The central concern of this thesis is the study of non-equilibrium behaviour in magnetic materials and its interpretation within the framework of the Preisach model of hysteresis. Comprehensive experimental characterizations of the field and temperature and time dependence of a suite of standard magnetic response functions have been performed on a variety of magnetic materials, including a naturally occurring mineral of nanodimensional titanomagnetite particles embedded in volcanic glass, a compressed powder of nanodimensional magnetite particles immobilized in an organic binder, a thin film of nanodimensional Fe particles embedded in alumina, and a series of sintered, bond-disordered CaxSr1-xRuO3 ferromagnets. The measurements were compared with numerical simulations based on a model Preisach ensemble of thermally activated two-level subsystems, characterized individually by a double well free energy profile in a two-dimensional configuration space, an elementary moment reversal, a dissipation field and a bias field, and characterized collectively by a distribution of these characteristic fields. Our efforts were concentrated on two principal spheres of investigation. (1) By performing detailed numerical simulations of the relaxation response of model Preisach collections of two-level subsystems under the same field and temperature protocols used to probe experimentally the relaxation dynamics of spin glasses, we have been able to show that aging, memory and rejuvenation effects are ubiquitous features of all materials which possess a broad distribution of free energy barriers which block the approach to thermal equilibrium. (2) We propose a general strategy for isolating and quantifying the two principal mechanisms, thermal fluctuations and barrier growth, which are jointly responsible for shaping the measured temperature dependence of the magnetic properties of all magnetic materials which exhibit a history dependent response to an external field excitation, and is based on the analysis of viscosity isotherms and, in particular, on a plot of T ln(tr/0) versus Ha , where tr is the time at which a viscosity isotherm measured in a field Ha at temperature T reverses sign. When thermal activation dominates barrier growth, this plot will yield a universal curve while, in the opposite limit the plot fractures into a family of isothermal curves. The strategy is applied to the analysis of each magnetic material listed above.

The central concern of this thesis is the study of non-equilibrium behaviour in magnetic materials and its interpretation within the framework of the Preisach model of hysteresis. Comprehensive experimental characterizations of the field and temperature and time dependence of a suite of standard magnetic response functions have been performed on a variety of magnetic materials, including a naturally occurring mineral of nanodimensional titanomagnetite particles embedded in volcanic glass, a compressed powder of nanodimensional magnetite particles immobilized in an organic binder, a thin film of nanodimensional Fe particles embedded in alumina, and a series of sintered, bond-disordered CaxSr1-xRuO3 ferromagnets. The measurements were compared with numerical simulations based on a model Preisach ensemble of thermally activated two-level subsystems, characterized individually by a double well free energy profile in a two-dimensional configuration space, an elementary moment reversal, a dissipation field and a bias field, and characterized collectively by a distribution of these characteristic fields. Our efforts were concentrated on two principal spheres of investigation. (1) By performing detailed numerical simulations of the relaxation response of model Preisach collections of two-level subsystems under the same field and temperature protocols used to probe experimentally the relaxation dynamics of spin glasses, we have been able to show that aging, memory and rejuvenation effects are ubiquitous features of all materials which possess a broad distribution of free energy barriers which block the approach to thermal equilibrium. (2) We propose a general strategy for isolating and quantifying the two principal mechanisms, thermal fluctuations and barrier growth, which are jointly responsible for shaping the measured temperature dependence of the magnetic properties of all magnetic materials which exhibit a history dependent response to an external field excitation, and is based on the analysis of viscosity isotherms and, in particular, on a plot of T ln(tr/0) versus Ha , where tr is the time at which a viscosity isotherm measured in a field Ha at temperature T reverses sign. When thermal activation dominates barrier growth, this plot will yield a universal curve while, in the opposite limit the plot fractures into a family of isothermal curves. The strategy is applied to the analysis of each magnetic material listed above.

