In this thesis, an approach is presented for the generation of grids suitable for the simulation of high Reynolds number viscous flows in complex three-dimensional geometries. The automatic and reliable generation of such grids is today on the biggest bottlenecks in the industrial CFD simulation environment.<p><p>In the proposed approach, unstructured tetrahedral grids are employed for the regions far from the viscous boundaries of the domain, while semi-structured layers of high aspect ratio prismatic and hexahedral elements are used to provide the necessary grid resolution inside the boundary layers and normal to the viscous walls. The definition of the domain model is based on the STEP ISO standard and the topological information contained in the model is used for applying the hierarchical grid generation parameters defined by the user. An efficient, high-quality and robust algorithm is presented for the generation of the unstructured simplicial (triangular of tetrahedral) part of the grid. The algorithm is based on the Delaunay triangulation and the internal grid points are created following a centroid or frontal approach. For the surface grid generation, a hybrid approach is also proposed similar to the volume.<p>Semi-structured grids are generated on the surface grid (both on the edges and faces of the domain) to improve the grid resolution around convex and concave ridges and corners, by aligning the grid elements in the directions of high solution gradients along the surface. A method is also developed for automatically setting the grid generation parameters related to the surface grid generation based on the curvature of the surface in order to obtain an accurate and smooth surface grid. Finally, a semi-structured prismatic/hexahedral grid generation algorithm is presented for the generation of the part of grid close to the viscous walls of the domain. The algorithm is further extended with improvements meant to increase the grid quality around concave and convex ridges of the domain, where the semi-structured grids are known to be inadequate.<p><p>The combined methodology is demonstrated on a variety of complex examples mainly from the automotive and aeronautical industry. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

J'ai étudié expérimentalement, à l'échelle de la taille des particules, les mécanismes à l'origine de l'intensification des transferts ayant lieu dans les suspensions cisaillées de particules non-inertielles et non-Browniennes. Dans un premier temps, l'expérience de Taylor est revisitée en étudiant l'évolution d'une goutte de colorant soumise à un cisaillement périodique. Au-delà d'une amplitude critique de déformation, la présence des particules brise la réversibilité du système et induit une forte dispersion de la goutte de colorant. Ensuite, en m'intéressant au transfert en proche paroi, j'ai montré que la rotation des particules sur la paroi induit un transport à flux constant d'un scalaire jusque dans le bulk de la suspension, brisant la couche limite diffusive. Une solution analytique du profil de concentration dans cette zone est proposée, en bon accord avec les expériences. Finalement, des mesures PIV haute résolution du fluide interstitiel dans le bulk de la suspension ont été réalisées. A partir de ces champs de vitesses, on a reconstruit l'historique d'étirement de lignes matérielles du fluide et ainsi déterminé les lois d'étirement, information fondamentale pour la compréhension du processus de mélange. La présence des particules change les lois d'étirement qui passent de linéaires dans un fluide pur, à exponentielles en présence de particules. Un modèle d'étirements multiplicatifs est proposé, qui prédit quantitativement l'évolution de la moyenne, de la variance, et la forme log-normale des distributions d'étirements mesurées expérimentalement. L'inhomogénéité des étirements dans les suspensions cisaillées implique une large distribution du temps de mélange. / Mainly based on experiments, I investigated at a particle scale the mechanisms at the origin of the transfer enhancement in sheared non-Brownian and non-inertial particulate suspensions. First, I revisited Taylor's experiment, investigating the evolution of a drop of dye in a periodic shear. Beyond a critical strain amplitude, the presence of the particles breaks the reversibility of the system and the drop of dye is rapidly dispersed in the surrounding medium. Then, investigating the transfer process in the wall vicinity, I showed that in this region, the rotation of the particles convectively transport a scalar at a constant rate directly from the wall towards the bulk of the suspension, breaking the diffusive boundary layer. An analytical solution for the concentration profile in this region is proposed, in good agreement with experimental measurements. Lastly, high-resolution PIV measurements of the fluid phase were performed in the bulk of the suspension. Using these velocity fields, we reconstructed the stretching histories of fluid material lines to determine the stretching laws, crucial for the understanding of the mixing process. The presence of the particles changes the very nature of the stretching laws from linear, in a pure fluid, to exponential in the presence of particles. A multiplicative stretching model is proposed, which quantitatively predicts the experimentally measured evolution of the mean and the variance of the elongations of the fluid material lines as well as their evolution towards a log-normal distribution. The strong stretching inhomogeneity in sheared suspensions results in a broad distribution of the mixing time.

