Important global hydrodynamic relationships for shallow spouted beds of high-density particles were characterized in terms of three features: minimum spouting velocity, overall bed pressure drop at minimum spouting velocity; and fountain height. Spouted bed literature is sparse for shallow beds (static particle depth to bed diameter ≤ 1) and beds with heavy particles (density > 3000 kg/m3). Correlations for such beds were developed here by varying column diameter, static bed height, particle diameter, particle density, gas density and gas flow in an ambient temperature and pressure bed. The degree of correlation between each of the observed hydrodynamic features and a set of selected dimensionless groups from the literature was evaluated with principal components analysis. The minimum spouting velocity correlated strongly with the ratios of particle to bed diameter, of particle to gas density, and of static bed height to particle diameter, and weakly with Archimedes number. Overall bed pressure drop at minimum spouting correlated strongly with Archimedes number, the ratio of static bed height to particle diameter and Froude number. Fountain height correlated strongly with the ratios of the superficial gas velocity to minimum spouting velocity, of static bed height to particle diameter and of the particle to the bed diameter. Principal component regression models were developed for minimum spouting velocity, bed pressure drop, and fountain height with respect to a selected set of dimensionless parameters. All models have regression coefficient values exceeding 85%. Predictions using models developed in this study were compared with correlations in the literature and found to give better results for the experimental conditions studied. Most likely the literature models were less accurate because they were extrapolated. Distinct bed pressure drop relationships with gas flow were observed for certain ranges of particle diameter and static bed height. In addition three dynamical spouting modes were observed, and named as regular, erratic and bimodal. A spouting regime map is proposed based on the spouting regimes defined in this investigation. The correspondence between bed pressure drop relationships and spouting regimes is still unclear.

Diblock-Copolymers (DBCs), created by covalently joining two chemically distinct polymer blocks, spontaneously form various nanoscale morphologies such as lamellae, cylinders, spheres, etc. due to the chemical incompatibility of its constituent blocks. This effect is called microphase separation in the literature. Because of this self-organizing property DBCs find applications in many areas e.g. in creating selective membranes, and in polymer based modern electronic devices like organic photovoltaics where the internal morphology plays an important role in determining the performance of the device. Many such modern devices are based on thin film technologies and uses copolymer nanocomposites as it exhibits advantageous electrical, optical, and mechanical properties. Also, DBC can direct the spatial distribution of nanoparticles (NPs) in the polymer matrix via microphase separation. Generally, two types of NPs are distinguished with respect to their monomer affinity: selective NPs which prefer one component of DBC, and non-selective NPs which interact equally with both components of DBC. In this work, using molecular dynamics simulations and analytical calculations, we explore the effect of adding both types of NP in the copolymer matrix considering a thin film (or confined) geometry. We consider a cylinder forming DBC melt confined by purely repulsive walls in slit geometry and study the behavior of the system upon adding non-selective NPs. Two models of non-selective interactions between the monomers and NPs are applied, i.e repulsive and weakly attractive interactions (athermal and thermal cases respectively). Spatial distribution of NPs in the copolymer matrix is sensitive to the NP-monomer interaction behavior. We focus on the thermal case and discuss, in particular, the following points: (1) role of diblock and polymer-wall interfaces, (2) spatial distribution of NPs, and (3) NP segregation and uptake behavior by the copolymer film. The uptake of NPs by the copolymer film in the thermal case displays a non-monotonic dependence on temperature which can be explained qualitatively using a mean-field model. In general, addition of non-selective NPs do not affect the copolymer morphology and the NPs are preferentially localized at the interface between microphase domains. Morphological transitions are observed when adding selective NPs to the copolymer matrix. By varying the amount of selective NPs and diblock composition we systematically explore the various structures