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Morphogenèse de structures de glace par solidification d’un ruisselet
No full textThis manuscript is dedicated to the study of the morphogenesis of ice structures formed by the solidification of rivulets. More broadly, it falls within the framework of the solidification of natural flows, which are responsible for spectacular formations in nature such as ice waterfalls, stalactites, or draperies found in caves. This field of study also has significant industrial relevance, particularly in connection with icing issues on aircraft or solidification processes in the metallurgy industry. To analyze these phenomena from both an experimental and theoretical perspective, we investigated the formation of ice structures generated by the flow of a rivulet over a cooled inclined plane. To characterize these structures, an optical profilometry method called Spatiotemporal Phase-Shifting Profilometry was adapted to reconstruct and characterize their morphology over time. This method also enabled the study of flow geometries at ambient temperature, such as rivulets, meanders and braided flows. The solidification experiments revealed that the rivulet moves laterally over the thermal exchanger due to a coupling between gravity, inertia, capillarity, and phase change. This lateral motion allows for the formation of structures that converge toward a stationary state, described by a symmetric triangular shape when viewed from above. The apparent angle of this geometry is independent of the flow rate and solely influenced by the control temperatures (those of the water and the substrate). These experiments also showed that the temperature field within the ice remains in quasi-static equilibrium with the motion of the flow at its surface. Additionally, a two-dimensional model was developed to study the evolution of the structure’s height toward a stationary state. This comparison revealed that the maximum height of the structure, along the transverse axis of the exchanger, evolves exponentially with the distance from the injection point. The dynamics of the structure’s evolution toward this stationary state were then studied by tracking the total volume of ice formed and its contact surface with the substrate. We also explored the case of a rivulet flowing over an initial block of ice, simulating conditions analogous to those encountered in supraglacial rivers. These experiments, combining melting and solidification, reveal a strong geometric similarity with the previously observed structures formed solely by solidification. Finally, we studied the transition toward two-dimensional drapery-like structures, which appear under particularly cold thermal conditions and for inclinations approaching 180°.Ce manuscrit est consacré à l’étude de la morphogenèse de structures glaciaires formées par la solidification de ruisselets. Il s’inscrit plus largement dans le cadre de la solidification d’écoulements naturels, à l’origine de formations spectaculaires dans la nature, telles que les cascades de glace, les stalactites ou les draperies rencontrées dans les grottes. Ce champ d’étude présente également un intérêt industriel marqué, notamment en lien avec les problèmes de givrage sur les aéronefs ou les procédés de solidification d’alliages dans l’industrie métallurgique. Afin d’analyser ces phénomènes sous un angle à la fois expérimental et théorique, nous avons étudié la formation de structures de glace générées par l’écoulement d’un ruisselet sur un plan incliné refroidi. Pour caractériser ces structures, une méthode de profilométrie optique, appelée Spatiotemporal Phase-Shifting Profilometry, a été adaptée afin de reconstruire et caractériser leur morphologie au cours du temps. Cette méthode a également permis d’étudier la géométrie d’écoulements à température ambiante tels que les méandres et les écoulements en tresse. Les expériences de solidification ont permis de mettre en évidence que le ruisselet se déplace latéralement sur l’échangeur thermique par un couplage entre gravité, inertie, capillarité et changement de phase. Ce déplacement latéral permet alors la formation de structures convergeant vers un état stationnaire, décrite par une forme triangulaire symétrique vue de dessus. L’angle apparent de cette géométrie est indépendant du débit de l’écoulement et uniquement influencé par les températures de contrôle (celle de l'eau et celle du substrat). Ces expériences ont également mis en évidence que le champ de température dans la glace est en équilibre quasi-statique avec le déplacement de l’écoulement à sa surface. De plus, un modèle bidimensionnel a été développé afin d’étudier l’évolution de hauteur de la structure vers un état stationnaire. Cette comparaison a révélé que la hauteur maximale de la structure, selon l’axe transverse de l’échangeur, évolue de manière exponentielle avec la distance à l’injection. La dynamique d’évolution des structures vers cet état stationnaire a ensuite été étudiée en suivant l’évolution du volume total de glace formée ainsi que la surface de contact avec le substrat. Nous avons également exploré le cas d’un ruisselet s’écoulant sur un bloc initial de glace, simulant des conditions analogues à celles rencontrées dans les rivières supraglaciaires. Ces expériences couplant fonte et solidification révèlent une forte analogie de géométrie avec les structures précédentes formées uniquement par solidification. Enfin, nous avons étudié la transition vers des structures bidimensionnelles de type draperies, qui apparaissent dans des conditions thermiques particulièrement froides et pour des inclinaisons proches de 180°
