La cochlée, organe responsable du sens de l’ouïe, est un capteur acoustique exceptionnel, capable de détecter des amplitudes sonores sur plus de 12 ordres de grandeur et couvrant trois décades en fréquence. Malgré sa sensibilité et sa sélectivité remarquables, elle constitue un détecteur intrinsèquement non linéaire et non fidèle, présentant des effets de distorsion. Ces caractéristiques suggèrent que la cochlée fonctionne comme un système actif et vivant, s’appuyant sur des dynamiques non linéaires pour améliorer le traitement du signal. Un modèle analytique a proposé que ce comportement pourrait résulter de la présence d’oscillateurs critiques non linéaires opérant à proximité d’une bifurcation de Hopf. Cette thèse met cette hypothèse à l’épreuve à travers la construction et l’étude expérimentale de modèles physiques reproduisant des phénomènes non linéaires essentiels observés dans le système cochléaire. Elle vise à évaluer la pertinence du cadre des oscillateurs critiques et à contribuer à une compréhension plus approfondie des mécanismes auditifs impliqués dans la perception. Le travail est structuré en plusieurs chapitres.Le Chapitre 1 présente une étude détaillée du processus auditif afin d’introduire les fondements nécessaires aux travaux expérimentaux qui suivent. Il identifie les principales caractéristiques du traitement auditif, ouvrant ainsi la voie à leur reproduction expérimentale.Le Chapitre 2 se concentre sur la fabrication et l’analyse d’un résonateur unique avec délai, opérant à proximité d’une bifurcation de Hopf. Ce résonateur, utilisant une boucle de rétroaction impliquant un microcontrôleur, un microphone et un haut-parleur, imite la réponse d’une cellule ciliée de grenouille taureau. Le système reproduit avec succès la courbe de sensibilité caractéristique de l’amplificateur cochléaire à faibles amplitudes. Il démontre également sa capacité à être accordé et à générer des effets auditifs non linéaires, tels que l’effet de masquage et les tons fantômes.Passant d’un à plusieurs résonateurs, le Chapitre 3 explore le couplage non linéaire de deux résonateurs. Une méthode de couplage numérique est étudiée expérimentalement pour analyser la dynamique non linéaire entre résonateurs. En faisant varier la force de couplage, l’étude analyse ses effets sur la synchronisation et le gain du système, posant ainsi les bases pour des modèles plus complexes destinés à l’étude future des systèmes auditifs.Le Chapitre 4 étend l’étude à une cochlée artificielle composée de 38 résonateurs de Helmholtz. Cinq résonateurs sont activés avec succès et couplés de manière non linéaire. Le comportement du système est analysé, allant de la réponse des résonateurs individuels aux interactions entre unités couplées. Le chapitre aboutit à une analyse des phénomènes de synchronisation et de divergence entre les cinq résonateurs actifs. Une calibration rigoureuse et une activation séquentielle sont essentielles pour assurer la stabilité des résonances. Une fois ces conditions remplies, les phénomènes non linéaires et les caractéristiques actives de la cochlée sont testés, soutenant l’hypothèse selon laquelle la cochlée fonctionne comme un réseau d’oscillateurs non linéaires actifs
Vascular leakage during circulatory failure: physiopathology, impact and treatments
No full textInternational audienceVascular leakage has emerged as a major factor during circulatory failure. Triggered by the inflammatory process following the recognition of both pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), it worsens circulatory failure through the hypovolemia it induces. It may also crucially participate in secondary microcirculation disorders and organ dysfunctions, due to interstitial edema resulting from extravascular fluid accumulation. Accordingly, fluid balance, i.e., the difference between fluid intake and output, is directly related with outcomes during the different types of shock. Moreover, controlling vascular leakage had beneficial effects in various animal models of circulatory failure. However, despite promising preclinical findings, no routine drug is currently available to control vascular leakage in humans. This review depicts the mechanisms involved in the maintenance of a quiescent endothelium and those implicated in the destabilization of its barrier function in various forms of shocks. It further describes available tools to explore vascular leakage and the most advanced treatments under development
Tailoring the spontaneous emission of magnetic dipole transitions with plasmonic multipods
No full textInternational audiencePlasmonic multipod structures with N metallic satellites on a dielectric core are designed and optimized to serve as bright, efficient and robust sources of magnetic light. Our approach based on quasi-normal modes reveals that structures with at least tetrahedral symmetry exhibit far superior behavior compared to structures with poor symmetry
Accelerated Training through Iterative Gradient Propagation Along the Residual Path
No full textInternational audienceDespite being the cornerstone of deep learning, backpropagation is criticized for its inherent sequentiality, which can limit the scalability of very deep models. Such models faced convergence issues due to vanishing gradient, later resolved using residual connections. Variants of these are now widely used in modern architecture. However, the computational cost of backpropagation remains a major burden, accounting for most of the training time. Taking advantage of residual-like architectural designs, we introduce Highway backpropagation, a parallelizable iterative algorithm that approximates backpropagation, by alternatively i) accumulating the gradient estimates along the residual path, and ii) backpropagating them through every layer in parallel. This algorithm is naturally derived from a decomposition of the gradient as the sum of gradients flowing through all paths and is adaptable to a diverse set of common architectures, ranging from ResNets and Transformers to recurrent neural networks. Through an extensive empirical study on a large selection of tasks and models, we evaluate Highway-BP and show that major speedups can be achieved with minimal performance degradation
Tunable Charge Transfer in Functionalised Betainoïd Pyridinium-Benzimidazole Scaffolds: Computational and Experimental Insights into Optical Properties
No full textInternational audienceDonor acceptor systems are promising systems for applications in optoelectronics. The zwitterionic nature of betainoid pyridinium compounds allows for a unique set of tunable electronic properties but their rationalisation remains challenging. Here, the optical properties of a series of five derivatives were studied experimentally and computationally to unveil and rationalize their distinctive intramolec-ular charge transfer properties. These compounds consist of 4-functionalised-pyridine with H, tert-butyl, dimethylamino, trifluoroborate and oxo linked through their nitrogen atom to benzimidazole at its C2 position (1-5 respectively). The transitions responsible for the absorption and emission proper-ties observed experimentally were investigated using DFT and TDDFT. Calculated absorption ener-gies systematically match the experimental λmax, while the prediction of emission energy seems to be less straightforward. By use of the molecular orbitals, charge distribution evolution, change in electric dipole moment and calculated vertical excitations, we are able to rationalise structure-properties relations involved in these transitions. This study allows to gain insights into the optoelec-tronic properties of a series of unique donor-acceptor systems using computationally cost-efficient methods
