Self-heating can have a detrimental effect on the performance and reliability of high power microwave devices. In this work, the thermal performance of the gallium arsenide (GaAs) Gunn diode was studied. Infrared (IR) thermal microscopy was used to measure the peak operating temperature of the graded-gap structured device. Temperature measurements were experimentally validated using micro-thermocouple probing and compared to values obtained from a standard 1D thermal resistance model. Thermal analysis of the conventionally structured Gunn diode was also undertaken using high resolution micro-Raman temperature profiling, IR thermal microscopy and electro/thermal finite element modeling. The accuracy of conventional IR temperature measurements, made on semiconductor devices, was investigated in detail. Significant temperature errors were shown to occur in IR temperature measurements made on IR transparent semiconductors layers and low emissivity/highly reflective metals. A new technique, employing spherical carbon microparticles, was developed to improve the measurement accuracy on such surfaces. The new ‘IR microparticle’ technique can be used with existing IR microscopes and potentially removes the need to coat a device with a high emissivity layer, which causes damage and heat spreading.

This study proposed a novel design of the passive micromixer which employed several quadrilateral shaped blocks in the micro channel to enhance mixing. Both numerical and experimental investigations have been carry out. Commercial software CFD-ACE was used to simulate the flows. The simulation results showed great agreement with the measured results, implying that Navier¡VStokes¡¦ equations still effectively governs the micro-scope flows in this scale. It is effective to enhance mixing efficiency over wide flow rate ranges. Mixing performance was characterized by Laser-induced-fluorescence system (LIF system) to quantity the concentration distribution in the micro channel . In addition, Microscopic flow visualization was also setup to visualize the flow field in the micro mixer. Micro-particle image velocimetry (Micro-PIV) was used to measure the flow fields in microchannel filled with deionized water (DI water) . The system utilizes an epifluorescent microscope, 3.3 £gm diameter seed particles, and an high speed CCD camera to record particle-image fields. The vector fields are analyzed using a double-frame cross-correlation algorithm. The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements.

No description available.

Red blood cells (RBCs) form approximately 40 to 45% of the human blood volume, and their behaviour and characteristics are the main determinant of blood properties, such as viscosity. RBCs are deformable species and stack together under low shear rate to form aggregates or rouleaux. Flowing RBCs migrate away from the wall leaving a cell-depleted layer known as the cell-free layer (CFL). This layer contributes to the blood viscosity and exchange between the RBCs and the target cells: a thinner CFL enhances the exchange process by reducing the diffusion distance. The formation of this CFL, however, is not yet completely understood. The goal of this study is to improve the understanding of the formation of the CFL in the micro-flow. This was accomplished by studying the effects of changing both the flow rate and the microchannel geometry on blood flow in microchannels. In this work, 10% hematocrit human blood suspensions were prepared in native plasma and flowed through poly-dimethylsiloxane (PDMS) microchannels of 100 μm x 34 μm cross-section. Investigation of the flowing cells was performed by using micro particle image velocimetry (μPIV) coupled with a high-speed camera. First, the high-speed camera images were processed with customized Matlab programs to detect and measure the CFL thickness and the RBC aggregates sizes. Second, the blood flow velocity profiles were measured using μPIV in order to determine the actual flow rate, the RBCs’ centerline velocity, and the shear rate. The results showed that the increase in both flow rate and shear rate significantly reduced the CFL thickness and RBC aggregates size. Comparison of the upstream and downstream measurements in the bifurcating microchannel showed that the change in microchannel geometry did not significantly influence CFL thickness and RBC aggregate size, while within the daughter branches, RBCs tended to flow close to the inner wall resulting in an undetectable CFL at the inner wall and in a larger CFL at the outer wall of the branch. These in vitro results quantitatively relate CFL thickness and RBC aggregate size at different shear rates. The findings are of immediate interest regarding the understanding of microcirculation and improved designs of microchips.