formed by the nanocomposites under confinement and constructed the corresponding phase diagram in diblock composition and NP concentration. We also discuss the NP induced orientation transition of lamellar structure and study the stability of lamellar phases formed by the nanocomposites. To study the commensurability and wetting transition of horizontally oriented lamellar phase formed by the nanocomposites we have developed a mean field model based on the strong segregation theory. Our model predicts that it is possible to reduce the frustration in a film of fixed thickness by properly tuning the NP-monomer interaction strength. Furthermore, the model predicts a discontinuous transition between the non-wetted phase (where a dense NP layer is present in the polymer-substrate interface) and wetted phase (where the substrate is covered by polymers). Finally, we extend our study to non-equilibrium where we apply a shear flow field to copolymer thin films. Here, we study the flow behavior, lamellae deformation and change of pair-wise interaction energy, and macroscopic response like kinetic friction coefficient and viscosity of the copolymer thin film with and without NPs. / Lösungen von Diblock-Copolymeren (DBC), welche durch die kovalente Bindung zweier chemisch unterschiedlicher linearer Polymerblöcke entstehen, können spontan mikroskopische Strukturen ausbilden, welche je nach dem Grad der chemischen Kompatibiliät der Blöcke beispielsweise lamellen-, zylinder- oder kugelartige Formen zeigen. Dieses Phänomen wird meist als Mikrophasenseparation bezeichnet. Aufgrund dieser selbstorganisierenden Eigenschaft finden DBCs Anwendungen in vielen Bereichen der Forschung und der Industrie. Beispielsweise zur Erzeugung selektiver Membranen oder in moderner polymerbasierter Elektronik, wie organischen Solarzellen, wo die innere Struktur eine wichtige Rolle spielt um die Leistungsfähigkeit zu erhöhen. Viele moderne Geräte basieren auf der Technologie dünner Schichten und nutzen Copolymer-Nanokomposite um elektrische, optische oder mechanische Eigenschaften zu verbessern. In Folge der Mikrophasenseparation kann man mit Hilfe von DBC die räumliche Verteilung von Nanopartikeln (NP) in der Polymermatrix kontrollieren. Man unterscheidet im Allgemeinen zwischen zwei Arten von NP: selektive NP, welche eine der beiden Komponenten der DBC bevorzugen und nicht-selektive NP, welche mit beiden Komponenten gleichartig wechselwirken. In der vorliegenden Arbeit nutzen wir molekulardynamische Simulationen und analytische Rechnungen um den Eigenschaften zu studieren, welche eine Zugabe von selektiven und nicht-selektiven NP auf eine dünnschichtige Copolymermatrix hat. Wir betrachten eine zylinderformende Schmelze aus DBC, welche in einem dünnen Film, zwischen zwei harten Wänden eingeschränkt ist, und untersuchen das Verhalten des Systems unter Zugabe nicht-selektiver NP. Zwei Modelle nicht-selektiver Wechselwirkungen werden angenommen: ausschließlich repulsive (athermische) Wechselwirkungen und schwach anziehende (thermische) Wechselwirkungen. Die räumliche Verteilung der NP ist abhängig von dem jeweiligen Wechselwirkungsverhalten. Wir konzentrieren uns hierbei auf den thermischen Fall und diskutieren speziell folgende Schwerpunkte: (1.) die Rolle der sich ausbildenden Grenzschichten, (2.) die räumliche Verteilung der NP und (3.) die Abscheidung der NP, sowie die Aufnahmefähigkeit derselben durch die Polymermatrix. Im thermische Fall zeigt die Aufnahme der NP durch die Copolymerschicht eine nicht-monotone Abhängigkeit von der Temperatur, was mit Hilfe eines Mean-Field Modells erklärt werden kann. Die Zugabe nicht-selektiver NP hat keinen Einfluss auf die Struktur der Copolymermatrix und die NP werden vorzugsweise an der Grenzschicht der jeweiligen Mikrophasen gefunden. Im Gegensatz dazu kann man durch die Zugabe selektiver NP eine Strukturveränderung in der Copolymermatrix feststellen. Durch Veränderung der Menge der NP und der Zusammensetzung der DBC können wir systematisch unterschiedliche Strukturen des räumlich eingeschränkten Nanokomposits erzeugen und ein entsprechendes Phasendiagram bezüglich der NP Konzentration und der DBC Zusammensetzung erstellen. Wir untersuchen auch die durch NP induzierte Orientierung der Lamellenstruktur und analysieren ihre Stabilität. Um den sogenannten Kommensurabilitäts- und Benetzungsübergang in horizontal orientierten Lamellenstrukturen zu untersuchen haben wir ein Mean-Field Modell entwickelt, welches auf der Annahme der 'starken Segregation' basiert. Unser Modell macht die Vorhersage, dass es möglich ist die Frustration in einem Kompositfilm zu reduzieren, indem man die NP-Monomer-Wechselwirkung entsprechend anpasst. Zusätzlich sagt das Modell einen diskontinuierlichen Übergang zwischen der unbenetzten Phase (Ausbildung einer dichten NP Konzentration an der Polymer-Substrat Grenzschicht) und der benetzten Phase (das Substrat ist ausschließlich vom Polymerkomposit bedeckt) voraus. Abschließend weiten wir unsere Untersuchungen auf Nicht-Gleichgewichtszustände aus und induzieren durch Scherung der Substratwände einen Strömungprofil im Kompositfilm. Dabei analysieren wir das Strömungsverhalten, die Lamellendeformation und die Änderung der paarweisen Wechselwirkungsenergie. Wir untersuchen auch makroskopische Größen, wie den kinetischen Reibungskoeffizienten und die Viskosität, je in An- und Abwesenheit von Nanopartikeln.

