<p>The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by <i>in-situ</i> X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V₂H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.</p>

The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by in-situ X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V2H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.

This licentiate thesis addresses two important aspects of the van der Waals and the Casimir-Polder ground-state and excited-state (resonance) interactions between two atoms or molecules. The first is the finite-size effect and the second is the non-perturbative effect. Going beyond the usual assumption of atoms and molecules as point particles and adopting a description of finite size, the divergence inherent in such interaction energies in the limit of zero separation distance between the two interacting atoms or molecules is removed. The attainment of finite interaction energy at such close separation distance facilitates the estimation of van der Waals force contribution to the binding energy of the molecules, and towards surfaces. This is particularly important for noble atoms. We investigate in detail for a pair of helium (He) atoms and krypton (Kr) atoms, and for a pair of methane (CH4) molecules considering its environmental importance. The application of finite size further leads to finite self energies of the atoms. The expression of the interaction energy, as is discussed in detail in this thesis, typically contains a logarithmic factor of the form ln(1-x). Formerly, in evaluating the interaction energies, this factor is customarily series-expanded and truncated in the leading order with certain assumptions. This thesis explores the effect of using the full expression, which we refer to as the non-perturbative (or, the non-expanded) theory, analytically wherever possible as well as numerically. The combined application of the finite-size theory and the non-perturbative theory results in as much as 100% correction in the self energy of atoms in vacuum. This may give rise to significant physical consequences, for example, in the permeabilities of atoms across dielectric membranes. The non-perturbative theory, in addition, exhibits interesting behaviour in the retarded resonance interaction. / <p>QC 20160509</p>

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Dans cette thèse, nous étudions la robustesse dans le contexte de la modélisation de systèmes complexes par les automates cellulaires. En effet, si l'on cherche à reproduire un comportement émergent à partir d'un modèle d'automate cellulaire, il nous semble nécessaire de se demander si les comportements observés sont bien le résultat d'interactions entre entités constituantes, ou bien s'ils dépendent d'une définition particulière du modèle. Nous allons ainsi être amenés à considérer la robustesse du modèle, à savoir la résistance de son comportement à de petites variations sur les attributs de sa définition. Dans un premier temps, nous montrons la pertinence de cette approche en considérant plusieurs définitions possibles d'une perturbation de la mise à jour globale et en les appliquant à une classe simple et représentative de modèles d'automates cellulaires, les Automates Cellulaires Elémentaires. Nous observons que, malgré le fait que nos perturbations soient proches et qu'une majorité des modèles considérés ne change pas de comportement, quelques cas particuliers montrent des changements qualitatifs du comportement que nous étudions plus en détail. Dans un second temps, nous appliquons cette approche en nous penchant sur un modèle particulier d'automate cellulaire, qui simule le phénomène de formation d'essaim à partir d'un modèle évolué d'automate cellulaire, le gaz sur réseau. Nous explorons la robustesse du comportement du modèle en considérant la perturbation de deux attributs du modèle, la forme de la grille cellulaire et la mise à jour globale, et en tirons les conclusions sur la relation entre l'observation du comportement et la définition précise du modèle / In this thesis, we study the role of robustness in the context of the modelling of complex systems by cellular automata. Indeed, if we consider a cellular automaton which aims at reproducing an emergent behaviour from a similar structure, we want to determine whether its observed dynamics are the result of the interaction of entities, or whether it depends a precise definition of the model. We thus consider the model's robustness, that is, the resistance of the behaviour to small perturbations on the model features. First, we show the relevance of this approach by considering several definitions of a perturbation of the global updating and by applying them to a simple and representative class of cellular automata, the Elementary Cellular Automata. We observe that, despite the fact that most models show little or no change between the different perturbations, some particular cases show qualitative changes that we study in detail. Second, we apply this approach to a particular model of cellular automata, which simulates a swarming behaviour based on a lattice-gas model. We then explore the model robustness by considering the pertubations of two of the model's attributes, the lattice shape and the global updating, and discuss the relationship between the observation of the behaviour and the precise definitions of the model