Due to the requirements for high blade loading, modern turbo‐machine blades operate very close to the stall regime. This can lead to flow separation with periodic shedding of vortices, which could lead to self induced oscillations or stall flutter of the blades. Previous studies on stall flutter have focused on flows at high Reynolds number (Re ~ 106). The Reynolds numbers for fans/propellers of Micro Aerial Vehicles (MAVs), high altitude turbofans and small wind turbines are substantially lower (Re < 105). Aerodynamic characteristics of flows at such low Re is significantly different from those at high Re, due in part to the early separation of the flow and possible formation of laminar separation bubbles (LSB). The present study is targeted towards study of stall flutter in a cascade of blades at low Re. We experimentally study stall flutter of a cascade of symmetric NACA 0012 blades at low Reynolds number (Re ~ 30, 000) through forced sinusoidal pitching of the blades about mean angles of incidences close to stall. The experimental arrangement permits variations of the inter‐blade phase (σ) in addition to the oscillation frequency (f) and amplitude; the inter‐blade phase angle (σ) being the phase difference between the motions of adjacent blades in the cascade. The unsteady moments on the central blade in the cascade are directly measured, and used to calculate the energy transfer from the flow to the blade. This energy transfer is used to predict the propensity of the blades to undergo self‐induced oscillations or stall flutter. Experiments are also conducted on an isolated blade in addition to the cascade. A variety of parameters can influence stall flutter in a cascade, namely the oscillation frequency (f), the mean angle of incidence, and the inter‐blade phase angle (σ). The measurements show that there exists a range of reduced frequencies, k (=πfc/U, c being the chord length of the blade and U being the free stream velocity), where the energy transfer from the flow to the blade is positive, which indicates that the flow can excite the blade. Above and below this range, the energy transfer is negative indicating that blade excitations, if any, will get damped. This range of excitation is found to depend upon the mean angle of incidence, with shifts towards higher values of k as the mean angle of incidence increases. An important parameter for cascades, which is absent in the isolated blade case is the inter‐blade phase angle (σ). An excitation regime is observed only for σ values between ‐450 and 900, with the value of excitation being maximum for σ of 900. Time traces of the measured moment were found to be non‐sinusoidal in the excitation regime, whereas they appear to be sinusoidal in the damping regime. Stall flutter in a cascade has differences when compared with an isolated blade. For the cascade, the maximum value of excitation (positive energy transfer) is found to be an order of magnitude lower compared to the isolated blade case. Further, for similar values of mean incidence angle, the range of excitation is at lower reduced frequencies for a cascade when compared with an isolated blade. A comparison with un‐stalled or classical flutter in a cascade at high Re, shows that the inter‐blade phase angle is a major factor governing flutter in both cases. Some differences are observed as well, which appear to be due to stalled flow and low Re.