Les chocs non-collisionnels jouent un rôle majeur dans de nombreux événements astrophysiques à haute densité d'énergie (sursauts gamma, restes de supernovæ, vents de pulsar...), et seraient responsables de la génération de particules supra-thermiques et de radiations. Les simulations ont démontré qu'en l’absence de champs magnétiques externes, des instabilités électromagnétiques peuvent prendre place lors de la collision de plasmas à haute vitesse. Les instabilités du type Weibel sont en effet capables de faire croître, dans ces milieux, une turbulence électromagnétique potentiellement en mesure de défléchir et d'accélérer des particules par des processus du type Fermi. En plus d'une compréhension théorique toujours croissante, la génération expérimentale de tels chocs est maintenant étudiée à l'aide de lasers de puissance. Les fluctuations thermiques électromagnétiques constituent les germes des instabilités se développant dans un plasma. Nous nous sommes attelés à leur description dans le cas d’un plasma relativiste régi par une fonction de distribution de type Maxwell-Jüttner. Des formules exactes de la densité spectrale ont pu être obtenues pour différentes orientations du vecteur propre. Ces résultats ont pu être confrontés aux prédictions d’un code de simulation particle-in-cell (PIC). Un très bon accord a été démontré.Ces résultats ont été exploités lors d'une collaboration internationale dont le but était d'estimer le temps de saturation de l'instabilité cinétique de Weibel, générant des fluctuations magnétiques. Les estimations obtenues ont pu être validées par des simulations PIC sur trois ordres de grandeur d'énergie de dérive.Nous avons ensuite mené une étude théorique et numérique des collisions de plasma d'électrons-ions en régime non-collisionnel ayant lieu lors d'événements astrophysiques tels que les restes de supernovæ. Par-delà un intérêt académique pour la compréhension des processus de transfert/transport d’énergie au sein des plasmas, la récente génération de tels plasmas en laboratoire ouvre des perspectives inédites en astrophysique des hautes énergies. La zone de recouvrement de ces faisceaux de particules est sujette à des instabilités cinétiques du type Weibel, générant des champs magnétiques intenses.Nous avons modélisé l'évolution non-linéaire d'un système soumis à l'instabilité de Weibel, et obtenu des formules analytiques de l'évolution des paramètres plasmas (températures et vitesse de dérive) et des champs magnétiques. Le modèle prédit ainsi l’évolution du système jusqu’à un stade proche de l’isotropisation complète des populations de particules et donc jusqu'à la formation d’un choc non-collisionnel. Ce modèle, en accord avec des simulations du type « particle-in-cell », pu aussi être comparé à des résultats expérimentaux récents. L'étude de la propagation des chocs non-collisionnels, m'a permis de généraliser le précédent modèle au cas de la turbulence magnétique ayant lieu en amont du front de choc.Nous nous sommes consacrés enfin aux chocs non-collisionnels créés dans un plasma dense (opaque) irradié par un laser intense. L’interaction laser-plasma qui en résulte donne lieu à un important courant d'électrons relativistes qui sont à l’origine d’instabilités cinétiques (de filamentation notamment) susceptibles d'évoluer en choc non-collisionnel. Une observation originale, contrastant avec les premières publications sur le sujet est que pour les paramètres considérés (un laser d’éclairement ~1021 Wcm-2, interagissant avec une cible solide), le choc résulte de la turbulence magnétique produite par l’instabilité électronique, plutôt que par l’instabilité ionique (dont la croissance est plus tardive). En d’autres termes, compte tenu de l’énergie très élevée des électrons accélérés par le laser, la turbulence qu'ils génèrent s’avère assez forte pour rapidement défléchir les ions. / Collisionless shocks play a major role in powerful astrophysical objects (e.g., gamma-ray bursts, supernova remnants, pulsar winds, etc.), where they are thought to be responsible for non-thermal particle acceleration and radiation. Numerical simulations have shown that, in the absence of an external magnetic field, these self-organizing structures originate from electromagnetic instabilities triggered by high-velocity colliding flows. These Weibel-like instabilities are indeed capable of producing the magnetic turbulence required for both efficient scattering and Fermi-type acceleration. Along with rapid advances in their theoretical understanding, intense effort is now underway to generate collisionless shocks in the laboratory using energetic lasers. In a first part we study the (w,k)-resolved electromagnetic thermal spectrum sustained by a drifting relativistic plasma. In particular, we obtain analytical formulae for the fluctuation spectra, the latter serving as seeds for growing magnetic modes in counterstreaming plasmas. Distinguishing