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Biomécanique multi-échelles de la peau : étude expérimentale du rôle de l'organisation microstructurale du collagène
No full textSkin is a complex organ consisting of three main layers, namely the epidermis, dermis and hypodermis. The dermis is responsible for most of the complex mechanical properties of skin, including non-linearity, anisotropy and viscoelasticity. Like all soft collagenous tissues, the dermis is constituted mostly of extracellular matrix proteins, fibrillar collagens being the major structural components. Modelling efforts using a scaling-up approach for skin generally lack appropriate micro-mechanical experiments to clarify the link between macroscopic mechanical properties and microstructural behaviour. The goal of this research was to measure the evolution of skin's microstructure during mechanical stimulation to identify the relevant mechanisms at the microscopic scale. Uniaxial tensile tests were carried out on ex vivo mice skin under a multiphoton microscope with Second Harmonic Generation detection. This technique allows for specific imaging of collagen fibres in the depth of the dermis. We were then able to simultaneously monitor the tissue's mechanical response and image the microstructural reorganisation of the fibrillar collagen network, using quantitative characterisations at both scales. We showed that the collagen fibres continuously align in the direction of traction with stretch, generating the observed mechanical response. A general framework of hypothetical microstructural mechanisms was proposed to account for the features observed experimentally. Genetic mutations inducing a decreased or abnormal collagen synthesis can result in defective mechanical properties in skin. For instance, patients suffering from Ehlers-Danlos syndrome, a general collagenous tissue disorder caused by mutations in the genes coding for a minor form of collagen, typically present hyperelastic skin. We applied our multiscale approach to two genetically-modied mice strains created in the context of investigating the Ehlers-Danlos syndrome. The ageing process is also a factor of change in skin's mechanical properties, and was investigated in this work through experiments on aged mice skin. Genetically-modied and aged mice skin exhibited altered collagen reorganisation and mechanical response during a tensile test. The variations were interpreted in the context of the microstructural interpretation developed for control mice, and can be used for phenotyping. These findings show that our multiscale approach provides new crucial information on the biomechanics of skin. It can be generalised to study other pathologies, other collagenous tissues, or other mechanical properties, such as the biaxial or viscoelastic response.La peau est un organe complexe constitué de 3 couches : l'épiderme, le derme et l'hypoderme. Le derme confère à la peau la plupart de ses propriétés mécaniques, comme la non-linéarité, l'anisotropie et la viscoélasticité. Comme tous les tissus conjonctifs, le derme se compose majoritairement de matrice extracellulaire, dont les composants structurants majeurs sont les fibres de collagène. Les efforts de modélisation de la peau s'inscrivant dans une démarche de "scaling-up", comme l'homogénéisation, se heurtent en général à l'absence de caractérisation expérimentale multi-échelles permettant de clarifier le lien entre propriétés mécaniques macroscopiques et réorganisation microstructurale.L'objectif de ce travail de thèse est de mesurer l'évolution de la microstructure de la peau lors d'un essai mécanique pour identifier les mécanismes pertinents à l'échelle microscopique. Des échantillons de peau de souris ex vivo ont été soumis à des tests de traction uniaxiale, in situ sous un microscope multiphoton avec détection du signal de Génération de Seconde Harmonique. Cette technique nous permet de suivre simultanément la réponse mécanique du tissu et la réorganisation du réseau de fibres de collagène, et de quantifier le comportement aux échelles macroscopique et microscopique. Nous avons démontré que les fibres de collagène s'alignent continuellement dans la direction de traction pendant l'essai, ce qui produit la réponse mécanique du tissu. Des mécanismes microstructuraux ont été proposés pour constituer une interprétation générale permettant de rendre compte des résultats observés.Certaines mutations génétiques induisent une synthèse réduite ou anormale du collagène, ce qui aboutit à des propriétés mécaniques altérées. Ainsi, les patients atteints du syndrome d'Ehlers-Danlos, une maladie des tissus conjonctifs provoquée par une mutation du gène codant pour un collagène minoritaire, présentent typiquement une peau hyper-élastique. Nous avons appliqué notre protocole multi-échelles à deux lignées de souris génétiquement modifiées, créées dans le contexte de l'étude de la pathologie d'Ehlers-Danlos. De façon similaire, l'effet du vieillissement sur les propriétés mécaniques et microstructurales de la peau a été étudié à travers des expériences sur peau de souris âgées. Ceci a permis de mettre en évidence une réponse mécanique ainsi qu'une réorganisation microstructurale différentes entre souris génétiquement modifiées, souris âgées et souris contrôles. Ces modifications ont été evaluées dans le contexte de l'interprétation microstructurale précédemment développée.Notre technique permet d'étudier la biomécanique de la peau avec une approche multi-échelles novatrice. Elle peut être généralisée à l'étude d'autres pathologies et d'autres tissus conjonctifs, ou à l'évaluation d'autres propriétés de la peau comme sa réponse biaxiale ou viscoélastique
SCC crack initiation in nickel based alloy welds in hydrogenated steam at 400°C
No full textInternational audienc
Discrete time McKean-Vlasov control problem: a dynamic programming approach
No full textWe consider the stochastic optimal control problem of nonlinear mean-field systems in discrete time. We reformulate the problem into a deterministic control problem with marginal distribution as controlled state variable, and prove that dynamic programming principle holds in its general form. We apply our method for solving explicitly the mean-variance portfolio selection and the multivariate linear-quadratic McKean-Vlasov control problem
Shock assisted ionization injection in laser-plasma accelerators