The flow field and forces on an isolated oscillating NACA 0012 airfoil in a uniform flow is studied using viscous vortex particle method. The simulations are carried out at very low chord (c) based Reynolds number (Re=1000), motivated by the current interest in development of Micro Air Vehicles (MAV). The airfoil is forced to oscillate in both heave and pitch at different normalized oscillation frequencies (f), which is represented by the non-dimensional reduced frequency fc/U).( From the unsteady loading on the airfoil, the net energy transfer to the airfoil is calculated to determine the propensity for the airfoil to undergo self-induced oscillations or flutter at these very low Reynolds numbers. The simulations are carried out using a viscous vortex particle method that utilizes discrete vortex elements to represent the vorticity in the flow field. After validation of the code against test cases in the literature, simulations are first carried out for the stationary airfoil at different angles of attack, which shows the stall characteristics of the airfoil at this very low Reynolds numbers. For the airfoil oscillating in heave, the airfoil is forced to oscillate at different reduced frequencies at a large angle of attack in the stall regime. The unsteady loading on the blade is obtained at different reduced frequencies. This is used to calculate the net energy transfer to the airfoil from the flow, which is found to be negative in all cases studied. This implies that stall flutter or self-induced oscillations are not possible under the given heave conditions. The wake vorticity dynamics is presented for the different reduced frequencies, which show that the leading edge vortex dynamics is progressively more complex as the reduced frequency is increased from small values. For the airfoil oscillating in pitch, the airfoil is forced to oscillate about a large mean angle of attack corresponding to the stall regime. The unsteady moment on the blade is obtained at different reduced frequencies, and this is used to calculate the net energy transfer to the airfoil from the flow, which is found to be positive in all cases studied. This implies that stall flutter or self-induced oscillations are possible in the pitch mode, unlike in the heave case. The wake vorticity dynamics for this case is found to be relatively simple compared to that in heave. The results of the present simulations are broadly in agreement with earlier stall flutter studies at higher Reynolds numbers that show that stall flutter does not occur in the heave mode, but can occur in the pitch mode. The main difference in the present very low Reynolds number case appears to be the broader extent of the excitation region in the pitch mode compared to large Re cases studied earlier. region in the pitch mode compared to large Re cases studied earlier.

In comparison to the flow in a rigid channel, there is a multi-fold reduction in the transition Reynolds number for the flow in a micro channel when one of the walls is made sufficiently soft, due to a dynamical instability induced by the fluid-wall coupling. The flow after transition is characterized using Particle Image Velocimetry (PIV) in the x − y plane where x is the stream-wise direction and y is the cross-stream co-ordinate along the small dimension of the channel of height 0.2 − 0.3mm. For the two different soft walls of shear modulus 18 kPa and 2.19 kPaused here, the transition Reynolds number is about 250 and 330 respectively. The deformation of the microchannel due to the applied pressure gradient is measured in the experiments, and is used to predict the laminar mean velocity profiles for comparison with the experimental results. The mean velocity profiles in the microchannel are in quantitative agreement with those predicted for the laminar flow before transition, but are flatter near the centerline and have higher gradients at the wall after transition. The flow after transition is characterized by a mean velocity profile that is flatter at the center and steeper at the walls in comparison to that for a laminar flow. The root mean square of the stream-wise fluctuating velocity shows the characteristic sharp increase from the wall and a maximum close to the wall, as observed in turbulent flows in rigid-walled channels. However, the profile is asymmetric with a significantly higher maximum close to the soft wall in comparison to that close to the hard wall, and the Reynolds stress is found to be non-zero at the soft wall, indicating that there is a stress exerted by fluid velocity fluctuations on the wall. The turbulent energy production profile has a maximum at the soft wall, in contrast to the flow at a rigid surface where the turbulent energy production is zero at the wall (due to the zero Reynolds stress). The maximum of the root mean square of the velocity fluctuations and the Reynolds stress (divided by the fluid density) in the soft-walled microchannel for Reynolds numbers in the range 250-400, when scaled by suitable powers of the maximum velocity, are comparable to those in a rigid channel at Reynolds numbers in the range 5000-20000. The near-wall velocity profile shows no evidence of a viscous sub-layer for (yv∗/ν) as low as 2, but there is a logarithmic layer for (yv∗/ν) up to about 30, where the von Karman constants are very