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Characterization of soaked soft porous materials under cyclic compression for eX-Poro-HydroDynamic lubrication
No full textInternational audienceeX-Poro-HydroDynamic (XPHD) lubrication is an innovative method in which soaked soft porous materials are used to store and provide lubricant to moving surfaces. Storing the fluid in the pores of the porous media eliminates the need for an external source of lubricant, thus making this approach environmentally friendly and enabling the use of lubricants such as water or other environmentally friendly lubricants. This study aims to carry out experimental and numerical characterizations of the mechanism of dynamic re-imbibition of porous materials. An experimental device is developed to cyclically compress an open-pore foam soaked in water at different frequencies. The experimental results show a direct link between the variation in the normal force, the appearance of air inside the imbibed foam, and the frequency of stress. These parameters influence the ability of the foam to re-soak in the decompression phase, and hence control the maximum normal load that can be generated in the contact. A numerical model based on the Darcy model is also developed, which takes into account the phenomenon of film rupture. The use of a numerical model is made possible by the preceding experimental determination of the water permeability of the foam. Initial comparisons with experimental results show good similarity
Smectic order-driven total wall defect formation
No full textInternational audienceWe study experimentally and theoretically liquid crystal structure of smectic oily streaks, focusing on planar wall defects hosted within smectic flattened hemicylinders (SFHs). The wall configuration is singular both in orientational and translational order and we refer to it as the Total Wall Defect (TWD). Here "singular" refers to nematic director field and smectic phase field. In theoretical analysis of the TWD we use a mesoscopic Landau-de Gennes-Ginsburg approach in terms of the nematic tensor order parameter and smectic A (SmA) complex order parameter field. The smectic layer structure is experimentally determined using polarizing optical microscopy and X-ray diffraction measurements at Synchrotron facilities. We demonstrate theoretically and numerically that the experimentally observed abrupt change of the SmA layering in the centre of the wall defect is realized via nematic order reconstruction mechanism. Our experiments reveal that smectic layer spacing above and below the wall are almost similar. The theoretical analysis suggests that lateral SFH boundary conditions determine the vertical position of TWD
X-Ray Diffraction Reveals the Consequences of Strong Deformation in Thin Smectic Films: Dilation and Chevron Formation
No full textInternational audienceSmectic liquid crystals can be viewed as model systems for lamellar structures for which there has been extensive theoretical development. We demonstrate that a nonlinear energy description is required with respect to the usual Landau-de Gennes elasticity in order to explain the observed layer spacing of highly curved smectic layers. Using x-ray diffraction we have quantitatively determined the dilation of bent layers distorted by antagonistic anchoring (as high as 1.8% of dilation for the most bent smectic layers) and accurately described it by the minimal nonlinear expression for energy. We observe a 1°tilt of planar layers that are connected to the curved layers. This value is consistent with simple energetic calculations, demonstrating how the bending energy impacts the overall structure of a thin distorted smectic film. Finally, we show that combined x-ray measurements and theoretical modeling allow for the quantitative determination of the number of curved smectic layers and of the resulting thickness of the dilated region with unprecedented precision
Low stress grain boundary mediated plasticity and early fracture at basal twist grain boundaries in a titanium alloy
