Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

The Feynman path integral formulation of quantum mechanics is a path integral representation for a propagator or probability amplitude in going between two points in space-time. The wave function is expressed in terms of an integral equation from which the Schrodinger equation can be derived. On taking the limit h — 0, the method of stationary phase can be applied and Newton's second law of motion is obtained. Also, the condition the phase vanishes leads to the Hamilton - Jacobi equation. The secondary objective of this paper is to study ways of relating quantum mechanics and classical mechanics. The Ehrenfest theorem is applied to a particle in an electromagnetic field. Expressions are found which are the hermitian Lorentz force operator, the hermitian torque operator, and the hermitian power operator.

Automotive industry is faced with numerous power train Noise, Vibration and Harshness issues. Particularly, in the driveline area of vehicles a noise commonly referred as differential axle whine which is a tonal response and becomes apparent under cruising conditions. This is one of the key concerns in rear wheel drive commercial vehicles. Although not a failure state, it is regarded as a quality issue and a source of annoyance, which can lead to warranty concerns. The associated cost of palliation to Ford Motor Company was estimated to be $25,000,000 in 2003. There have been several ways of studying axle whine through experimentation and numerical analysis. In this thesis, a new approach for investigating axle whine is highlighted, which is more integrative and detailed. Multi-body dynamics model of a light truck s driveline is developed with all the appropriate components, using constrained Lagrangian dynamics. Component flexibility is included for driveshaft pieces, rear axle half-shafts and the suspension elements. The connectivity of the components is accurately modelled such as the floating effect of rear half-shafts, linear bushings between driveline components to chassis connections, as well as the non-linear effect of tapered roller bearings, supporting the wheel hubs and gears. Furthermore, integrated to the previously described large scale model a detailed hypoid gear pair model is devised. This incorporates micro-scale physics for tooth contact analysis to predict geometric properties and deflections for the gear pair. At the same time thermo-elastohydrodynamic lubrication theory with non-Newtonian friction is applied. All these phenomena at different physical scales, such as large displacement rigid body dynamics and analytical equations for the detailed model are solved simultaneously, all within the same modelling environment. This multi-physics, multi-scale approach has not hitherto been reported in the literature, and constitutes a significant contribution to knowledge. Comparative studies of the model predictions and detailed vehicle tests are carried out, the combination of which points to resonant conditions in system responses and flexible component behaviour, coincident with the adverse conditions in the hypoid gear meshing. It is shown that vehicle drive and coast conditions, promoting teeth pair separations lead to irregular (improper) meshing of the differential gears. Such conditions induce impulsive actions that promote the axle whine phenomenon. This is a major finding of the research and contributes to a better understanding of the axle whine problem.

Dans le domaine de la construction, le béton constitue le matériau le plus consommé. Afin de favoriser le remplissage des coffrages in situ, les bétons sont liquéfiés ponctuellement suite à l'insertion successive d'une aiguille vibrante. Malgré l'arrivée sur le marché des bétons très fluides à auto-plaçant, les bétons ordinaires représentent plus de 90% des formulations employés sur les chantiers. Cependant, les recommandations traditionnelles actuelles se basent sur des études établies au cours de la première moitié du siècle dernier. Ainsi, nous choisissons de les revisiter afin d'incorporer les progrès actuels sur la rhéologie des matériaux cimentaires. A partir d'une étude de la littérature, nous établissons les liens entre les caractéristiques mécaniques des matériels vibrants et le comportement en écoulement des bétons. Puis, dans le chapitre deux, nous mettons en évidence pour quelles consistances de matériau la vibration est réellement nécessaire. Par la suite, nous développons un modèle analytique simple afin de prédire le diamètre d'action d'un vibreur et nous le comparons à deux configurations de mises en œuvre. Enfin, nous proposons un temps minimal de vibration nécessaire au compactage du matériau et un temps maximal afin d'assurer un parement de qualité / In the field of construction, concrete is the most used material. In order to facilitate the casting process, concretes are liquefied punctually following the successive insertion of an internal vibrating poker. Despite the introduction of very fluid to self-compacting concrete, ordinary concrete represents more than 90% of the mix-design used on building sites. However, international recommendations are based on studies carried out during the first half of last century. Thus, we choose to investigate theses recommendations in order to incorporate the current progress on the rheology of cementitious materials. From a study of the literature, we establish the relationship between the mechanical properties of vibrating poker and the fresh behavior of concrete. Then, in chapter two, we determine, for which consistency, the vibration is really needed. Thereafter, we develop a simple analytical model to predict the diameter of action of internal poker and we compare two configurations of casting. Finally, we propose a minimum time of vibration required for compaction of the material and a maximum time to ensure a surface quality