Ce travail a été développé dans le cadre du projet européen FACETS-ITN, dans le domaine des Neurosciences Computationnelles. Son but est d’améliorer la compréhension des réseaux de neurones stochastiques de taille finie, pour des sources corrélées à caractère aléatoire et pour des matrices de connectivité biologiquement réalistes. Ce résultat est obtenu par l’analyse de la matrice de corrélation du réseau et la quantification de la capacité de codage du système en termes de son information de Fisher. Les méthodes comprennent diverses techniques mathématiques, statistiques et numériques, dont certaines ont été importés d’autres domaines scientifiques, comme la physique et la théorie de l’estimation. Ce travail étend de précédents résultats fondées sur des hypothèses simplifiées qui ne sont pas réaliste d’un point de vue biologique et qui peuvent être pertinents pour la compréhension des principes de travail liés cerveau. De plus, ce travail fournit les outils nécessaires à une analyse complète de la capacité de traitement de l’information des réseaux de neurones, qui sont toujours manquante dans la communauté scientifique. / The brain is the most complex system in the known universe. Its nested structure with small-world properties determines its function and behavior. The analysis of its structure requires sophisticated mathematical and statistical techniques. In this thesis we shed new light on neural networks, attacking the problem from different points of view, in the spirit of the Theory of Complexity and in terms of their information processing capabilities. In particular, we quantify the Fisher information of the system, which is a measure of its encoding capability. The first technique developed in this work is the mean-field theory of rate and FitzHugh-Nagumo networks without correlations in the thermodynamic limit, through both mathematical and numerical analysis. The second technique, the Mayer’s cluster expansion, is taken from the physics of plasma, and allows us to determine numerically the finite size effects of rate neurons, as well as the relationship of the Fisher information to the size of the network for independent Brownian motions. The third technique is a perturbative expansion, which allows us to determine the correlation structure of the rate network for a variety of different types of connectivity matrices and for different values of the correlation between the sources of randomness in the system. With this method we can also quantify numerically the Fisher information not only as a function of the network size, but also for different correlation structures of the system. The fourth technique is a slightly different type of perturbative expansion, with which we can study the behavior of completely generic connectivity matrices with random topologies. Moreover this method provides an analytic formula for the Fisher information, which is in qualitative agreement with the other results in this thesis. Finally, the fifth technique is purely numerical, and uses an Expectation-Maximization algorithm and Monte Carlo integration in order to evaluate the Fisher information of the FitzHugh-Nagumo network. In summary, this thesis provides an analysis of the dynamics and the correlation structure of the neural networks, confirms this through numerical simulation and makes two key counterintuitive predictions. The first is the formation of a perfect correlation between the neurons for particular values of the parameters of the system, a phenomenon that we term stochastic synchronization. The second, which is somewhat contrary to received opinion, is the explosion of the Fisher information and therefore of the encoding capability of the network for highly correlated neurons. The techniques developed in this thesis can be used also for a complete quantification of the information processing capabilities of the network in terms of information storage, transmission and modification, but this would need to be performed in the future.

Nanostructured materials are the next generation of high-performance materials, harnessing the novel properties of their nanosized constituents. The controlled assembly of nanosized particles and the design of the optimal nanostructure require a detailed understanding of particle interactions and robust methods to tune them. This thesis describes innovative approaches to these challenges, relating to the determination of Hamaker constants for iron oxide nanoparticles, the packaging of nanopowders into redispersible granules, the tuning of the wetting behavior of nanocrystals and the simulation of collective magnetic properties in arrays of superparamagnetic nanoparticles. The non-retarded Hamaker constants for iron oxides have been calculated from their optical properties based on Lifshitz theory. The results show that the magnitude of vdW interactions in non-polar solvents has previously been overestimated up to 10 times. Our calculations support the experimental observations that oleate-capped nanoparticles smaller than 15 nm in diameter can indeed form colloidally-stable dispersions in hydrocarbons. In addition, a simple procedure has been devised to remove the oleate-capping on the iron oxide nanoparticles, enabling their use in fluorometric assays for water remediation, with a sensitivity more than 100 times below the critical micelle concentration for non-ionic surfactants. Nanosized particles are inherently more difficult to handle in the dry state than larger micron-sized powders, e.g. because of poor flowability, agglomeration and potential toxicity. The rheology of concentrated slurries of TiO2 powder was optimized by the addition of sodium polyacrylate, and spray-dried into fully redispersible micron-sized granules. The polymer was embedded into the granules, where it could serve as a re-dispersing aid. Monte Carlo (MC) simulations have been applied to the collective magnetic behavior of nanoparticle arrays of various thicknesses. The decrease in magnetic susceptibility with the thickness observed experimentally was reproduced by the simulations. Ferromagnetic couplings in the arrays are enhanced by the finite thickness, and decrease in strength with increasing thickness. The simulations indicate the formation of vortex states with increasing thickness, along with a change in their orientation, which becomes more and more isotropic as the thickness increases. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p>