between subluminal and supraluminal thermal fluctuations, we derived analytical formulae of their respective spectral contributions. Comparisons with particle-in-cell (PIC) simulations are made, showing close agreement in the subluminal regime along with some discrepancy in the supraluminal regime. Our formulae are then used to estimate the saturation time of the Weibel instability of relativistic pair plasmas. Our predictions are shown to match 2-D particle-in-cell (PIC) simulations over a three-decade range in flow energyWe then develop a predictive kinetic model of the nonlinear phase of the Weibel instability induced by two counter-streaming, symmetric and non-relativistic ion beams. This self consistent, fully analytical model allows us to follow the evolution of the beams' properties up to a stage close to complete isotropization and thus to shock formation. Its predictions are supported by 2D and 3D particle-in-cell (PIC) simulations of the ion Weibel instability in uniform geometries, as well as shock-relevant non-uniform configurations. Moreover, they are found in correct agreement with a recent laser-driven plasma collision experiment. Along with this comparison, we pinpoint the important role of electron screening on the ion-Weibel dynamics, which may affect the results of simulations with artificially high electron mass. We subsequently address the shock propagation resulting from the magnetic Weibel turbulence generated in the upstream region. Generalizing the previous symmetric-beam model to the upstream region of the shock, the role of the magnetic turbulence in the shock-front has been analytically and self-consistently characterized. Comparison with simulations validates the model. The interaction of high-energy, ultra-high intensity lasers with dense plasmas is known to produce copious amounts of suprathermal particles. Their acceleration and subsequent transport trigger a variety of Weibel-like electromagnetic instabilities, acting as additional sources of slowing down and scattering. Their understanding is important for the many applications based upon the energy deposition and/or field generation of laser-driven particles. We investigate the ability of relativistic-intensity laser pulses to induce Weibel instability-mediated shocks in overdense plasma targets, as first proposed by Fiuza in 2012. By means of both linear theory and 2D PIC simulations, we demonstrated that in contrast to the standard astrophysical scenario previously addressed, the early-time magnetic fluctuations (Weibel instability) generated by the suprathermal electrons (and not ions) are strong enough to isotropize the target ions and, therefore, induce a collisionless electromagnetic shock.

<p>Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and <i>I–V</i> measurements.</p><p>In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.</p><p>The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa₂Cu₃O₇ (YBCO) and MgB₂ thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.</p><p>In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.</p><p>The <i>I–V</i> characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. </p><p>From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.</p>

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements. In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs. The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length. In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices. The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

Les transitions de phase près d'un point critique - dites du second ordre - sont un sujet toujours d'actualité en raison des nombreux phénomènes critiques intéressants tels que la force de Casimir critique, les problèmes de confinements ou les phénomènes hors d'équilibre suivant une trempe au point critique. Cette thèse vise à étudier expérimentalement certains phénomènes engendrés près d'un point critique. La thèse est divisée en deux axes : le premier consiste à développer plusieurs systèmes expérimentaux qui permettront de mesurer essentiellement la viscosité, par l'intermédiaire des fluctuations thermiques à l'échelle micrométrique. Le deuxième axe consiste à trouver et caractériser des mélanges binaires présentant une transition de phase du second ordre dans lesquelles on souhaite faire des mesures. Les enjeux de ces systèmes expérimentaux sont d'avoir une régulation en température précise, une sonde de mesure sensible aux fluctuations thermiques et/ou à des forces de l'ordre du pN, et un échantillon fiable et reproductible présentant un point critique accessible expérimentalement. Nous avons ainsi monté à partir d'un microscope à force atomique (AFM) déjà présent au laboratoire, un système de mesure de viscosité à sonde AFM fibrée. Malgré sa faible efficacité en terme de sonde de mesure métrologique, nous avons pu décrire et développer un modèle de couplage