Lorsqu'un fluide complexe s'écoule à travers un milieu poreux, à la non-linéarité de l'écoulement s'ajoute la spécificité de la structure poreuse, qui est souvent multi-échelle. Il émerge alors un grand nombre de problématiques fondamentales liées à l'interaction entre le fluide et la structure poreuse. L'interprétation et la modélisation de la grande variété des phénomènes physiques à petite échelle ainsi que leurs répercussions à grande échelle soulèvent de nombreuses questions. Dans cette thèse, les fluides étudiés sont des solutions de polymères, et les milieux poreux sont, entre autre, des roches issues de réservoirs pétroliers. Dans le contexte des méthodes de récupération améliorée pour les gisements pétroliers, l’injection d’eau avec polymères fait en effet partie des méthodes couramment utilisées, permettant d’augmenter l’efficacité du balayage et donc la production d’huile sur différents types de réservoirs. De part la rhéologie non-Newtonienne ainsi que les phénomènes particuliers proches de la paroi que développent les molécules de polymères, les fluides modélisés dans cette thèse peuvent être qualifiés de complexes. L'objectif de cette thèse est d'étudier la rhéologie non-Newtonienne ainsi que le comportement des molécules de polymère proches de la paroi. On relie alors ces phénomènes aux propriétés effectives macroscopiques. Pour cela, on simule numériquement les écoulements à travers desimages tomographiques de milieux poreux. Ainsi, on souhaite mieux cerner la physique qui est en jeu et également proposer des pistes d'amélioration des modèles actuellement implémentés dans les simulateurs de réservoirs. Dans une première partie de ce travail de thèse, on s'intéresse à la transition du régime d'écoulement macroscopique, de Newtonien à non-Newtonien, induite parune solution de polymère. Par des simulations numériques de l'écoulement à travers un large panel de milieux poreux, on étudie la transition entre ces deux régimes. Une analyse de la mécanique de l'écoulement permet de proposer un modèle simple et d'évaluer en ordre de grandeur la vitesse moyenne de transition. Ensuite, on étudie le glissement apparent induit par un mécanisme de répulsion des chaînes de polymère à la paroi liquide/solide. On propose un modèle à l'échelle des pores de ce phénomène et, par comparaison avec des données expérimentales,on montre que ce modèle permet de retrouver avec une précision acceptable les comportements macroscopiques observés. Enfin, avec des simulations directes sur des milieux périodiques, on relie les phénoménologies micro- et macroscopiques d'écoulements non-Newtoniens. D'un point de vue fondamental, on étudie notamment la compétition entre la non-linéarité induite par la rhéologie non-Newtonienne et le désordre inhérent à la structure poreuse. Les modèles actuellement utilisés dans les simulateurs de réservoirs sont reconsidérés au vu des résultats. / When a complex fluid is flowing through a porous medium, in addition to the fluid intrinsic physics, the multi-scale properties of the porous structure play a significant role. From the interaction between these features arise a great number of complex physical phenomena. The understandingand the modelling of the variety of these phenomena involved at the small scales and their impact on the large scales is the subject of intense work. In this thesis, the fluids we consider are polymer solutions and the porous media are typical sandstones met in petroleum applications. In petroleumengineering, the injection of a polymer slug into the oil-bearing reservoir is indeed a method commonly used in enhanced oil recovery. This method allows to increase the sweep efficiency, hence to improve the oil production of the reservoir. Due to the non-Newtonian rheology induced by the polymer molecules as well as specific mechanisms occurring at the liquid/solid interface, a polymer solution may be qualified as a complex fluid. The goal of this thesis is to investigate the non-Newtonian rheology and the behaviour of the polymer molecules near the liquid/solid interface. These phenomena are related to macro-scale effective properties. To thoroughly address this goal, we perform numerical simulations of flows through porous media. The goal is to obtain a better understanding of the underlying physics and, furthermore, we wish to propose possible improvements of the models that are currently used in reservoir simulators. Primarily, we are interested in the macro-scale transition from a Newtonian to a non-Newtonian flow. This transition is induced by the non-Newtonian rheology. By simulating flows through a wide panel of porous media, we study the macro-scale transition. An analysis of the fluid mechanics involved allows us to propose a simple model for the critical average velocity at which the transition occurs. In addition, we study the apparent slip induced by a repulsive mechanism of the polymer chains from the liquid/solid interface. We propose a pore-scale model to this mechanism. By performing comparisons with experimental datasets, we show that the model allows for a good description of the observed macro-scale behaviour. Finally, with direct simulations over periodic media, we link the phenomenology at the micro- and macro-scale for the flow of non-Newtonian fluids. On a fundamental level, we study the competition that emerges between the nonlinearity induced by the rheology and the disorder inherent to the porous structure. The results are related to modelscommonly used in reservoir simulators.