No full textTitanium alloys are high-performance materials critical for demanding applications in aerospace, defense, and energy sectors. Basal twist grain boundaries (BTGBs) were recently identified as key microstructure configurations leading to failures under different conditions. In the present study, we examined deformation and fracture in these specific locations to shed light on the mechanical behavior of BTGBs in relation to grain boundary characteristics. In situ characterization using high resolution digital image correlation revealed deformation at BTGBs for macroscopic stress levels as low as 100 MPa, which corresponds to approximately 10 % of the yield strength. As a comparison, conventional intragranular slip activity was only detected from 600 MPa. Although the mechanical response of BTGBs was insensitive to a tilt component up to ~ 4°, a twist angle higher than ~ 10° was required for low-stress deformation to occur. Complementary molecular dynamic simulations revealed that this specific behavior is governed by the pattern of the interfacial dislocation network. Simulations also highlighted no major role of nanoscale defects, or chemical segregation at BTGBs. Upon further deformation, in-situ experiments revealed cleavage-like fracture along BTGBs for macroscopic strains as low as 1.5 %. Early cracking was triggered at BTGBs prone to intense preliminary deformation, and subjected to a high normal stress level. The clarified influence of grain boundary characteristics on their mechanical response offers a new understanding of the detrimental role of basal twist grain boundaries on the performance of titanium alloys
Antennerie microphonique tridimensionnelle en soufflerie pour l’analyse du bruit aéroacoustique généré par des obstacles en écoulement
No full textThis study focuses on the aerodynamic noise generated by obstacles within a flow. The identification of these aeroacoustic sources is generally carried out in a plane using a planar array. In this thesis, to analyze these sources in a volume, a three-dimensional array is used, composed of 1024 microphones forming a tunnel around the test section of an anechoic wind tunnel. The aeroacoustic sources from acoustically compact objects are dipolar, and for some, their orientation varies in spaces (e.g., for curved obstacles or combinations of obstacles). An inverse method is thus implemented in this thesis, which does not require any prior assumptions about the orientation of the sources, and allows to determine the orientation of dipoles and their level in space. This method is adapted to the three-dimensional domain, but it is more effective when the focus area is restricted to the contours of the studied obstacles. Specifically, in this study, arrangements of cylinders with various cross-sections, as well as a ring, are experimentally investigated. The noise sources are then placed along the cylinders and have a dipolar directivity orthogonal to the flow. The orientation of the dipoles, as well as the coherence length of the aeroacoustic sources along the axis of the cylinder, are estimated using the inverse method and compared to the literature. Finally, identification tools are used to study the direction of the acoustic dipole generated by cylinders with square and rectangular cross-sections, depending on the angle of incidence of the flow. This study confirms numerical predictions on the directivity of the radiation of these cylinders, based on Curle's solutions.Cette étude porte sur le bruit aérodynamique généré par des obstacles au sein d’un écoulement. L’identification de ces sources aéroacoustiques se fait généralement dans un plan, au moyen d’une antenne plane. Dans cette thèse, afin d’analyser ces sources dans un volume, une antenne tridimensionnelle est utilisée, composée de 1024 microphones formant un tunnel autour de la veine d’essai d’une soufflerie anéchoïque. Les sources aéroacoustiques issues d'objets acoustiquement compacts sont dipolaires, et pour certaines, leur orientation varie dans l’espace (par exemple pour des obstacles courbés ou des associations d'obstacles). Une méthode inverse est ainsi mise en œuvre dans cette thèse, qui ne nécessite aucune hypothèse préalable sur l’orientation des sources, et permet de retrouver l’orientation de dipôles et leur niveau dans l’espace. Cette méthode est adaptée au domaine tridimensionnel, mais est plus efficace lorsque la zone de focalisation est restreinte aux contours des obstacles étudiés. Plus particulièrement, dans cette étude, sont étudiés expérimentalement des agencements de cylindres de sections diverses, ainsi qu’un anneau. Les sources de bruit sont alors disposées le long des cylindres, et ont une directivité dipolaire orthogonale à l’écoulement. Cette orientation des dipôles, ainsi que la longueur de cohérence des sources aéroacoustiques dans la direction de l'axe des cylindres, sont estimés grâce à la méthode inverse, et comparées à la littérature. Enfin, des outils d'identification développés sont utilisés pour étudier la direction du dipôle acoustique généré par des cylindres de sections carrée et rectangulaire, en fonction de l'angle d'incidence de l'écoulement. Cette étude vient confirmer des prédictions numériques sur la directivité du rayonnement de ces cylindres, basées sur les solutions de Curle