Micro particle image velocimetry (microPIV) is a non-intrusive tool for visualizing flow in micron-scale conduits. Using this investigative instrument, two experimental studies were performed to understand flow behaviors in microfluidic channels - a sudden expansion step flow and laminar velocity profile variation in diffusion driven mixing. First, flow in a backward facing step feature (1:5 expansion ratio) in a microchannel was taken as the subject of microPIV flow visualization. The onset and development of a recirculation flow was studied as a function of flow rate. This flow pattern was further used to investigate two major parameters affecting microPIV measurements; the depth-of-focus and recording time-intervals between images in a microPIV image pair. The onset of recirculation was initiated at flow rates that correspond to Reynolds numbers, Re > 95, which is well beyond the typical working range of microfluidic devices (Re=0.01-10). The recirculation flow has a 3D structure due to the dimensions of the microchannel and the effect of no slip condition on the walls. Ensemble cross-correlation was found not to be sensitive to variations of depth-of-focus and the output flow fields were similar as long as the overall optical focus remained within the upper and lower bounds of the microchannel. However, variations of time intervals between images in a microPIV pair, resulted in quantitatively and qualitatively different flow patterns for a given constant flow rate and depth-of-focus. In the second experiment, the effect of the laminar velocity profile and its variation on mixing phenomena at the reduced scale is studied. It is shown that the diffusive mass flux between two miscible streams, flowing in a laminar regime in a microchannel, is enhanced if the velocity at their diffusion interface is increased. Based on this idea, an in-plane passive micromixing concept is proposed and implemented in a working device (sigma micromixer). This mixer shows considerable mixing performance by periodically varying the flow velocity profile, such that the maximum of the profile coincides with the transversely progressing diffusion fronts repeatedly throughout the mixing channel. microPIV has been used to visualize the behavior of laminar flow inside the micromixer device and to confirm the periodic variation of the velocity profile through the mixing channel.

This thesis presents models and simulations of dielectrophoretic separation of micro and nano particles. The fluid dynamics involved and the dielectric properties of water inside single-walled carbon nanotube are studied as well. Based on the effective dipole moment method, the particle dynamic model focuses on the translational motions of micro particles. The hydrodynamic force between the particles and the particle-particle electrostatic interactions are considered as well. By comparing the dimensionless parameters, the dominating force can be determined. Based on a simplified version of the particle dynamic model, two numerical simulations are carried out to predict the efficiency of dielectrophoretic separation of micro size particles. The first calculation suggests a strategy to improve the trapping efficiency of E.coli bacteria by applying superimposed AC electric fields. The second calculation discusses the concept of mobility and improves the separation rate of particles by a multi-step trapping-releasing dielectrophoresis strategy. The model is extended down scale to calculate the separation of metallic and semiconducting single-walled carbon nanotubes by the modified effective dipole moment method for prolate ellipsoids. The steeply changed gradient of electric field results in the local joule heating therefore creates gradient of dielectric properties in the solution. As a result, certain pattern of fluid flow with a considerable strength is created and affects the motion of carbon nanotubes especially close to the electrode gap, which indicates that the so-called electrothermal flow should be considered in designing the experiment to separate single-walled carbon canotubes. When the length scale of particles is comparable to that of the electrodes, the calculation of dielectrophoretic force by the effective dipole moment is considered not to be accurate since only the electric field in the center point is taken into account. Hence in the thesis a new method based on distributed induced charge is suggested. By approximating a straight slender body as a prolate ellipsoid, the electric field of multiple points along the centerline are all considered in the calculation and the interaction between particles could be concurrently taken care. This method is expected to be an improved method to calculate the dielectrophoretic force of rod-like virus, DNA, nanowires and carbon nanotubes. The dielectric property of water confined in carbon nanotubes is expected to be dramatically different from that of bulk water. The thesis also contains a molecular dynamics study to reveal the difference also a dependence on the diameter of carbon nanotubes. The results show that along the axial direction, both the static permittivity and the relaxation time are larger than the isotropic bulk water, and in the cross-section plane it is opposite. When the radius of the carbon nanotubes increases, the properties of water inside become closer to the bulk water. / QC 20100820