Cette thèse a pour objet le développement d’un modèle de transport sédimentaire avec la méthode SPH (Smoothed Particle Hydrodynamics). Si les modèles couramment proposés dans la littérature reposent sur un solveur hydrodynamique couplé à des lois semi-empiriques qui modélisent le transport sédimentaire, une approche différente est proposée ici. Dans le modèle proposé dans ce travail, la dynamique du sédiment est également résolue. Celui-ci est assimilé à un milieu continu dont la loi de comportement rend compte de la nature granulaire.Pour ce faire, le modèle élastique-viscoplastique d’Ulrich (2013) a été implémenté dans un code SPH préexistant programmé en langage Cuda, et amélioré du point de vue physico-numérique. Le comportement mécanique du sédiment dépend donc d’une contrainte de rupture déterminée conformément au critère de Drucker-Prager. Dans les zones du matériau où la rupture n’a pas eu lieu, les contraintes de cisaillement sont calculées selon la loi de Hooke généralisée. Dans les zones où la contrainte de rupture a été dépassée, le matériau est assimilé à un fluide rhéofluidifiant. Numériquement, la transition entre les deux états est opérée à l’aide d’une fonction de raccord qui dépend notamment du l’amplitude du taux de déformation et des propriétés granulaires du sédiment.L’eau et le sédiment sont modélisées comme deux phases immiscibles, dans le cadre d’une formulation SPH multi-phase. Pour ce faire, le modèle de Hu et Adams (2006) a été adapté aux modèles de conditions limites semi-analytiques (Ferrand, 2013). Enfin, un schéma d’intégration implicite des forces visqueuses a été développé dans ce contexte, afin d’améliorer les performances du solveur lors de modélisation d’écoulement à bas Reynolds.Plusieurs cas tests sont proposés pour valider le modèle multiphasique, le schéma implicite et le modèle élastique-viscoplastique. De manière générale, les résultats sont en bon accord avec les données expérimentales et analytiques. Le modèle permet de représenter des écoulements multi-fluide avec une bonne précision, même en présence de grand rapport de densité entre les phases. Il en va de même pour les écoulements de fluide non-newtonien et les écoulements à bas Reynolds, pour lesquels le schéma implicite conduit à des résultats très satisfaisants. Enfin, le modèle élastique-viscoplastique a été appliqué à divers cas d’écoulements granulaires, dans le cas d’un matériau sec et saturé, ainsi qu’à des cas d’érosion et d’affouillement. Là encore, les résultats sont globalement en bon accord avec l’expérience / This thesis presents the development and application of a Smoothed Particle Hydrodynamics (SPH) model to bed-load transport. While state of the art simulation methods commonly rely on a fluid dynamics solver coupled to semi-empirical relationships to model the sediment transport, a completely different approach is investigated in this work. The sediment is treated as a continuum whose behaviour law takes account for its granular nature. citepos{ulrich2013smoothed} elastic-viscoplastic model is thus implemented in an in-house code based on the Cuda language, and improved on physical and numerical aspects. The sediment behaviour depends on a yield stress determined according to Drucker-Prager's criterion. In unyielded regions, the shear stresses are calculated in line with the linear elastic theory. In yielded regions, a shear thinning rheological law is used and the transitions between solid and liquid states are ensured by a blending function driven by the strain rate magnitude and sediment granular properties. Water and sediment are modelled as two immiscible phases in the frame of a multi-phase SPH model with semi-analytical wall boundary conditions cite{ferrand2013unified}. An implicit viscous forces integration scheme is also developed to improve the code performance as for low-Reynolds flows.The multi-phase model, as well as the implicit viscous forces integration scheme, were validated on analytical test cases and good agreement was obtained. The multi-phase formulation has also proven its capability to handle flows involving high density ratio, while the implicit viscous forces integration scheme was successfully applied to the simulation of a non-Newtonian flow. The elastic-viscoplastic model was tested on dry and submerged granular flow problems. The model was able to correctly capture the liquid and solid states of the granular material, as well as the failure and the regime transitions. It was also applied to bed-load transport problems for which a good agreement with the experiment was generally found