Modèle géométrique, statistique et mécanique de la contraction musculaire
No full textThe myosin II head is a biological nanomotor that converts the chemical energy derived from ATP hydrolysis into mechanical work. The state during which the myosin head generates both force and motion is referred to as the Working Stroke (WS). During the WS, the myosin head is mechanically modeled as three rigid segments articulated in pairs: (i) the catalytic domain, which is strongly bound to the actin filament, (ii) the lever, on which a motor moment is exerted by the transducer within the catalytic domain, and (iii) the rod, which transmits force to the myosin filament. In this framework, the muscle fiber can be simultaneously described as a thermodynamic system composed of millions of chemical reactors located within the catalytic domains of the myosin heads, and as a mechanical structure comprising millions of interconnected rigid bodies capable of moving in space.When the muscle fiber is tetanized under isometric conditions, the stochastic nature of the chemical reactions—driven by entropy—and the spatial constraints imposed by filament geometry result in a uniform angular distribution of the levers of WS myosin heads within each half-sarcomere. Based on the swinging lever arm hypothesis, the torque produced by each myosin head during WS is mechanically quantified. By integrating probabilistic descriptions of cross-bridge cycle events with the principles of rigid body mechanics, the model accounts for and predicts the various transient phase scenarios that follow the shortening of a tetanized fiber after a step change in length or force. In particular, the model provided a theoretical expression describing the relationship between force and shortening velocity, offering an explanation for the biphasic nature of the force–velocity curve. Notably, the model highlights the presence of viscosity during rapid shortening phases, especially at low temperaturesLa tête de myosine II est un nanomoteur biologique qui convertit en énergie mécanique l’énergie chimique délivrée par une molécule d’ATP. L’état d’une tête de myosine durant laquelle force et mouvement sont générés est nommé Working Stroke (WS). Durant le WS, la tête de myosine est modélisée mécaniquement par trois segments rigides articulés entre eux, deux à deux : (1) le domaine catalytique fixé fortement au filament d’actine, (2) le levier sur lequel s’exerce un moment moteur engendré par le transducteur du domaine catalytique, (3) la tige tirant le filament de myosine. Ainsi, la fibre musculaire se présente tout à la fois comme un système thermodynamique composé de millions de réacteurs chimiques localisés dans le domaine catalytique des têtes de myosine, et comme un objet mécanique formé de millions de segments matériels susceptibles de se déplacer dans l’espace.Lorsque la fibre est tétanisée en conditions isométriques, la nature entropique et donc aléatoire des processus chimiques, associée à la géométrie inter-filamentaire, mène à l’uniformité de la distribution des positions angulaires des leviers appartenant aux têtes de myosine en WS dans chaque demi-sarcomère. A l’appui du postulat du swinging lever arm, le moment moteur est quantifié mécaniquement. En caractérisant les probabilités de réalisation des évènements associés aux réactions du cross-bridge cycle et en appliquant les théorèmes de la mécanique du solide à la tête de myosine en WS, le modèle explique et prédit les divers scenarii des phases transitoires qui suivent le raccourcissement d’une fibre tétanisée après une perturbation selon un échelon de longueur ou de force. En particulier, le modèle fournit une équation théorique qui relie force (F) et vitesse de raccourcissement (V) de la fibre et qui interprète l’aspect bi-phasique de la courbe F-V. Il est à noter la présence de la viscosité lors de raccourcissements effectués à vitesse rapide et à basse température
Numerical Simulation of the Unsteady 3D Flow in Vertical Slot Fishway—The Impact of Macro-Roughness
No full textInternational audienceVertical slot fishways (VSFs) are crossing devices that are built on rivers or streams. They were initially designed to help salmons to complete their migratory cycle by crossing permanent obstructions. In order to favor the passage of smaller or benthic species, stones or concrete cylinders, called macro-roughnesses, are often inserted at the bottom of the fishway. To study the effects of macro-roughnesses on the flow inside a VSF, three-dimensional unsteady simulations were carried out using the volume of fluid method to model the free surface. In this paper, kinematic quantities obtained by CFD are used to detail the flow inside a VSF with and without macro-roughnesses. It can provide valuable information about the flow characteristics, especially in areas where the experimental measurements are difficult to implement