deferent from those for a rigid-walled channel. Here, v∗ is the friction velocity, ν is the kinematic viscosity and y is the distance from the soft surface. . The surface of the soft wall in contact with the fluid is marked with dye spots to monitor the deformation and motion along the fluid-wall interface. The measured displacement of the surface in the stream-wise direction, which is of the order of 5 − 12µm, is consistent with that calculated on the basis of linear elasticity. Low-frequency oscillations in the displacement of the surface are observed after transition in both the stream-wise and span-wise directions, indicating that the turbulent velocity fluctuations are dynamically coupled to motion in the solid. Modification of soft-wall turbulence in a micro channel due to the addition of small amounts of polymer The modification of soft-wall turbulence in a microchannel due to the addition of small amounts of polymer is experimentally studied using Particle Image Velocimetry (PIV) to measure the mean and the fluctuating velocities. The micro channels are of rectangular cross-section with height about 160 µm, width about 1.5 mm and length about 3 cm, with three walls made of hard Poly-dimethylsiloxane (PDMS) gel, and one wall made of soft PDMS gel with an elasticity modulus of about 18 kPa. A dynamical instabilty of the laminar flow due to the fluid-wall coupling, and a transition to turbulence, is observed at a Reynolds number of about 290 for the flow of pure water in the soft-walled microchannel (Verma and Kumaran, J. Fluid Mech., 727, 407-455, 2013). Solutions of polyacrylamide of molecular weight 5 × 106 and mass fraction up to 50 ppm, and of molecular weight 4 × 104 and mass fraction up to 1500 ppm, are used in the experiments. In all cases, the solutions are in the dilute limit be-low the critical concentration where the interactions between polymer molecules become important. The modification of the fluid viscosity due to addition of polymer molecules is small; the viscosity of the solutions with the highest polymer concentration exceed those for pure water by about 10% for the polymer with molecular weight 5 × 106, and by about 5% for the polymer with molecular weight 4 × 104. Two distinct types of flow modifications below and above a threshold mass fraction for the polymer, cTHRESHOLD , which is about 1 ppm for the polyacrylamide with molecular weight 5 × 106, and about 500 ppm for the polyacrylamide with molecular weight 4 × 104. As the polymer mass fraction increases up to the threshold value, there is no change in the transition Reynolds number, but there is significant turbulence attenuation the root mean square velocities in the stream wise and cross-stream directions decrease by a factor of 2, and the Reynolds stress decreases by a factor of 4 in comparison to that for pure water. When the polymer concentration increases beyond the threshold value, there is a decrease in the decrease in the transition Reynolds number by nearly one order of magnitude, and a further decrease in the intensity of the turbulent fluctuations. The lowest transition Reynolds number of about 35 for the solution of polyacrylamide with molecular weight 5 × 106 and mass fraction 50 ppm. For the polymer solutions with the highest concentrations, the fluctuating velocities in the stream wise and cross-stream direction are lower by a factor of 5, and the Reynolds stress is lower by a factor of 10, in comparison to pure water. Despite the significant turbulence attenuation, a sharp increase in the intensity of the fluctuating velocities is evident at transition for all polymer concentrations. Transitions to deferent kinds of turbulence in a channel with soft walls The flow in a rectangular channel with walls made of soft polyacrylamide gel is studied to examine the effect of soft walls on transition and turbulence. The width of the channel is much larger than the height, so that the flow can be considered approximately two-dimensional, the wall thickness is much larger than the channel height (smallest dimension), the bottom wall is fixed to a substrate and the top wall is unrestrained. The fluid velocity is measured using Particle Image Velocimetry, while the wall motion is studied by embedding beads in the soft wall, and measuring the time-variation of the displacement both parallel and perpendicular to the surface. As the Reynolds number increases, two different flow regimes are observed in sequence. The first is the ‘soft-wall turbulence’ resulting from a dynamical instability of the base flow due to the fluid-wall coupling. The flow in this case exhibits many of the features of the turbulent flow in a rigid channel, including the departure of the velocity profile from the parabolic profile, and the near-wall maxima in the stream-wise root mean square fluctuating velocity. However, there are also significant differences. The turbulence intensities, when scaled by suitable powers of the mean velocity, are much larger than those after the hard-wall laminar-turbulent transition at a Reynolds number of about 1000. The Reynolds stress profiles do not decrease to zero at the walls, indicating that the wall motion plays a role in the generation of turbulent fluctuations. There is no evidence of a viscous sub-layer close to the wall to within the experimental resolution. The