As a result of recent advances in micro and nanotechnology, the tiny movements of nanoscale active and passive objects in fluids can be probed with ultrahigh sensitivity and time resolution. The overarching theme of this dissertation is to harness these movements in fluids in order to study fundamental fluid dynamics and develop novel biomedical devices. First, we use the oscillatory movements of nanocantilevers to investigate the scaling behavior of unsteady fluid flow. Here, our expansive experimental data and rigorous theoretical analysis suggest that a generalized scaling parameter combining the length and time scales of the flow governs the scaling. Second, we turn our attention to nanoscale movements of bacteria in a buffer. We develop a simple, robust and sensitive experimental method to detect and track the random movements of bacteria. Using this method, we show evidence that these random movements of bacteria correlate with their antibiotic susceptibility. In the first part of this thesis, we explore, through experimental and theoretical work, the breakdown of the Navier-Stokes equations in oscillatory fluid flows. The Navier-Stokes equations of hydrodynamics are based on two crucial assumptions. First, the fluid is approximated as a continuum, with a well-defined ``fluid particle." Second, the stress in the fluid is assumed to be a linear function of the rate-of-strain, resulting in a so-called Newtonian fluid. If a fluid such as an ideal gas is gradually rarefied, the Navier-Stokes equations begin to fail and a kinetic description of the flow becomes appropriate. The failure of the Navier-Stokes equations can be thought to take place via two different physical mechanisms: either the continuum hypothesis breaks down as a result of a finite size effect; or the local equilibrium is violated due to the high rate of strain. Our experimental approach is to create an unsteady flow by oscillating a finite-sized body in a gas and to measure the dissipation (or the drag force) acting on the body. By using micro and nanofabrication techniques, we independently tune the relevant linear dimensions and the frequencies of the oscillating bodies. We then measure the pressure-dependent dissipation of these micro/nano oscillators in three different gases, Helium, Nitrogen, and Argon. We observe that the scaling of the fluidic dissipation is governed by a subtle interplay between the length scale and the frequency, embodied respectively in the dimensionless Knudsen (Kn) and the Weissenberg ( Wi) numbers. We collapse all the experimental data using a single scaling parameter: Wi + Kn. This new dimensionless parameter, which can be regarded as a generalized Knudsen number, combines the relevant linear dimension and the frequency of the body; it is rooted in Galilean invariance and can be obtained rigorously from the Chapman-Enskog expansion of the Boltzmann equation. In the second part of the thesis, we turn to the movements of active micro-swimmers in a buffer. This portion of the work is motivated by a serious global public health problem: the rise of multi-drug resistant bacteria. One way to prevent this threat from growing is to treat bacterial infections with effective antibiotics using the minimum dosage. However, currently-used antibiotic susceptibility tests (ASTs), which determine whether or not bacterial isolates from a patient are susceptible to administered antibiotics, take too long. Here, we aim to develop a robust and rapid AST by exploiting a recently-observed microbiological phenomenon: various nanomechanical movements of bacteria subside promptly (within minutes) when the bacteria are exposed to an effective antibiotic. Our approach is to transduce bacterial movements into electrical voltage fluctuations in a microchannel filled with a buffer solution. When a small but constant current is driven through the microchannel, bacterial movements are converted into strong voltage fluctuations due to the fact that they modulate the effective microchannel diameter. Our experiments with E. coli show that the proposed detection method can provide antibiotic susceptibility results in ~1 hour, making it a promising rapid AST. Because this approach is based on a simple electrical measurement and does not require delicate process steps and instrumentation, it may eventually be used at the point of care. / 2019-03-09T00:00:00Z