Les liquides confinés présentent beaucoup de comportements fascinants, très différents de ceux qui sont observés dans leur volume. Le confinement peut induire un déplacement de l'équilibre des phases (par exemple de la transition liquide-vapeur, aussi appelé condensation capillaire), il peut modifier la température de transition vitreuses des polymères, ou bien imposer un ordre dans l'arrangement moléculaire du fluide. Les modifications des propriétés mécaniques des liquides aux interfaces sont particulièrement importantes au niveau des applications. Cependant au niveau de la compréhension, le simple cas des liquides newtoniens est toujours sujet à controverse, avec d'une part des simulations numériques montrant que la viscosité ne devrait pas être modifiée pour des confinements supérieurs à quelques tailles moléculaires, et d'autre part des expériences non unanimes, montrant parfois des modifications qualitatives des propriétés rhéologiques sous confinement. Récemment nous avons montré que les méthodes d'impédance hydrodynamique en géométrie sphère-plan constituent une méthode privilégiée, non-intrusive et non-ambigüe, pour aborder la nano-mécanique des liquides aux interfaces (1,2). S'agissant d'interphases, cad de couches fluides dont les propriétés sont modifiées par la proximité d'un solide, il est possible d'accéder à leur module sans contact, donc sans la perturbation apportée par une seconde surface. S'agissant de l'effet du confinement sur la rhéologie, nous avons montré que la déformation élastique à l'échelle du pico-mètre des surfaces confinantes, donne une forte modification de la rhéologie apparente du fluide, même en l'absence de tout effet intrinsèque. Le sujet de thèse vise à mettre en oeuvre les méthodes d'impédance hydrodynamique pour étudier la rhéologie de solutions de polymères confinés. On étudiera plus précisément deux systèmes modèles d'importance fondamentale aussi bien que pratique : les brosses de polymères greffés, dont les propriétés mécaniques sont un enjeu dans les applications de lubrification aussi bien que pour les écoulements biologiques, et les solutions de polymères hydro-solubles d'intérêt pour la récupération assistée du pétrole, en vue de comprendre les effets de fluidification sous confinement et de faire la part entre modification de la viscosité et couche de déplétion induite par l'écoulement. Au niveau instrumental, un des enjeux de la thèse sera de mettre en oeuvre les mesures d'impédance hydrodynamique sur deux types d'instruments complémentaires au niveau de l'échelle de la sonde: l'appareil de mesure de forces dynamique (SFA) du Liphy, et l'AFM à détection interférométrique développé à l'Institut Néel. Ces différentes échelles d'investigation devront permettre de préciser les propriétés moyennes mécaniques moyennes des liquides confinés et leurs gradients au voisinage de la paroi. Une perspective du travail sera de mettre en regard les propriétés mécaniques et rhéologiques intrinsèques des brosses polymères déterminées directement sur SFA ou AFM, avec leur propriétés fonctionnelles: propriétés de lubrification des contacts frottants, ou de modification des écoulements des dans micro-canaux. Ceci sera poursuivi sur la plateforme expérimentale mise en place par Lionel Bureau au Liphy : SFA de friction, systèmes micro-fluidiques à visée biomimétique (parois fonctionnalisées par des brosses polymères). L'enjeu sera alors de comprendre comment les propriétés mécaniques et rhéologiques des brosses déterminent celles des systèmes dans lesquels elles interviennent. / Liquids confined present many fascinating behaviors very different from those observed in their volume. Confinement can induce a shift in the balance of phases (eg the liquid-vapor transition, also called capillary condensation), it can change the glass transition temperature of the polymer, or impose order on the molecular arrangement of fluid. The changes in the mechanical properties of liquid interfaces are particularly important in applications. However the level of understanding, the simple case of Newtonian liquids is still controversial, with one hand, numerical simulations