mean velocity profile does satisfy a logarithmic law close to the surface within a region between 2-30 wall units from the surface, but the von Karman constants are very different from those for the hard-wall turbulence. The wall displacement measurements indicate that there is no observable motion perpendicular to the surface, but displacement fluctuations parallel to the surface are observed after transition, coinciding with the onset of velocity fluctuations in the fluid. The fluid velocity fluctuations are symmetric about the center line of the channel, and they show relatively little downstream variation after a flow development length of about 5 cm. As the Reynolds number is further increased, there is a second ‘wall flutter’ transition, which involves visible downstream traveling waves in the top (unrestrained) wall alone. Wall displacement fluctuations of low frequency (less than about 500 rad/s) are observed both parallel and perpendicular to the wall. The mean velocity profiles and turbulence intensities are asymmetric, with much larger turbulence intensities near the top wall. There is no evident logarithmic profile close to either the top or bottom wall. Fluctuations are initiated at the entrance of the test section, and the fluctuation intensities decrease with downstream distance, the fluctuation intensities first rapidly increase and then decrease as the Reynolds number is increased. For a channel with relatively small height (0.6 mm), the transition Reynolds number for the soft-wall instability is lower the hard-wall transition Reynolds number of about 1000, and the laminar flow becomes unstable to the soft-wall instability leading to soft-wall turbulence and then to wall flutter as the Reynolds number is increased. For a channel with relatively large height (1.8 mm), the transition Reynolds number for the soft-wall instability is higher than 1000, the flow first undergoes the hard-wall laminar-turbulent transition at a Reynolds number of about 1000, the turbulent flow undergoes the soft-wall transition leading to soft-wall turbulence, and then to wall flutter.

Este trabalho é dedicado à análise e implementação de esquemas "upwind" de alta ordem modernos e o modelo de turbulência k-epsilon padrão no Freeflow-2D; um ambiente integrado para simulação numérica em diferenças finitas de problemas de escoamentos incompressíveis com superfícies livres. O propósito do estudo é a simulação de escoamentos de fluidos newtonianos incompressíveis, bidimensionais, confinados e/ou com superfícies livres e a altos valores do número de Reynolds. O desempenho do código Freeflow-2D atual é avaliada na simulação do escoamento numa expansão brusca e de um jato livre incidindo perpendicularmente sobre uma superfície rígida impermeável. O código é então aplicado na simulação de um jato planar turbulento em uma porção de fluido com superfície livre e estacionário. Os resultados numéricos obtidos são comparados com dados experimentais, soluções analíticas e soluções numéricas de outros trabalhos. / This work is devoted to the analysis and implementation of modern high-order upwind schemes and the standard k-epsilon turbulence model into the Freeflow-2D; a finite difference integrated environment for the numerical simulation of incompressible free surface flow problems. The purpose of this study is the two-dimensional simulation of high-Reynolds incompressible newtonian confined and/or free surface flows. The performance of the current Freeflow-2D code is assessed by applying it to the simulation of flow over a backward facing step and of an impinging free jet onto an impermeable rigid surface. The code is then applied to a turbulent planar jet into a pool. The numerical results are compared with experimental data, analytical solution, and numerical simulations of other works.

Microorganisms and drops are ubiquitous in nature: while drops can be found in sneezes, ink-jet printers, oceans etc, microorganisms are present in our stomach, intestine, soil, oceans etc. In most situations they are present in complex conditions: drop spreading on a rigid or soft substrate, drop covered with impurities that act as surfactants, marine microbe approaching a surfactant laden drop in density stratified oceanic waters in the event of an oil spill etc. In this thesis, we extract the physics underlying the influence of two such complicated effects (surfactant redistribution and density-stratification) on the motion of drops and swimming microorganisms when they are in isolation or in the vicinity of each other. This thesis is relevant in understanding the bioremediation of oil spill by marine microbes.