Ce mémoire présente des travaux de thèse consacrés à la caractérisation et la modélisation des hétérogénéités spatio-temporelles dans les écoulements granulaires cisaillés entre deux parois rigides. De nombreuses simulations ont permis de révéler le rôle crucial des dimensions de l’écoulement et l’influence des interactions des grains avec les parois (avec leur rugosité) sur le comportement global. Pour des systèmes allant jusqu'à une épaisseur de cent diamètres, des profils de vitesse non homogènes ont été mis en évidence, alors que la contrainte cisaillante est constante dans le volume, mettant en défaut le modèle viscoplastique au sein de l’écoulement. Pour réconcilier ces deux observations, nous avons enrichi le modèle viscoplastique en reliant la viscosité à une variable interne qui porte la perturbation due à la présence des parois. Cette nouvelle formulation de la viscosité permet de rétablir la validité d'une loi de comportement locale prenant en compte simultanément l’épaisseur de l’écoulement, la rugosité des parois et le nombre inertiel.Cette dépendance de la viscosité à une variable interne liée à la connectivité des grains ou à leur agitation à compacité fixée, suggère que, d’une manière générale,les écoulements granulaires doivent être décrits en termes d’au moins trois paramètres en fonction du nombre inertiel : le coefficient de frottement, la compacité et la connectivité. / This PhD work is devoted to the description and modeling of spatiotemporal inhomogeneities in granular flows sheared between two rigid walls. Our extensive simulations reveal the crucial role played by flow dimensions and the interactions of the grains with the walls and their roughness. For granular systems with increasingly larger thickness, non uniform strain profiles are evidenced while the shear stress remains uniform. This observation contradicts the common viscoelastic approach based on inertial number in the bulk of the flow. In order to reconcile these observations, we propose a viscoplastic model by a introducing an internal variable carrying the wall-induced perturbations of the flow. This re-formulation of granular viscosity reaffirms the local rheology by accounting for flow thickness, wall routines and inertialeffects. The well-defined dependence of the viscosity on an internal variable pertaining to grain connectivity or kinematic randomness at constant packing fraction, suggests that granular flows should be described by at least three parameters as a function of the inertial number:friction coefficient, packing fraction and connectivity.

Le but de ce travail de thèse est l’étude de la dynamique de sphères de grande taille dans un écoulement fortement turbulent. Pour ce faire, nous avons développé une nouvelle technique optique permettant de suivre la dynamique à 6 dimensions – position et orientation absolues – de plusieurs particules dans un écoulement complexe. Bien que la taille des particules soit comparable à l’échelle intégrale de l’écoulement, nous trouvons que sa dynamique de rotation et de translation est intermittente. De plus, nous observons que la translation et la rotation sont reliées par la force de Magnus. La répartition statistique de l’accélération n’est pas gaussienne et l’échange d’énergie avec le fluide est gouverné par la théorie mathématique des grandes déviations. Nous trouvons que le diamètre influence fortement la manière dont la particule explore l’écoulement. Nous avons ensuite appliqué le suivi de position et d’orientation à une particule instrumentée. Ce système mesure en permanence l’accélération lagrangienne qu’il subit via un accéléromètre embarqué et émet l’information à travers une électronique radio fréquence. L’orientation absolue est nécessaire pour exprimer les signaux de l’accéléromètre et ceux du suivi optique dans un repère commun; cela nous permet de comparer rigoureusement les mesures issues de ces deux techniques indépendantes. À partir de ces résultats nous avons développé des méthodes pour inférer des propriétés de l’écoulement à partir des signaux d’accélération de la particule instrumentée. / The aim of this dissertation is to gain a better understanding of the Lagrangian dynamics of solid large spheres in a complex turbulent environment. Therefore, a novel measurement technique to optically track the 6–dimensional dynamics – position and absolute orientation – of large spheres advected by a complex flow is developed. Although the sphere’s diameter is comparable to the integral length of the underlying flow, we find intermittency for both the translation and the rotation. Moreover, rotation and translation couple in agreement with a lift force. Apart from the fact that the acceleration statistics are not gaussian, and the exchange of energy between the particle and the carrier flow falls into the mathematical theory of large deviations. Additionally, we find that the particle diameter has a surprisingly strong influence on how a particle samples the flow. The 6D–tracking technique is then applied to an instrumented particle, which embarks a 3D–accelerometer and a radio-transmission system to constantly emit the felt Lagrangian acceleration as it is advected in the flow. Measuring the particle’s absolute orientation is a crucial step here to project the acceleration measured by the particle into the laboratory reference frame and enables us to compare the forces obtained by the two independent measurements. Based thereon methods for interpreting the acceleration signals of the instrumented particle are developed and tested.