show that the viscosity should not be changed for some higher molecular sizes containment, and secondly non-unanimous experiences, sometimes showing qualitative changes in rheological properties under confinement. Recently we have shown that the methods of hydrodynamic impedance sphere-plane geometry is a privileged, non-intrusive method and unambiguous, to discuss the mechanics of nano-liquid interfaces (1,2). As interphase, ie fluid layers whose properties are modified by the proximity of a solid, it is possible to accede their contactless module, so without the disturbance caused by a second surface.S As regards the effect of confinement on the rheology, we have shown that the elastic deformation across the pico meter of confining surfaces, gives a strong modification beyond apparent rheology of the fluid, even in the absence of any intrinsic effect. The thesis aims to implement the hydrodynamic impedance methods to study the rheology of polymer solutions confined. We specifically consider two models of fundamental importance as well as practical systems: brushes grafted polymer whose mechanical properties are an issue in lubrication applications as well as for biological flows and solutions of water-soluble polymers interest in enhanced oil recovery, in order to understand the effects of thinning containment and to distinguish between changes in viscosity and depletion layer induced by the flow. At the instrumental level, one of the challenges of the thesis is to implement the hydrodynamic impedance measurements on two complementary instruments at the level of the probe: the measuring dynamic power (SFA) of Liphy, and AFM interferometric detection developed at the Institut Néel. These different scales of investigation will help to clarify the medium average mechanical properties of liquids confined and their gradients near the wall. A view of work will be to look mechanical and rheological properties of polymer brushes intrinsic determined directly on SFA or AFM with their functional properties: lubricating properties of sliding contacts, or modification of the flow in microchannels. This will continue on the implementation by Lionel Bureau Liphy experimental platform: SFA friction advised biomimetic micro-fluidic systems (walls functionalized polymer brushes). The challenge will be to understand how the mechanical and rheological properties of brushes determine those systems in which they operate.

Les suspensions d'objets microscopiques ayant la faculté de se déplacer par eux-mêmes dans le fluide qui les entoure sont des systèmes qui présentent un intérêt croissant dans la communauté scientifique. Du fait de leur dynamique intrinsèquement hors-équilibre au sens de la physique statistique, ils génèrent des effets particulièrement complexes. Parmi les micro-objets autopropulsés existants, les micro-algues vertes représentent une part importante de la biomasse de la Terre et participent activement au retraitement du CO2 par leur activité photosynthétique. Elles présentent de plus un remarquable potentiel dans les domaines de la production de bio-carburants, du retraitement des déchets, de la fabrication de cosmétiques et de compléments alimentaires. La compréhension de la dynamique de nage de ce type de microorganisme est d'un intérêt primordial d'un point de vue industriel. Cet ouvrage présente l'étude de la dynamique de la micro-algue Chlamydomonas Reinhardtii. En utilisant un système de suivi de particules en imagerie optique que nous avons développé, nous analysons ici le mécanisme fondamental de nage utilisé par cette algue jusqu'à ses implications en terme d'effets collectifs sur la dynamique de nage d'une suspension semi-diluée. / The suspensions of microscopic objects with the ability to propel themselves into the surrounding fluid are systems of growing interest in the scientific community. Due to their intrinsic out-of-equilibrium dynamics in the sense of statistical physics, they generate complex effects. Among the existing self-propelled micro-objects, green micro-algae are an important part of the biomass of Earth and they actively participate to the recycling of CO2 by their photosynthetic activity. Moreover they have remarkable potential for the production of bio-fuels, waste reprocessing, cosmetics and dietary supplements production. From an industrial point of view, understanding the dynamics of this type of swimming microorganism is of primary interest. This work presents the study of the dynamics of microalgae Chlamydomonas Reinhardtii. Using a system of particle tracking with optical imaging that we have developed, we analyze the mechanism of stroke used by the algae up to its implications in terms of collective effects on the dynamics of swimming in a semi-dilute suspension.