<div><br></div><div>We divide this thesis into two themes. In the first theme, we analyze the motion of motile microorganisms near a surfactant-laden interface in homogeneous fluids. We begin by calculating the translational and angular velocities of a swimming microorganism outside a surfactant-laden drop by assuming the surfactant is insoluble, incompressible, and non-diffusing, as such system is relevant in the context of bioremediation of oil spill. We then study the motion of swimming microorganism lying inside a surfactant-laden drop by assuming the surfactant is insoluble, compressible, and has large surface diffusivity. This system is ideal for exploring the nonlinearities associated with the surfactant transport phenomena and is relevant in the context of targeted drug delivery systems wherein one uses synthetic swimmers to transport the drops containing drug. We then analyze the motion of a swimming organism in a liquid film covered with surfactant without making any assumptions about the surfactant and this system is relevant in the case of free-standing films containing swimming organisms as well as in the initial stages of the biofilm formation. In the second theme, we consider a density-stratified background fluid without any surfactants. In this theme, we examine separately a towed drop and a swimming microorganism, and find the drag acting on the drop, drop deformation, and the drift volume induced by the drop as well as the motility of the swimming microorganism.</div>

La clairance mucociliaire est un processus physico-chimique qui sert à transporter et éliminer le mucus bronchique. Pour cela, des milliards d'appendices de taille micrométrique, que l'on nomme cils, recouvrent l'épithélium respiratoire. Ces cils propulsent le mucus en suivant un motif périodique comprenant une phase de poussée où leur pointe peut pénétrer dans le mucus, et une phase de récupération où ils sont totalement immergés dans le fluide périciliaire. Un dysfonctionnement de ce processus peut engendrer de nombreux problèmes de santé. Il a été observé expérimentalement que les cils ne battent pas aléatoirement, mais synchronisent leurs battements avec leurs voisins, formant ainsi des ondes métachronales. Toutefois, les observations in vivo sont extrêmement difficiles à réaliser, et les propriétés de ces ondes restent mal connues. Dans cette thèse, nous utilisons un solveur Lattice Boltzmann - Frontière Immergée afin de reproduire un épithélium bronchique et étudier l'émergence, ainsi que les capacités de transports et de mélanges, de ces ondes / The mucociliary clearance process is a physico-chemical process which aims is to transport and eliminate bronchial mucus. To do so, billions of micro-sized appendages, called cilia, cover the respiratory epithelium. These cilia propel the mucus by performing a periodical pattern composed of a stroke phase where their tips can enter the mucus layer, and a recovery phase where the cilia are completely immersed in the periciliary liquid layer. A failure of this process may induce numerous health problems. It has been experimentally observed that cilia do not beat randomly, but instead adapt their beatings accordingly to their neighbours, forming metachronal waves. However, in vivo observations are extremely difficult to perfom, and the properties of these waves remain poorly understood. In this thesis, we use a Lattice Boltzmann - Immersed Boundary solver to reproduce a bronchial epithelium and study the emergence, as well as the transport and mixing capacities, of these waves

Multiphase flows abound in nature and enterprises. Our daily interactions with fluids - washing, drinking, and cooking, for example - occur at a free surface and within the realm of multiphase flows. The applications of multiphase flows within the context of emulsions, which are caused by mixing two immiscible fluids, have been of interest since the nineteenth century: compartmentalizing one fluid in another is particularly of interest in applications in pharmaceutical, materials, microfluidics, chemical, and biological engineering. Even more control in compartmentalization and delivery can be obtained through the usage of double emulsions, which are emulsions of smaller drops (i.e., inner drop) within larger drops (i.e., outer drop). The goal of this work is to understand the dynamic behavior of compound drops in confined flow at low Reynolds numbers. These behaviors include the migration patterns, limit cycles, and equilibrium locations in confined flows such as channel flows.<br> <br>Firstly, we look at non-concentric compound drops that are subject to simple shear flows. The eccentricity in the inner drop is either within the place of shear, normal to the plane of shear, or mixed. We show unreported motions that persist throughout time regardless of the initial eccentricity, given that the deformations of the inner and outer drops are small. Understanding the temporal dynamics of compound drops within the simple shear flow, one of the simplest background flows that may be imposed, allows us to probe at the dynamics of more complicated background flows.<br> <br>Secondly, we look at the lateral migration of compound drops in a Poiseuille flow. Depending on the initial condition, we show that there are multiple equilibria. We also show that the majority of initial configurations results in the compound drop with symmetry about the short wall direction. We then show the time it takes for the interfaces to merge if a given initial configuration does not reach the aforementioned symmetry.<br> <br>Thirdly, while the different equilibria of compound drops offer some positional differences at different radii ratio, we show that the lift force profiles at non-equilibrium locations offer distinctly different results for compound drops with different radii ratio. We then look at how this effect is greater than changes that arise due to viscosity ratio changes, and offer insights on what may create such a change in the lift force profile.