Oriented networks of gold nanorods in smectic topological defects
No full textInternational audienceA liquid crystal (LC) is a mesophase where the molecules are free to move as in a fluid, but show a certain degree of order as in a crystal lattice. Under specific conditions, a thin film of smectic LC can form a pattern of flattened hemicylinders that present different kinds of topological defects (1D and 2D defects) strictly oriented along the hemicylinders direction. These defects are able to confine nanoparticles (NPs) [1], leading to NP networks that are well oriented along the defects [2]. Our aim is to exploit the intrinsic order of this LC matrix to create plasmonic composites with gold nanorods coated with thiol-terminated polystyrene.Methods: We investigate the optical properties of the composite measuring light absorption polarizing the incident light either parallel and perpendicular to the hemicylinders. Absorption spectra are combined with FDTD simulations and X-ray diffraction experiments performed in a transmission configuration at synchrotron facilities.Results and discussion: Absorption measurements reveal that these nanorods self-assemble side-to-side, either perpendicular or parallel to the hemicylinders. The well-defined orientation of these networks imposed by the flattened hemicylinders orientation provides the composite with polarized plasmonic properties (see figure). We show that by playing with the LC’s thickness and the nanorod concentration we can modulate the side-by-side assembly anisotropy, eventually leading to almost perfectly oriented nanorods as confirmed by X-ray diffraction measurements. Comparing the results with FDTD simulations, the plasmonic light absorption is in agreement with the formation of small assemblies of oriented nanorods, where the absorption wavelength appears strongly dependent on the size and shape of the assemblies.*****References:[1] Do, Nano Letters 20,3,1598-1606 (2020).[2] Jeridi et al. Soft Matter 18, no. 25 (2022): 4792–480
Visual Assistance in VR-based Robot Control: Towards a Reproducible Evaluation Scenario
No full textNational audienceImmersive technologies enhance industrial applications by creating virtual representations of manufacturing environments and providing visual assistance for tasks. Most studies in the literature are based on ad hoc scenarios, making comparison across studies challenging. In this research, 99 participants controlled a remote industrial robot using a VR headset in a reproducible maintenance task. They performed the task under three conditions: without assistance (control), text-based assistance, or attentional cueing (highlighting objects). We manipulated task difficulty (easy versus difficult) and assessed performance based on completion time and failure rate. Results showed visual assistance significantly shortened task completion time, with findings discussed
Experimental Investigations on Carbon Segmented Seals With Smooth and Pocketed Pads
No full textInternational audienceAbstract The present paper introduces experimental results for radial segmented seals operating at high rotating speed. These seals are made of three or more interlocking, carbon-graphite segments, in permanent contact with the rotor while also accommodating its dynamic displacements. Currently, many efforts are made to model these behaviors and for obtaining accurate leakage flowrate and friction torque predictions. A novel test rig fitted with a casing for measuring leakage and friction torque produced by two carbon segmented seals was developed. The tested seals were composed of three segments. Two types of segmented seals were tested: two seals with flat pads and one seal with pocketed pads. The seals were installed two by two in a casing and were simultaneously tested in a back to back arrangement. The process fluid was air, with a maximum inlet pressure of 0.3 MPa, while it was set to ambient at the outlet. The rotor diameter was 100 mm with a maximum rotating speed of 24,000 rpm. The main measurements were the inlet mass flowrate and friction torque produced by the two seals. Results show that the seal with pocketed pads produced higher leakage rates but lower friction torque compared to the smooth pads. This suggests that the seal with pocketed pads operated with segments completely lifted from the rotor surface. Mass flow rates measurements during run-up/coast-down tests revealed a hysteresis effect, which could be the consequence of segment tilt. The experimental results could also be useful to validate theoretical models