Neste trabalho realizamos dois estudos independentes sobre a termodinâmica de modelos de água e o dobramento de proteínas em rede. / On this work we develop two independent studies on lattice models for water thermodynamics and protein folding.

Neste trabalho caracterizamos dispersões aquosas de vesículas lipídicas por espalhamento de luz. Utilizamos os diferentes métodos da técnica: espalhamento de luz estático (SLS) e espalhamento de luz dinâmico (DLS) no estudo desses sistemas. No SLS a intensidade da luz espalhada foi coletada em vários ângulos de espalhamento para diferentes concentrações, e obtivemos os seguintes parâmetros: peso molecular, Mw, raio de giração, Rg, que esta relacionado _a forma do centro espalhador e segundo coeficiente do virial, A2, que está relacionado a interações entre partículas. No DLS a correlação da luz espalhada foi obtida em função do tempo, para vários ângulos e concentrações. As medidas de DLS fornecem o coeficiente de difusão dos centros espalhadores, que estão relacionados a seus diâmetros efetivos. Em especial a técnica de SLS foi amplamente estudada para desenvolvimento de uma apostila coordenada pela Profa. Vera B. Henriques. Nesta apostila a teoria de espalhamento de luz estático está apresentada através de uma abordagem diferente da convencional, sob o ponto de vista da mecânica estatística. O principal objeto de estudo neste trabalho foram as dispersões aquosas de dimiristoil fosfatidil glicerol (DMPG). O DMPG, lipídio aniônico, saturado, com 14 carbonos nas cadeias hidrocarbônicas, em baixa força iônica, exibe um comportamento termo-estrutural muito peculiar, apresentando uma larga região de transição de fase, entre as fases gel e fluida. O lipídio dimiristoil fosfatidil colina (DMPC) com as mesmas cadeias carbônicas mas cabeça polar neutra, foi utilizado como controle, pois não apresenta esta anomalia. Os tamanhos efetivos obtidos pelos diferentes métodos (SLS) e (DLS) são semelhantes nas fases gel e fluída de ambos os lipídios, mas isso não ocorre na região de transição do DMPG. Ambos os métodos indicam um aumento na dimensão das vesículas de DMPG, nesta região de temperatura. No entanto, por SLS temos que o Rg triplica na região de transição, levando a um raio efetivo muito maior que o observado por DLS. Como é sabido que não há fusão entre as vesículas, considerando os dados de espalhamento de luz e a literatura, discutimos um modelo em que o agregado lipídico apresente grandes deformações isotrópicas e grandes flutuações de forma. / Light scattering used to structurally characterize lipid dispersions. In the present work light scattering was used to characterize aqueous dispersions of lipid vesicles. Two different methods were applied: static light scattering (SLS) and dynamic light scattering (DLS). With SLS, the intensity of the scattered light was measured at several scattering angles and different lipid concentrations. The following parameters were obtained: molecular weight (Mw) and radius of gyration (Rg), related to the mass and form of the scattering particle, and the second Virial coefficient (A2), related to interactions among particles. With DLS, the autocorrelation of the intensity of the scattered light was obtained, also at several scattering angles and lipid concentrations. Measurements of DLS provide the difusion coefficient of the particles, which are related to their hydrodynamic radii. In particular, the theory of SLS was largely studied to the elaboration of a manuscript, under the supervision of Dr. Vera B. Henriques, which presents an unconventional approach to the technique. Here, the main object of investigation was the aqueous dispersion of the anionic lipid dimyristoyl phosphatidylglycerol (DMPG), a saturated anionic lipid, with 14 C-atoms in the hydrophobic chains. At low ionic strength, this lipid exhibits a very peculiar thermo-structural behavior, presenting a large region of transition, between the gel and fluid phases. The lipid dimyristoyl phosphatidylcholine (DMPC), with the same hydrocarbon chains but zwitterionic polar headgroup, was used as a control, since it does not present such anomaly. Lipid dispersions were extruded through 100 nm filters (pore diameter). In the gel