de modes de vibration permettant de comprendre la mécanique de microleviers AFM fibrés. J'ai également développé au laboratoire la mesure de microscopie dynamique différentielle qui permet de faire des mesures à sondes multiples contrairement au premier montage. J'ai discuté de la précision de la mesure dans le cadre de notre objectif d'étude des fluctuations critiques. En ce qui concerne l'échantillon de mesure, nous avons étudié plusieurs mélanges binaires que nous avons caractérisés par des méthodes classiques de turbidité et diffusion statique de la lumière. Cette caractérisation nous a permis de connaître les mélanges binaires pour les utiliser dans un troisième système de mesure : billes micrométriques piégées dans des pinces optiques déjà monté au laboratoire. Nous y avons rajouté un système de régulation thermique fait maison pour être exploité avec les contraintes de la pince optique. Ces tests ont fait apparaître un phénomène inattendu d'oscillations de transition de phase induites par laser. Nous avons développé un modèle pour les décrire. Enfin, des expériences préliminaires - toujours avec les pinces optiques dans les mélanges binaires - nous ont permis d'observer qualitativement des effets de l'approche au point critique par des mesures de viscosité et d'interaction type force de Casimir critique. / Phase transitions near a critical point, or second order phase transitions, are still a recent object of studies because of the large amount of interesting critical phenomena as the critical Casimir force, confinements problems or out of equilibrium phenomena following a quench at the critical point. This thesis experimentally studies phenomena near a critical point. This manuscript is divided in two parts : the first one consists in building several experimental set-up which measure viscosity through thermal fluctuation at micrometric scale. The second part consists in finding and characterize binary mixtures which show a second order phase transition. Preliminary results have been done in these samples. One of the principal points of these experimental set-up are a well regulated temperature, a probe sensitive to thermal fluctuation and/or pN forces and a reproducible binary mixture which presents a critical point easy to reach experimentally. We mounted from an Atomic Force Microscope (AFM) already built in the laboratory, a hanging-fiber probe to measure viscosity of liquids. Despite its weak efficiency as a metrologic probe, we described and developed a mode coupling model which let us understand mechanics of hanging-fiber probes. I also developed in the lab the dynamic differential microscopy technique (DDM) which do measurements with several probes. I discussed about the measure precision with in mind the aim of studying critical fluctuations. For the choice of the sample, we studied several binary mixtures. We characterized them by classical methods as turbidity measurements and static light scattering. These characterizations let us learn about binary mixtures in order to use them in a third experimental set-up : beads trapped in an optical tweezers already built in the lab. We added to it a home-made thermal regulation which can be used with the constraints of optical tweezers. These tests showed an unexpected phenomenon of oscillating phase transition induce by laser. We developed a model to describe it. At last, preliminary experiments with optical tweezers in binary mixtures showed qualitative effects of an approach near a critical point on the viscosity and on interactions between beads as critical Casimir force.

Mankind has come a long way since the invention of wheel to accessing information in the quintillionth of a second. At the heart of every invention ever made, there has been only one objective, to ease the way of living. The progeny of this philosophy automatically came to be known as technology. It was technology that led to the design of the wheel for fast human transportation and the same motivation let him design more sophisticated machines. In mankind’s journey to improve technology, it began to learn efficient or correct ways to utilize and understand resources around it, creating a whole new philosophy called science. Ingeniously, it was science that let humans understand what they were made of: matter, to discovering what matter itself was composed of: atoms and what puts these together: forces. Science and technology has been of tremendous comfort for mankind and has helped it evolve throughout history. However, it is not always that science and technology go hand in hand. Technology has always helped man design devices and instruments which often bring physical comfort. Science on the other hand has made sure that loss in manual labor is compensated by increased inquisitiveness. There were times when technology was more developed than science. This was the time when machines were taking mankind by