L’imperméabilisation des surfaces et l’augmentation des débits de pointe sont liés au développement croissant des environnements urbains. Avec cette croissance urbaine, les processus de ruissellement sont largement affectés et représentent un secteur dans lequel des actions doivent être entreprises afin d’assurer à la fois le développement économique et la protection de l’environnement et des ressources en eau. Le ruissellement est fréquemment associé à des pollutions diffuses (Non Point Source) qui ne sont généralement pas prises en compte dans les dispositifs classiques de régulation hydraulique. L’objectif de cette recherche était d’évaluer les performance de dispositifs de filtration sur les micro particules polluantes diffuses issues des chaussées routières et des déversoirs d’orages des réseaux d’assainissement unitaires (Combined Sewer Overflows – CSOs). Le protocole expérimental implique de mesurer, dans un premier temps, la taille et les caractéristiques des particules polluantes diffuses et ensuite, de réaliser une procédure identique à la sortie des installations de filtration et de traitement. Les modifications dans la distribution de la taille des particules et de leurs caractéristiques sont alors analysées. Les quatre principaux dispositifs de filtration disponibles - Expanded Polypropylene (EPP), Expanded polystyrene (EPS), Perlite pour les particules flottantes et Zeolite pour les non flottantes – ont été implanté sur un équipement de test et les résultats comparés. Une modélisation 3D (CFD) a été réalisée afin d’optimiser la conception de la disposition de filtration. / The impermeable area and the peak discharge volume have been consistently increasing due to widening of city development and renewal. Thus, distortion of water cycle has been serious problem compared to the past. When it comes to NPS pollution, lack of information on the load and characteristics of pollutants led to the insufficient reduction measures. The purpose of this study is to investigate the effectiveness of filters to control micro particles from non-point sources, especially from road runoff and Combined Sewer Overflows (CSOs). The study first examined the particle size and characteristics of NPS pollutants, and then analyzed the head loss, treatment efficiency and changes in particle size distribution before and after the treatment using device type facilities. The four most commonly used filters - Expanded Polypropylene (EPP), Expanded polystyrene (EPS), and Perlite as floatability filtering materials, and Zeolite as non-floatability - were compared. The upward-type filtering equipment was manufactured for the lab experiment, and CFD modeling was conducted to identify optimized design parameters. The results showed that the ratio of micro particles(＜100 ㎛), to which the filtering process can be applied, was highest in samples from the road runoff, followed by one from the bridge, and from the parking lot. The results using road runoff displayed that EPP was most efficient with ratio of 89% to treat SS and COD.

<p dir="ltr">Particle tracking velocimetry, also known as PTV, is a technology to measure velocity and study the flow field in fluid by observing change in position of individual tracer particles over time. A laser sheet illuminates a thin layer of the sample, in which particles emit fluorescent light and are visible to the camera. Particles at different distances from the microscope lens focal plane are visible, because particle diameter is much smaller than the thickness of laser sheet in micro-scale. The defocused distance changes the shape of particle seen by the camera. Analyzing particle shapes and obtaining the defocused distance of particles completes the third dimension of PTV with the use of a single camera. One approach to obtain defocused distance from particle shape is by comparing particle shapes with calibration images of known defocused distance. The accuracy of PTV relies on the collection of proper calibration images. There are three methods involved in this work. The first approach is to use synthetic images generated by solving Lommel differential equations, which describe the intensity distribution of particles under the impact of defocusing aberration. It was later discovered that the point source assumption inherent in Lommel function causes inaccuracy in generated calibration images. The second approach captures particle images while manually shifting the microscope stage in the z-direction. This approach causes systematic error by ignoring the refractive index of the immersion medium. The third approach is to use a microscale reference ramp as calibration target. Results are experimentally compared with particle shapes obtained from pressure driven flow with known velocity profile.</p>