A Universal Method for Achieving Ultra‐Low Contact Resistances in Organic Electrochemical Transistors
No full textInternational audienceOrganic ElectroChemical Transistors (OECTs) are intensively studied for enabling their use in organic bioelectronics, neuromorphic systems, and biosensors. Beyond device geometry, reaching optimal operation of organic electronic circuits requires the optimization of the physico‐chemical properties of the channel. Toward this end, the effects of a “bulk” doping of the channel material and its influence on the contact resistance (R C ) at the interface between a Polymeric Mixed Ionic‐Electronic conductors (PMIECs) and the Source (S) and Drain (D) electrodes are presented. An easy‐to‐implement method to achieve ultra‐low contact resistances in OECTs is introduced. By incorporation of LiTFSI, a 4x transconductance improvement is achieved, and a decrease of R C by a factor of ≈2 and ≈40 has been observed for p‐ type or n‐ type PMIECs, respectively. It reaches an unprecedented width‐normalized contact resistance value as low as 1 Ohm.cm with the p(g2T‐T) polymer. The formation of very localized domains in the polymeric matrix in the vicinity of the electrodes, as a result of the reduction of TFSIˉ anions, which modulates the energy barrier at the S/D interface, is suggested here. Furthermore, both p(g2T‐T) and p(gNDI‐gT2) polymers exhibit low water uptake with minute amounts of LiTFSI. Worth noticing, doped p(g2T‐T) preserves its volumetric capacitance and demonstrates an exceptional long‐term stability. Finally, a universal strategy to fine‐tune OECT performances, drawing prospects for implementing next‐generation applications in organic bioelectronics and neuromorphics, is proposed
Expressive Rendering for 2D Animations of Liquids
No full textInternational audienceWe describe a new rendering technique for expressive liquid surface 2D animation that can be used on top of existing particlebased simulations. We introduce a hybrid particle model that carries both water and air density distribution and can evolve through particle history. These material quantities combined with the kinematics information are then used to generate a scalar field, which can be parameterized to create an implicit iso-surface capturing stylized geometry commonly seen in paintings and cartoons. We propose, in particular, to represent behavior highlighting the dynamical aspect of the scene, such as elongated droplet behavior and curl-like shapes found in breaking waves
Efficient multi-fidelity Gaussian process regression for noisy outputs and non-nested experimental designs
No full textInternational audienceThis paper presents a multi-fidelity Gaussian process surrogate modeling that generalizes the recursive formulation of the auto-regressive model when the high-fidelity and low-fidelity data sets are noisy and not necessarily nested. The estimation of high-fidelity parameters by the EM (expectation-maximization) algorithm is shown to be still possible in this context and a closed-form update formula is derived when the scaling factor is a parametric linear predictor function. This yields a decoupled optimization strategy for the parameter selection that is more efficient and scalable than the direct maximum likelihood maximization. The proposed approach is compared to other multi-fidelity models, and benchmarks for different application cases of increasing complexity are provided
Steps toward the 5FGL Fermi-LAT source catalog
No full textInternational audienceThe current Fermi-LAT source catalog (4FGL-DR4: 7194 sources over 14 years) was built incrementally from the 8-year catalog. In a survey mission like Fermi, data accumulate on each source over time, so after 16 years (reached in August 2024) and twice the data for the original 4FGL sources we have much more precise localization. It is then time to generate a new original catalog (5FGL), which implies, beyond adding the sources newly detectable after twomore years, changing the existing source names (derived from their coordinates) and reviewing the associations.The systematic errors due to our imperfect knowledge of the Galactic diffuse emission, which dominates the gamma-ray sky, are the major limiting factor of the sensitivity and quality of the point source catalog at low energies (a few 100 MeV over the extragalactic sky, up to a few GeV in the Galactic ridge). Two parallel efforts are ongoing to improve on this situation, using more advanced methods to modulate the interstellar gas templates with the LAT data themselves. In parallel an early 16-year list (FL16Y) of 7217 sources is presented, which relocalizes all sourcesand improves a few aspects of the catalog analysis, but still uses the same diffuse model as 4FGL-DR4
UNBIASED ESTIMATION OF MULTI-WAY GRAVITY MODELS
No full textMaximum likelihood estimators, such as the Poisson Pseudo-Maximum Likelihood (PPML), suffer from the incidental parameter problem: a bias in the estimation of structural parameters that arises from the joint estimation of structural and nuisance parameters. To address this issue in multi-way gravity models, we propose a novel, asymptotically unbiased estimator. Our method reframes the estimation as a series of classification tasks and is agnostic to both the number and structure of fixed effects. In sparse data environments -common in the network formation literature -it is also computationally faster than PPML. We provide empirical evidence that our estimator yields more accurate point estimates and confidence intervals than PPML and its bias-correction strategies. These improvements hold even under model misspecification and are more pronounced in sparse settings. While PPML remains competitive in dense, low-dimensional data, our approach offers a robust alternative for multi-way models that scales efficiently with sparsity. The method is applied to estimate the effect of a policy reform on spatial accessibility to health care in France