and fluid phases of DMPC and DMPG, both SLS and DLS indicate similar dimensions for the lipid vesicles, assuming them spherical. For DMPC, the obtained effective radius (Reff ) values were around 53 and 62nm for the gel and fluid phases, respectively, indicating the expected expansion of the uid bilayer, and radii compatible with the extrusion process. For DMPG, the fluid vesicle (Reff _ 30nm) was also found to be larger than the gel vesicle (Reff _ 27nm), but much smaller than the dimensions of the filter pore (R = 50nm). More interesting, over the DMPG transition region, SLS indicated a three times increase for the vesicle radius of gyration, whereas the hydrodynamic radius, measured by DLS, increased by 30% only. Besides, no light depolarization could be detected, indicating that the DMPG aggregates are isotropic particles, in average, at all temperatures. Considering the results presented here, and those in the literature, the presence of large bilayer fluctuations over the DMPG transition region will be discussed, possibly including the existence of bilayer pores.

Com base em resultados de DSC, análise estrutural por difração de raios X e observações de textura, analisamos os efeitos da mistura do solvente apolar hexadecano com cristais líquidos formados pelas moléculas banana 1,3-fenilenobis[4-(4-tetradecilfeniliminometil)benzoato] (MB14) e 4-cloro-1,3-fenilenobis[4-(4-tetradecilfeniliminometil) benzoato] (MB14Cl). Propusemos um modelo estrutural para explicar as modificações causadas no arranjo molecular da fase B2 pelo acréscimo gradual do solvente. Observamos a diminuição da temperatura de transição entre esta fase e a fase isotrópica, porém a transição entre a fase B2 e a fase a temperaturas mais baixas não sofre alteração significativa. Para concentrações em massa de hexadecano de 45 % no MB14 e de 55 % no MB14Cl, a fase B2 já não é mais observada. Para o MB14Cl, resultados de difração de raios X revelaram que as moléculas de hexadecano penetram entre as camadas esméticas, aumentando a distância intercamada em torno de 3 Å. Acima de 5 % de concentração do solvente, o aumento da distância intercamada satura e ocorre segregação de fases em escala nanométrica. O comportamento da fase B2 sob a ação de um campo elétrico variável também foi analisado para o MB14 puro. Apresentamos um modelo para a linha de base do sinal de corrente de polarização, que leva em conta a não linearidade da condutividade para valores altos de campo aplicado, devido à movimentação iônica na amostra. Para o cálculo da viscosidade, consideramos a não linearidade da constante dielétrica com o campo aplicado, e adaptamos outro modelo, inicialmente utilizado para cristais líquidos ferroelétricos sob a ação de um campo quadrado, para o caso de um cristal líquido antiferroelétrico sob um campo triangular. Quanto aos dois tipos de arranjo molecular da fase B2, o arranjo homoquiral se mostrou bem mais estável que o racêmico, mesmo sob aplicação de campo triangular, quando este último é inicialmente favorecido. O arranjo racêmico se apresentou mais viscoso que o arranjo homoquiral, contrariando nossas previsões. / Based on DSC results, structural analysis by X-ray diffraction and texture observations, we observed the effects of mixing the nonpolar solvent hexadecane with the banana molecules liquid crystals ,3-phenilenebis[4-(4-tetradecilpheniliminometil)benzoate] (MB14) and 4-chloro-1,3-phenilenebis[4-(4-tetradecilpheniliminometil) benzoate] (MB14Cl). We propose a structural model to explain the changes in the molecular ordering of the B2 phase caused by the gradual increase of the solvent. We observed a decreasing of the transition temperature between B2 and isotropic phases, however the transition between B2 and lower temperature phases did not change significantly. For hexadecane concentrations above 45 wt% in MB14 and 55 wt% in MB14Cl, the B2 phase is no longer present. In MB14Cl, X-ray diffraction results showed that the hexadecane molecules penetrate between the smectic layers, increasing the interlayer spacing by about 3 Å. Above 5 wt% of solvent concentration, the increasing of the interlayer spacing saturates, and a phase segregation in nanometric scale occurs. The behavior of