Nas pesquisas que buscam a inserção da História e Filosofia da Ciência no Ensino de Física aparecem dois papeis principais para a Filosofia, a saber, o de atuar como estratégia didática para a compreensão conceitual e o de ser necessária para a compreensão da Natureza da Ciência. Com respeito ao primeiro, investigamos se a Filosofia pode ser mais do que apenas uma estratégia e passar a contribuir de maneira essencial, uma vez que identificamos nas propostas mencionadas que elas não trouxeram um modo de compreensão diferente dos que antes se criticava. Começamos analisando as relações entre Física e Filosofia e encontramos que o pensamento físico possui intersecções constitutivas com o pensamento filosófico, tanto na atribuição de significações, ao conectar as teorias formalizadas com o mundo físico, quanto na crítica de suas próprias teorias. Em seguida, exploramos a concepção de compreensão na Filosofia da Linguagem tardia de Wittgenstein. Para ele, a compreensão de um conceito não está em processos ou estados mentais, mas sim nos usos que fazemos das palavras em diferentes contextos, fundamentadas em uma normatividade presente na própria linguagem. O pensamento filosófico, como parte desta normatividade é também condição para a compreensão e formação de conceitos físicos em um grau suficiente para permitir a autonomia do sujeito. Exemplificamos nossa defesa a partir de um estudo teórico da mecânica newtoniana, explorando as questões metafísicas relacionadas ao problema do espaço e do movimento e através da análise de discussões entre professores em formação inicial, procurando observar o papel dos pressupostos filosóficos para a significação. Concluímos que, sem eles, havia grandes lacunas de significação que eram preenchidas por conceitos pertencentes a contextos não físicos, além de que a mecânica clássica era sinônima de um conjunto de pressupostos para a resolução de exercícios. Estes permitiram que os futuros professores fizessem descrições, deduções e classificações de fenômenos físicos, mas apenas em conjunto com as proposições filosóficas, passaram a permitir um grau mais alto de compreensão, bem como a formação de novas habilidades, tais como a elaboração de hipóteses, argumentações, deduções e críticas. / Most proposals that advocate the inclusion of history and philosophy of science in physics curriculum award two main roles to philosophy: as a teaching strategy for conceptual understanding and as a method to understand the Nature of Science. With respect to the first role, we inquire whether philosophy can be more than just a teaching strategy and to contribute in an essential way instead, as we find that the above-mentioned proposals do not yield better results in terms of conceptual understanding then the methods criticized by them. We begin by analysing the relationship between physics and philosophy and we find that physical thinking has constitutive intersections with philosophical thought, so much in assigning meanings in order to connect formalized theories to the world as in the criticism of its own theories. Then we explore the conception of understanding in the philosophy of language of the late Wittgenstein. For him, understanding of a concept is not a mental process or state, but it rather consists in the use we make of the words in different contexts, based on the normativity present in language itself. Philosophical thought, as a part of this normativity is also a condition for the comprehension and development of physicals concepts in a level sufficient for the subject\'s autonomy. We exemplify our conclusions in theoretical study of newtonian mechanics, exploring the metaphysical questions related to the problem of space and movement and analyzing discussions among teachers in initial training, trying to observe the role of philosophical assumptions in shaping meanings. We conclude that, without them, there were great significance gaps, which were filled with concepts belonging to non-physical contexts, and that classical mechanics was synonymous to a set of assumptions for solving exercises. These assumptions enabled the prospective teachers to make descriptions, deductions and classifications of physical phenomena, but only together with philosophical propositions, did they increased the degree of understanding and enable formation of new skills, such as development of hypothesis, argumentation and criticism.