Cassini CAPS‐ELS Observations of Low‐Energy Electron Beams Within Enceladus Mid‐Latitude Flux Tubes
No full textInternational audienceAbstract The electrodynamic interaction between Saturn's magnetosphere and Enceladus accelerates electrons along magnetic field lines. These electrons propagate inside magnetic flux tubes connecting the moon to the giant planet, generating distinctive auroral hiss and auroral footprint signatures, both previously observed by the Cassini spacecraft. In this study, we analyze low‐energy electron measurements made during multiple mid‐latitude crossings of magnetic flux tubes connected to Enceladus' wake. We show that the properties of the observed electrons are consistent with those of electrons inducing Enceladus' auroral hiss, and discuss the physical processes responsible for their pitch‐angle distributions and acceleration. Field‐aligned electron beams have very different properties from those triggering the Enceladus ultraviolet footprint, with a much lower characteristic energy and energy flux. Observations of electron beams resulting from the moon‐magnetosphere interactions up to 30° downstream of the moon reveal that the coupling system between Enceladus and Saturn is significantly more extended than previously anticipated
Determination of the accuracy of actinometry and line ratio techniques in an O <sub>2</sub> glow discharge: part II, Electric field measurements with Ar and Xe admixtures
No full textInternational audienceA line-ratio method for determining the reduced electric field is benchmarked against independent measurements from electrostatic probes and cavity ring-down spectroscopy (CRDS). The method is applied to oxygen DC glow discharges with trace admixtures of argon and xenon. A corona model incorporating fluorescence quenching by heavy species is used to simulate the emission, with electron-impact excitation rates calculated using the LisbOn KInetics Boltzmann solver (LoKI-B). The excitation cross sections and quenching coefficients are those proposed and validated for actinometry in part one of this combined study [1]. The reduced electric field is determined over a pressure range of 0.55 to 5 Torr (at 40 mA) and a current range of 15 to 50 mA (at 5 Torr). Consistent agreement with measured emission line intensities is achieved when applying a correction factor of κ c,Ar = 3 ± 0.5 to the excitation cross sections for the argon lines at 750 nm and 811 nm. With this correction, the reduced electric field values obtained from the line-ratio method are in good agreement with direct measurements. A comparison of different line ratios is presented, showing that the best performance is achieved using the ratio of the Ar 750 nm and Xe 828 nm lines. This ratio is particularly sensitive to changes in the electron energy distribution function, due to the large difference in excitation thresholds, while remaining independent of the knowledge of species densities
Homoepitaxial growth of device-grade GaAs using low-pressure remote plasma CVD
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Decadal responses of global land monsoon to two surface thermal modes in the last millennium
No full textInternational audienceThis study investigates the influence of natural external forcing over the last millennium (950–1850) on global land monsoon variability at decadal scale. Two major players operate through changes in global mean surface temperature (GMST) and in the tropical Pacific temperature gradient (TPTG). They modulate precipitation, either individually or jointly in the form of cool-GMST and weak-TPTG episodes. A diagnostic with moisture budget decomposition is performed to separate dynamic and thermodynamic contributions leading to monsoon precipitation. Cool-GMST primarily suppresses the summer precipitation via thermodynamic contributions, with a complex winter response featuring competing thermodynamic gains and evaporation losses. In contrast, weak-TPTG involves significant dynamic adjustments, driving an inter-hemispheric dipole for the summer land monsoon precipitation: decreases in the Northern Hemisphere via anomalous subsidence, and increases in the Southern Hemisphere via the thermodynamic contributions and evaporation. The enhanced winter monsoon precipitation (especially in the Southern Hemisphere) in the case of weak-TPTG is largely driven by the dynamics and evaporation. Under compound conditions, the overall monsoon intensity response can be largely interpreted as a linear superposition of the individual drivers. Summer precipitation is substantially reduced, primarily due to cooling-induced thermodynamic suppression, with additional dynamic contributions. The winter response shows a distinct inter-hemispheric pattern, with precipitation increasing in the Southern Hemisphere and slightly decreasing in the Northern Hemisphere, reflecting different controls by cool-GMST and weak-TPTG. This study provides a valuable framework for interpreting the past monsoon variability and projecting its future variation