the B2 phase under variable electric field was also analysed for the pure MB14. We present a model for the baseline of the polarization current signal, which considers the non-linearity of the conductivity for high values of applied field, due to the presence of ions in the sample. In order to calculate the viscosity, we considered the non-linearity of the dielectric constant with the applied field, and adapted another model, initially used in ferroelectric liquid crystals under rectangular field, for the case of an antiferroelectric liquid crystal under triangular field. Concerning the two kind of molecular ordering in the B2 phase, the homoquiral ordering proved to be far more stable than the racemic, even under triangular field, when the latest is favored. Our measurements resulted in a racemic ordering more viscous than the homoquiral, going against our predictions.

Les fluides complexes tels que les gels, les pâtes ou les émulsions présentent une large gamme d'applications, tant dans la vie quotidienne que dans l'industrie. Parmi ces fluides, les fluides dits ”à seuil” se comportent soit comme des solides élastiques, soit comme des fluides nonnewtoniens selon la contrainte qu'on leur applique. Ces propriétés sont décrites par des lois phénoménologiques (telle que la loi d'Herschel Bulkley), bien que leurs origines soient encore mal comprises. Dans cette thèse, nous nous intéressons à la question des petites échelles, où la structure et les phénomènes interfaciaux jouent des rôles prépondérants dans les écoulements. Ce travail expérimental a été essentiellement mené sur du carbopol, un microgel de polymères et un fluide à seuil modèle. L'étude se concentre essentiellement sur trois points : – La caractérisation rhéologique et structurale des échantillons. L'accent est porté sur l'influence du protocole de préparation sur les propriétés des gels.– Le cas des écoulements confinés de fluides complexes. Ce travail met en évidence l'existence d'effets non-locaux aux échelles micrométriques, faisant intervenir une taille caractéristique interprétée dans le cadre du modèle de fluidité. – Les ascensions capillaires de fluides complexes, aux échelles millimétriques. En revisitant l'expérience classique des ascensions capillaires dans le cas des fluides complexes, cette étude propose une extension de la loi de Jurin au cas des fluides à seuil. Il est montré que cette expérience simple permet de mesurer la tension de surface et quelques paramètres rhéologiques de fluides complexes tels que le seuil et l'exposant d'Herschel-Bulkley / Complex fluids such as gels, pastes or emulsions have a wide range of applications, both in everyday life and in the industry. Among them, Yield Stress Fluids (YSF) behave either as solids or as non-Newtonian fluids depending on the shear stress applied. These features are modeled by phenomenological laws ( such as the Herschel-Bulkley one ) although their origins are still poorly understood. In this thesis, we focus on the issue of small lengthscales where the structure and interfacial phenomena play predominant roles in the flows. This experimental work was essentially carried out on carbopol, a polymer microgel and a model YSF. The study mainly focuses on three points : – The rheological and structural characterization of the samples. The emphasis is put on the influence of the preparation protocol on the microgel properties. – The case of confined flows of complex fluids. This work highlights the existence of nonlocal effects at the micrometer scales, involving a characteristic length, interpreted in the frame of the fluidity model. – Capillary ascents of complex fluids, involving millimeter scales. This work suggests an extension of Jurin’s law to the case of YSFs. It is shown that this simple experiment allows to measure the surface tension and some rheological parameters of complex fluids such as the yield stress and the Herschel-Bulkley exponent