fire, resulting in the first and second industrial revolutions. During that same time, science was develop-ing slowly by increasing human curiosity to learn the way nature functioned at finer details. This led to the discovery of the electron by Joseph John Thomson, who proved the electron to be a negatively charged particle. Consequently, he was awarded the 1906 Nobel Prize in Physics for his work on electricity conduction in gases. Later, his son, George Paget Thomson, counter-proved that electrons are actually waves. He was also awarded the 1937 Nobel Prize in Physics, along with Clinton Joseph Davisson for their discovery of electron diffraction caused by crystals. Despite the ambiguity, mankind today accepts electrons to have dual properties. It is both a wave and a particle. This duality is not limited to electrons but is applicable to all matter, as proposed by Louis de Broglie and is one of the fundamental principles in science. With the help of well-developed technology, mankind can now design machines that allow controlled flow of electrons establishing the world of electronics, allowing faster human communication. The study of electronic properties and its usage in designing efficient devices is what electronics is all about. Electrons are the protagonist of mankind today. The presence of electrons is unanimously accepted by everyone. All physical and chemical processes are a result of electrons getting transported. Electron transfer processes are ubiquitous in nature, be it in photosynthesis or energy production in mitochondria . It is the fundamental process in all chemical reactions and all physical processes related to electricity. Every piece of hi-tech gadget practically uses the electron, and the whole of humanity is being serviced by it. In fact, a life without utilizing the electrons is abysmally mundane. Electronics has evolved from designing the first millimeter sized point contact transistor to silicon chip processors that contain billions of nanosized transistors. Studying electron transport has also led to the discovery of light emission during conduction popularly known as LED, an abbreviation for light emitting diode. Heating up of devices during electron transport forced mankind to study heat transport and design materials that have highly efficient electron transfer processes. Electron transfer is also the basic principle behind the Scanning Tunneling Microscope (STM), Scanning Electron Microscope (SEM) and the Transmission Electron Microscope (TEM) which replaced the conventional idea of using light (photons) as a source to observe matter at the nanolevel. However, mankind is still in the process of developing a technology which exploits both properties of the electron simultaneously. Today, science and technology work together to overcome this barrier. Indeed, science and technology today have come as far as controlling electron transport up to a single atomic level where quantum effects (discretization and interference of states that make up the system) are very pronounced. This branch can be referred to as quantum electronics or quantronics. It is one of the possible alternatives to conventional silicon based electronics, and is made of three separate fields. The first one that exploits the quantum nature of electron transport in nanoscopic systems, is usually called molecular electronics or moletronics. The second involves ex-ploiting the spin of the electron and is termed as spintronics. The third is the most challenging where neither science nor technology has been able to fully grasp the characteristics, i.e utilizing the heat quanta in designing thermal de-vices at the single atomic level. In general, for ultimate exploitation of both the wave and particle characteristics of the electron, a proper comprehension of the quantum effects during electron transport is necessary to design a quantronic device. Also, in any quantronic device, apart from quantum effects, fluctuations in temperature cause changes in the flow of electrons. Since electron flow is a random process, fluctuations need to be analyzed from a statistical point of view. Moreover, to address issues related to efficiency and power of these quantronic devices, a proper understanding of the thermodynamic aspects is required. The aim of the work in the thesis is to theoretically analyze the fluctuations, quantum effects and thermodynamics, that in principle, affect the basic physics and chemistry during electron and heat transport in a specific class of out of equilibrium quantum systems. This class of quantum systems are prototypes for designing quantronic devices, where both wave and particle nature of the electrons are pronounced. These are called molecular junctions or quantum junctions. It will in turn help the field of quantronics in the long run. However, in this thesis, it is the science that I address and not the technological aspects.