La compréhension des mécanismes qui pilotent la transmission des contraintes aux interfaces déformables est au centre de nombreuses problématiques touchant des applications actuelles utilisant un film mince de polymère souple comme couche interfaciale. Arriver à caractériser de tels films fins est encore un défi aujourd’hui car l’analyse des mesures expérimentales destinées à extraire les contributions dues aux films est complexe et délicate et les techniques usuelles de caractérisation sont peu adaptées aux systèmes. Ce travail étudie la réponse mécanique de deux types de systèmes modèles au moyen de deux techniques de caractérisation différentes. Le premier système que nous avons élaboré et caractérisé mécaniquement par le test JKR, est constitué de films d’élastomère réticulé d’épaisseurs micrométriques (de 5 à 100µm) et déposés sur des wafers de silicium. Les mesures expérimentales ont été analysées par comparaison à un modèle semi-analytique récent proposé par E. Barthel dans le but d’extraire le module élastique de chaque film et de répondre à la question de savoir si l’épaisseur du film influe sur la valeur de ce module. Nous avons montré que ce modèle permet de rendre compte quantitativement du raidissement lié à la présence d’un solide supportant le film mais que la précision sur les mesures de modules de Young reste limitée (de l’ordre de 35 %).Le deuxième système modèle est constitué de brosses de polymères greffées (PDMS) par une extrémité à la surface de wafers de silicium et gonflées dans un bon solvant (47V20). Nous avons analysé la réponse mécanique dans plusieurs régimes de distance et de fréquence en utilisant un appareil à forces de surface (SFA) dans lequel on contrôle l’approche d’une sphère millimétrique d’un plan sur lequel sont greffées les polymères. En statique, nous avons vérifié que la réponse en compression était celle d’une brosse de type Alexander-de Gennes. En mode dynamique, quand la sphère est loin de la couche gonflée, nous avons vérifié que la réponse dissipative était celle d’un écoulement de Reynolds qui décrit normalement l’écoulement d’un fluide simple newtonien entre une sphère et un plan solide. Ceci nous a permis de montrer que l’écoulement du solvant pénètre partiellement à l’intérieur de la couche greffée sur une profondeur de l’ordre du tiers de l’épaisseur gonflée de la couche. Dans le régime ou les brosses sont comprimées, il n’y a pas d’accord entre les mesures réalisées et le modèle classique de Fredrickson et Pincus. Ceci s’explique par les expériences que nous avons réalisées sur un substrat nu (sans polymère) montrant pour la première fois la déformation des substrats solides qui sont indentés par l’écoulement de liquide et qu’il faut prendre en compte cette déformation dans les analyses de nanorhéologie. Finalement, une annexe est consacrée à la fabrication de surfaces hydrophobes silanisées optimisées en vue d’étudier le glissement d’un liquide simple et d’électrolytes à la paroi. / Understanding how stresses are transmitted to deformable interfaces is a key-point in numerous issues having everyday life applications which use a thin polymer film as an interfacial layer. Still, characterizing the mechanical properties of such elastic films remains a challenge because the usual employed techniques are destructive of the surface and because of the complexity of the associated analysis. In this work, we study the mechanical response of two types of home-made model systems using two different characterization techniques. The first system – studied with a JKR test- is composed of reticulated elastomeric films of micrometric thickness (5 to 100 µm) and stuck to a silicon wafer. We analyse the experimental data with E.Barthel’s recently published semi-analytical model in order to determine the elastic modulus of each indented film and see if the thickness of the film had any influence on its value. We show that this model is in a quantitatively good agreement with our data but that we only have a 35% accuracy on the elastic modulus values thanks to the set-up. The second system we studied consists in polymer brushes end-grafted onto the surface of silicon wafers and of nanometric thickness. To characterize the mechanical response of those brushes and the effect of both their molecular organization and ingredients on their ability to transmit stresses at the interface, we use a surface force apparatus in the dynamic mode as a soft fluid indenter. We use a millimetric sphere to create a liquid flow of the solvent in which the brushes are immerged and swollen. This flow induces hydrodynamic forces whose range we can control by varying the excitation frequency and the distance of approach. We obtain the following results : first with the static response we checked that the response of the polymer layers are well-described by the Alexander-de Gennes approximation. In the dynamical mode, when the sphere is far from the solid surface, we showed that the dissipative response was well-described by the Reynolds force. Thanks to those results, we succeeded in localizing the limit of penetration of the liquid flow inside the brushes at one third of the thickness of the swollen brush; second, when the brushes are compressed, we showed that the existing models (Fredrickson & Pincus) are insufficient to explain the dynamic responses of the brushes. This disagreement is explained by experiments we performed on the bare solid substrate, which show for the first time, the deformation of the substrate due to the liquid. Thus, the mechanical response of the underlying substrate has to be taken into account in the analysis of the nanorheological results on the brushes even though the substrate is much stiffer than the polymer layers. Finally, we present how we fabricated hydrophobic (silanized) surfaces in order to study the sliding of simple liquids at the wall with the same surface force apparatus.