Quand on augmente le nombre de Reynolds d’un écoulement, il passe progressivement d’un état ordonné, laminaire, à un état chaotique, turbulent. Lors de cette transition, on voit apparaître des structures hydrodynamiques ordonnées qui se désordonnent sous l’effet de déstabilisations successives. Ces structures peuvent poser problème lorsqu’on souhaite par exemple homogénéiser des suspensions car des particules peuvent s’y retrouver piégées et générer des zones sur ou sous-concentrées. La plupart des fluides naturels et industriels sont non newtoniens. Pour mieux comprendre et contrôler l’apparition des structures hydrodynamiques dans de tels fluides, nous nous sommes intéressés dans le cadre de cette thèse, aux mécanismes d’instabilités gouvernant la transition vers la turbulence de fluides complexes dans la configuration simplifiée de Taylor-Couette. Dans un premier volet théorique et numérique, les concepts d’instabilités hydrodynamiques ont été appliqués à l’écoulement de Taylor-Couette pour trois modèles rhéologiques : la loi de puissance, le modèle de Carreau et le modèle de Bingham. Deux rapports de rayons et différentes vitesses de rotation des deux cylindres ont été considérés. La déstabilisation de l’écoulement de Couette circulaire a été étudiée pour prédire l’apparition de structures telles que les rouleaux de Taylor-Couette aux temps longs. Une analyse aux temps courts a également été effectuée pour étudier les phénomènes de croissance transitoire dans l’écoulement. Dans un second volet expérimental, un dispositif de Taylor-Couette en plexiglas a été élaboré. Dans un premier temps, des mesures de couple avec un fluide newtonien (glycérol) ont été réalisées et comparées avec succès avec les résultats de la littérature. Des mesures rhéométriques ont ensuite été réalisées pour choisir des fluides répondant aux lois rhéologiques utilisées dans le volet théorique. Des visualisations ont été effectuées pour détecter l’apparition des instabilités et les conditions critiques ont été comparées aux prédictions de l’analyse linéaire de stabilité. Des mesures de vélocimétrie par image de particules (PIV) ont été aussi effectuées pour déterminer les champs de vitesse de l’écoulement. Ces différentes études ont permis de mettre en évidence le caractère sous-critique et l’apparition d’un écoulement chaotique engendrés par le caractère rhéofluidifiant des fluides. / When increasing the Reynolds number of a flow, it passes gradually from an ordered state, called laminar, to a chaotic state, called turbulent. During this transition, organized hydrodynamic structures appear which disorganize under successive destabilizations. These structures may cause problems for example in the relation to the homogenization of suspensions, because particles may be trapped in the structures, leading to under or over-concentrated zones. Furthermore, most of the fluids found in nature and in the industry are non-Newtonian. In order to better understand and control the apparition of hydrodynamic structures in such fluids, we focused during this PhD on the instability mechanisms governing the transition to turbulence of complex fluids in the simplified configuration of Taylor-Couette. In a theoretical and numerical approach, concepts of hydrodynamic instabilities were applied to the circular Couette flow for three rheological models: the power law, the Carreau and the Bingham models. Two radius ratios and different rotation velocities were considered for the cylinders. The destabilization of the circular Couette flow was studied to predict the apparition of structures such as Taylor vortex at long times. An analysis at short time was also performed to study the phenomena of transient growth. In an experimental approach, a Taylor-Couette device was designed. First, the torque on the inner cylinder was measured for a Newtonian fluid (glycerol) and successfully compared to previous results of the literature. Secondly, rheological measurements were conducted in order to select fluids showing rheological behaviors considered in the theoretical approach. Visualizations were then performed in order to detect the apparition of instabilities and the critical conditions were compared to the predicted by means of the linear stability analysis. Finally, PIV measurements provided velocity fields in the flow. These different studies evidenced a subcritical transition and chaotic flow due to the shear-thinning character of the fluids.

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