Este trabalho tem como propósito investigar como a variação geométrica de determinados parâmetros envolvidos na construção de uma geometria bifurcada de seção circular, em forma de Y, afeta a resistência ao escoamento, tanto de fluidos newtonianos como não newtonianos. As geometrias estudadas foram construídas utilizando-se o princípio do Design Construtal. Os parâmetros variados foram a relação entre os comprimentos dos dutos pais e filhos, a relação entre os diâmetros dos mesmos dutos, e o ângulo central da estrutura em forma de Y. Para as relações geométricas lineares foram utilizados os valores de 0,5; 0,6; 0,7; 0,8; 0,9 e 1, enquanto para os ângulos, foram utilizados os valores de 155°, 135°, 115°, 95°, 75°, 45°, 25° e 10°. Os fluidos utilizados foram do tipo newtoniano e não newtoniano, dentre estes últimos, foram estudados fluidos pseudoplásticos e dilatantes. O trabalho foi realizado através de simulações numéricas, implementadas com a utilização do software comercial Ansys Fluent, o qual resolve as equações governantes através do método dos volumes finitos. As malhas utilizadas foram do tipo poliédrica. Os resultados indicam que há uma diferença em relação ao que se espera da literatura para as relações entre os diâmetros e os comprimentos. A Lei Hess-Murray indica que estas relações ótimas seriam de 2-1/3 para as relações entre os diâmetros e comprimentos. No presente trabalho, foram determinadas relações entre os diâmetros próximas de 0,6, e entre os comprimentos, iguais a 1. Os ângulos ótimos ficaram localizados no intervalo entre 100° e 135°. / This work aims to investigate how the geometric variation of certain parameters involved in the construction of a bifurcated Y-shaped circular cross-section geometry affects the flow resistance of both Newtonian and non-Newtonian fluids. The geometries studied were constructed using the Constructal Design principle. The parameters were the relationship between the lengths of the daughter and parent ducts, the relationship between the diameters of the same ducts, and the central angle of the Y-shaped structure. For the linear geometric relations, values of 0.5; 0.6; 0.7; 0.8; 0.9 and 1 where used, for the angles, the values of 155 °, 135 °, 115 °, 95°, 75 °, 45 °, 25 ° and 10 ° were used. The fluids used were of the Newtonian and non-Newtonian type, among the latter, pseudo plastic and dilatant fluids were studied. The work was carried out through numerical simulations, implemented with the commercial software Ansys Fluent, which solves the governing equations through the finite volume method. The meshes used were of the polyhedral type. The results indicate that there is a difference in relation to what is expected from the literature for the relationships between diameters and lengths. The Hess-Murray Law indicates that these optimal relations would be 2-1/3 for the relationships between diameters and lengths. In the present work, relationships between the diameters close to 0,6 were found and s equal to 1 between the lengths. The optimum angles were located in the range between 100 ° and 135 °.

Many cells exploit the bending or rotation of flagellar filaments in order to self-propel in viscous fluids. Often swimming occurs in complex, nonlinear fluids, e.g. mucus. Futhermore even in simple Newtonian fluids, if swimming appendages are deformable then locomotion is subject to fluid-structure interactions. The fundamental question addressed in this thesis is how exactly locomotion is impacted, in particular if it is faster or slower, with or without these effects. First we study locomotion in shear-thinning and viscoelastic fluids with rigid swimming appendages. Following the introductory Chapter, in Chapter 2 we propose empirical extensions of the classical Newtonian resistive-force theory to model the waving of slender filaments in non-Newtonian fluids, based on experimental measurements for the motion of rigid rods in non-Newtonian fluids and on the Carreau fluid model. We then use our models to address waving locomotion in shear-thinning fluids, and show that the resulting swimming speeds are systematically lowered a result which we are able to capture asymptotically and to interpret physically. In Chapter 3 we consider swimming using small-amplitude periodic waves in a viscoelastic fluid described by the Oldroyd-B constitutive relationship. Using Taylor’s swimming sheet model, we show that if all travelling waves move in the same direction, the locomotion speed of the organism is systematically decreased. However, if we allow waves to travel in two opposite directions, we show that this can lead to enhancement of the swimming speed, which is physically interpreted as due to asymmetric viscoelastic damping of waves with different frequencies. A change of the swimming direction is also possible. Secondly we consider the affect of fluid-structure interactions. In Chapter 4, we use Taylor’s swimming sheet model to describe an active swimmer immersed in an Oldroyd-B fluid. We solve for the shape of an active swimmer as a balance between the external fluid stresses, the internal driving moments, and the passive elastic resistance. We show that this dynamic balance leads to a generic transition from hindered rigid swimming to enhanced flexible locomotion. The results are physically interpreted as due to a viscoelastic suction increasing the swimming amplitude in a non-Newtonian fluid and overcoming viscoelastic damping. In Chapter 5 we consider peritrichously flagellated bacteria, such as Escherichia coli. The rotation of each motor is transmitted to a flexible rod called the hook which in turns transmits it to a helical filament, leading to swimming. The motors are randomly distributed over the body of the organism, and thus one expects the propulsive forces from the filament to almost cancel out leading to negligible swimming. We show that the transition to swimming is an elasto-hydrodynamic instability arising when the flexibility of the hook is below a critical threshold.