This work is inspired by the peculiar behavior of the natural systems, namely the ability to produce self-sustained oscillations in the level of tens of Hertz in constant ambient conditions. This feature is one of the key signatures prescribed to living organisms. The firing rate of neuronal cells, a pulsating heart, or the beating of cilia and flagella are among many biological examples that possess amazing functionalities and unprecedented intelligence solely relying on bio-electro-chemical processes. Exploring shapeable polymeric technologies, new self-oscillating artificial microsystems were developed within this thesis. These microsystems rely on the novel nonlinear architecture that exhibits a negative differential resistance (NDR) within the parametric response that enables periodic oscillations. These systems are made of polymers and metals and were microfabricated in a planar fashion. The electrochemically deposited ionic electroactive polymers act as actuators of the system. Upon the self-assembly process, due to the interlayer strains, the planar device transforms into a three-dimensional soft nonlinear system that is able to perform self-sustained relaxation oscillations when subjected to a constant electric field while consuming extremely low powers (as low as several microwatts). The parameters of these systems were tuned for a high oscillation amplitude and frequency. This electro-mechanical parametric relaxation oscillator (EMPRO) can generate a rhythmic motion at stroke frequencies that are biologically relevant reaching up to ~95 Hz. The EMPRO oscillations at high frequencies generate a flow in the surrounding liquid, which was observed in the form of vortices around the micro actuators. This flow was further studied in ex-vivo conditions by measuring Doppler shifts of ultrasound waves. The EMPRO was made autonomous by integrating an electrochemical voltaic cell. Four different electrochemical batteries were tested to match the power consumption of the EMPRO system and electrochemical compatibility of the surrounding media. An Ag-Mg primary cell was then integrated with the EMPRO for autonomous operation without the need for external power sources, cables or controllers. This biomimicking self-powered self-sustaining oscillating microsystem is envisioned to be useful in novel application scenarios operating at low Reynolds numbers in biologically relevant conditions. Furthermore, as the system is electromechanical in nature, it could be integrated with electronic components such as sensors and communication devices in the next generation of autonomous microsystems.:  Table of contents Acronyms 7 1 Introduction 8 1.1 Motivation 9 1.2 Objectives 9 1.3 Thesis organization 10 2 Background 12 2.1 A brief review on nonlinear self-oscillation 12 2.2 Self-oscillating biological systems 13 2.3 Stimuli responsive materials 15 2.3.1 Electroactive polymers in electrochemical cells 16 2.3.2 Sources of electrical field for electroactive polymers 24 2.4 Self-oscillating synthetic systems 27 2.5 Movement in low Reynolds number regime 33 3 Materials and methods 38 3.1 Deposition methods 38 3.1.1 Photolithography 38 3.1.2 Plasma sputtering 41 3.1.3 Atomic layer deposition 42 3.1.4 Electrochemical polymerization 44 3.2 Shapeable polymeric platform technology 46 3.2.1 Sacrificial layer 46 3.2.2 Hydrogel swelling layer 47 3.2.3 Polyimide reinforcing layer 48 3.3 Characterization methods 49 3.3.1 Profilometry 49 3.3.2 Scanning electron and focused ion beam microscopy 50 3.3.3 Cyclic Voltammetry 52 3.3.4 Ultrasound and Doppler shift measurements 53 4 Electromechanical Parametric Relaxation Oscillators (EMPROs) 56 4.1 Relaxation oscillation in EMPROs 56 4.2 Theory of EMPRO relaxation oscillations 61 4.3 Realization of EMPROs 67 4.3.1 Design parameters of EMPROs 67 4.3.2 EMPRO on-chip battery integration 71 4.4 Fabrication of autonomous EMPROs 76 5 EMPRO performances 84 5.1 Externally biased EMPROs 84 5.2 Autonomous EMPROs 95 6 Conclusions and outlook 98 6.1 Outlook 99 Bibliography i List of Figures and Tables xi Versicherung xiii Acknowledgements xiv Scientific publications and contributions xvi Theses xvii Curriculum Vitae xix