No full textInternational audienceIonization injection is a simple and efficient method to trap an electron beam in a laser plasma accelerator. Yet, because of a long injection length, this injection technique leads generally to the production of large energy spread electron beams. Here, we propose to use a shock front transition to localize the injection. Experimental results show that the energy spread can be reduced down to 10 MeV and that the beam energy can be tuned by varying the position of the shock. This simple technique leads to very stable and reliable injection even for modest laser energy. It should therefore become a unique tool for the development of laser-plasma accelerators
Spatial-domain interferometer for measuring plasma mirror expansion
No full textInternational audienceWe present a practical spatial-domain interferometer for characterizing the electronic density gradient of laser- induced plasma mirrors with sub-30-femtosecond temporal resolution. Time-resolved spatial imaging of an intensity- shaped pulse reflecting off an expanding plasma mirror in- duced by a time-delayed pre-pulse allows us to measure characteristic plasma gradients of 10–100 nm with an ex- pansion velocity of 10.8 nm/ps. Spatial-domain interferom- etry (SDI) can be generalized to the ultrafast imaging of nm to μm size laser-induced phenomena at surfaces
Diffusive clock synchronization in highly dynamic networks
No full textInternational audienceThis paper studies the clock synchronization problem in highly dynamic networks. We show that diffusive synchronization algorithms are well adapted to environments in which the network topology may change unpredictably. In a diffusive algorithm, each node repeatedly (i) estimates the clock difference to its neighbors via broadcast of zero-bit messages, and (ii) updates its local clock according to a weighted average of the estimated differences. The system model allows for drifting local clocks, running at possibly different frequencies. We show that having a rooted spanning tree in the network at every time instance suffices to solve clock synchronization. We do not require any stability of the spanning tree, nor do we impose that the links of the spanning tree be known to the nodes. Explicit bounds on the convergence speed are obtained. In particular, our results settle an open question posed by Simeone and Spagnolini to reach clock synchronization in dynamic networks in the presence of nonzero clock drift. We also identify certain reasonable assumptions that allow for a significant higher convergence speed, e.g., bidirectional networks or random graph models
Investigating inhomogeneous electronic properties of radial junction solar cells using correlative microscopy
No full textInternational audienc
A bridged low band gap A-D-A quaterthiophene as efficient donor for organic solar cells
No full textInternational audience[small alpha],[small omega]-Bis(dicyanovinyl)quaterthiophene 1 with a median 4,4-diethyl-4H-cyclopenta[2,1-b:3,4-b[prime or minute]]dithiophene has been synthesized. UV-Vis absorption data show that the covalent bridging of the inner 2,2[prime or minute]-bithiophene leads to a significant reduction of the HOMO-LUMO gap essentially due to an increase of the HOMO level as confirmed by electrochemical and theoretical results. X-ray diffraction analysis of a single crystal of 1 shows that except for the out-of-plane ethyl groups, the conjugated system displays a quasi-planar geometry while the molecular packing exhibits strong [small pi]-stacking interactions and multiple short intermolecular contacts. Quaterthiophene 1 has been used as active donor material in organic solar cells of various architectures including bi-layer planar hetero-junctions and hybrid co-evaporated bulk hetero-junctions with C60 as electron acceptor material. A maximum conversion efficiency of 4.30% is obtained with a hybrid co-evaporated device. These results are discussed in terms of structure-properties relationships with reference to the open-chain parent [small alpha],[small omega]-bis(dicyanovinyl)quaterthiophene 2
A Proof-theoretic Characterization of Independence in Type Theory
No full textInternational audienceFor λ-terms constructed freely from a type signature in a type theory such as LF, there is a simple inductive subordination relation that is used to control type-formation. There is a related—but not precisely complementary—notion of independence that asserts that the inhabitants of the function space τ 1 → τ 2 depend vacuously on their arguments. Independence has many practical reasoning applications in logical frameworks, such as pruning variable dependencies or transporting theorems and proofs between type signatures. However, independence is usually not given a formal interpretation. Instead, it is generally implemented in an ad hoc and uncertified fashion. We propose a formal definition of independence and give a proof-theoretic characterization of it by: (1) representing the inference rules of a given type theory and a closed type signature as a theory of intuitionistic predicate logic, (2) showing that typing derivations in this signature are adequately represented by a focused sequent calculus for this logic, and (3) defining independence in terms of strengthening for intuitionistic sequents. This scheme is then formalized in a meta-logic, called G, that can represent the sequent calculus as an inductive definition, so the relevant strengthening lemmas can be given explicit inductive proofs. We present an algorithm for automatically deriving the strengthening lemmas and their proofs in G
Fourth order energy-preserving locally implicit time discretization for linear wave equations
No full textInternational audienceA family of fourth order coupled implicit-explicit time schemes is presented as a special case of fourth order coupled implicit schemes for linear wave equations. The domain of interest is decomposed into several regions where different fourth order time discretization are used, chosen among a family of implicit or explicit fourth order schemes. The coupling is based on a Lagrangian formulation on the boundaries between the several non conforming meshes of the regions. A global discrete energy is shown to be preserved and leads to global fourth order consistency in time. Numerical results in 1d and 2d for the acoustic and elastodynamics equations illustrate the good behavior of the schemes and their potential for the simulation of realistic highly heterogeneous media or strongly refined geometries, for which using everywhere an explicit scheme can be extremely penalizing. Accuracy up to fourth order reduces the numerical dispersion inherent to implicit methods used with a large time step, and makes this family of schemes attractive compared to second order accurate methods