A l’aide d’un interféromètre différentiel à quadrature de phase nous mesurons les fluctuations thermiques de la déflexion d’un micro-levier. Il est alors possible de déduire différentes propriétés mécaniques du levier telles que raideur, fréquences de résonance, facteurs de qualité etc. Dans un tel système, la précision maximale sur les mesures est limitée par le bruit de grenaille des photodiodes (shot-noise). Afin d’augmenter le rapport signal sur bruit, nous augmentons l’intensité lumineuse du laser de mesure, diminuant ainsi le bruit de fond des spectres de fluctuations thermique. En revanche, l’augmentation de l’intensité du laser a pour effet de décaler vers les basses fréquences les résonances du levier. Une première partie de ce travail de thèse a pour objectif la compréhension de ce phénomène. Ainsi, nous associons le décalage en fréquence à un échauffement du levier par le laser de l’interféromètre et au flux de chaleur associé le long du levier. Nous développons alors un modèle permettant de relier cet effet à la température de l’extrémité du levier en se basant sur un profil de température linéaire. Une seconde partie de ce travail vise à mesurer la température effective d’un levier à l’aide d’une extension du théorème fluctuation-dissipation. Nous montrons que les fluctuations de ce système hors équilibre sont plus faibles que celles attendues compte tenu du profil de température. Nous cherchons alors à identifier l’origine de ce déficit de fluctuations. Dans une dernière partie nous estimons les profils de température sur des leviers en faisant varier leurs paramètres géométriques ou leur coefficient d’absorption, ainsi que la position du laser chauffant le levier. / Thanks to a home made quadrature phase differential interferometer, we measure the thermal fluctuations ofa cantilever. It is then possible to infer various mechanical properties such as eigenfrequencies, stiffness,quality factor, etc. In such system, the maximal precision on the measure is limited by the shotnoise of thephotodiodes. To increase the signal-noise ratio we raise the light intensity of the laser, lowering thebackground noise. Doing so, the cantilever eigen frequencies shifts to lower values. A fisrt part of this thesiswork has for objective the understanding of this phenomenon. Thus, we associate this frequency shift with aheating of the cantilever by the laser. We develop a model linking this effect to the temperature at the freeend of the cantilever assuming a linear temperature profile.A second part of this thesis leads us to estimate the effective temperature of a cantilever using thefluctuation-dissipation theorem. We show that the fluctuations of our out of equilibrium system are lower thanthe fluctuations expected at equilibrium.In the last part, we estimate the temperature profiles on cantilevers by varying their geometry, absorptioncoefficient and laser position.

Ce travail porte sur l'étude théorique des interfaces entre deux fluides visqueux, soumis à un écoulement de Couette plan. Dans cette situation hors d'équilibre, les fluctuations thermiques de l'interface sont modifiées en raison du couplage par le cisaillement entre les effets visqueux et les effets de tension. Comme c'est le cas pour d'autres systèmes de matière molle (par exemple, les phases lamellaires), le cisaillement peut alors amplifier ou amortir les déformations interfaciales. On s'intéresse tout d'abord à la dynamique des fluctuations interfaciales. On montre que ces dernières vérifient une équation stochastique non-linéaire, dont la solution est contrôlée par un paramètre sans dimension qui contient toute l'information sur le système. La résolution à faible taux de cisaillement révèle que le déplacement quadratique moyen des fluctuations thermiques diminue avec l'écoulement, conformément aux observations expérimentales et numériques. Ensuite, on étudie l'influence des effets inertiels sur la stabilité de l'écoulement, dans le régime des fortes viscosités et des faibles tensions. Ce régime des grands nombres capillaires n'a été que très peu étudié, mais trouve sa pertinence par exemple dans les mélanges biphasiques de colloïdes et de polymères. Des critères de stabilité simples sont mis en évidence. Finalement, on réalise une étude numérique des propriétés des fluctuations interfaciales à grand cisaillement. Bien que les effets visqueux soient dominants, il en ressort une phénoménologie similaire à certains modèles de turbulence. / In this contribution, we investigate theoretically an interface between two newtonian fluids in a stationnary shear flow. The statistical properties of the interface are driven out of equilibrium due to the coupling by the shear flow between viscous and tension effects. The shear flow may either enhance or suppress interfacial deformations, as it is the case in others soft matter systems (for example, lamellar phases). The dynamics of thermal fluctuations is first considered. It is shown that fluctuation modes follow a stochastic nonlinear equation. The solution is then controlled by a single dimensionless parameter, that contains all the information of the system. The mean square displacement is obtained in the limit of small shear rates: it is found to be smoothed out by the flow, in qualitative agreement with experiments and simulations. Then, a stability analysis of the flow is achieved when inertial contibutions are taken into account. We focus on the regime of small surface tension and large viscosity. This regime has experienced a renewed interest in the last few years, in the context of phase-separated colloid-polymer mixtures. Simple criteria for the stability or instability of the flow are outveiled. Finally, a numerical study of fluctuation properties is performed in the limit of large shear rate. Although viscous effects are predominant, the results share some similarities with some turbulence models.