[pt] Alguns projetos pilotos de injeção de polímeros em campos de produção de petróleo mostraram um incremento na recuperação de óleo, embora os mecanismos que governam a dinâmica do escoamento não são bem compreendidos. Recentes investigações experimentais mostraram que as propriedades viscoelásticas de soluções poliméricas podem alterar o comportamento do escoamento na escala de poros e reduzir a saturação residual de óleo. Para entender esses fenômenos em meios porosos, é importante estudar o escoamento de soluções viscoelásticas através das gargantas dos poros. Este trabalho apresenta um estudo experimental do escoamento de uma solução viscoelástica de PEO (0,1 porcento em peso de óxido de polietileno) de alto peso molecular escoando através de uma constrição, utilizado como modelo de uma geometria de garganta de poro de um meio poroso. Medições de queda de pressão e campos de velocidade do escoamento são obtidos utilizando a técnica de velocimetria por imagem de partículas (Micro-PIV). Experimentos com uma solução viscosa de glicerina (45 porcento em peso de glicerina em água) de viscosidade similar à solução de PEO foram também realizados com a finalidade de estimar os efeitos elásticos da solução de PEO. O escoamento da solução de PEO exibiu uma queda de pressão extra (comportamento não linear) acima de uma condição crítica do escoamento, acima da qual os efeitos elásticos passam a ser preponderantes. Para toda a faixa de vazão explorada, os campos de velocidade da solução de glicerina mostraram um regime de escoamento Newtoniano, enquanto a solução de PEO apresenta instabilidade no escoamento a partir de um número de Weissenberg crítico, coincidindo com o aumento da queda de pressão. Esta instabilidade pode ser relacionada ao aumento da viscosidade extensional na entrada da garganta acima de uma determinada taxa de extensão. Os resultados obtidos indicam a variação do padrão do escoamento da solução polimérica de PEO devido à presença dos efeitos elásticos do polímero, e fornecem informações importantes sobre o comportamento das soluções poliméricas viscoelásticas em um meio poroso e que podem impactar sua utilização na recuperação melhorada de óleo. / [en] Some pilot projects of polymer injection in oil fiel ds have shown an increase in oil recovery, although the mechanisms that govern the flow dynamics are still not well understood. Recent experimental investigations have shown that the viscoelastic properties of polymer solutions may change the pore-scale flow behavior and reduce the residual oil saturation. To understand these phenomena in porous media, it is important to understand viscoelastic flow behavior through the pores-throats. This work presents experimental study of the flow of a high molecular viscoelastic PEO solution (0.1 wt percent Polyethylene Oxide) flowing through a constricted capillary, used as model for a pore-throat geometry of a porous media. Pressure drop measurements are performed and velocity fields are obtained using the micro-particle image velocimetry (Micro-PIV) technique. Experiments with a viscous solution of glycerin (45 wt percent glycerin in water), of similar shear viscosity to the PEO solution were also performed in order to isolate the elastic effects of the PEO solution. The flow of the PEO solution exhibited an extra pressure drop (nonlinear behavior) above a critical flow condition beyond which the elastic forces become relevant. For the entire flow rate range explored, the velocity field of the glycerin solution showed a Newtonian flow regime, while the PEO solution shows instability in the flow above a critical Weissenberg number, coinciding with the onset of the extra pressure drop. This instability in the flow is associated with the high extensional viscosity near the constriction at high enough extension rates. The results show the changes in the flow pattern of the PEO polymer solution due to the presence of the elastic effects of polymer, and provide important information on how viscoelastic polymer solutions behave in a porous media and can impact their use in Enhanced Oil Recovery operations.

The condition of no relative velocity of fluid past solid is termed as ‘no-slip boundary condition’. This condition is a general observation in fluid mechanics. However, several research groups have recently reported slip of water for surfaces with water repelling chemistry (referred to as hydrophobic surfaces). The effect has been attributed to disruption of H-bonding network of water molecules at such surfaces and resulting nucleation of dissolved gases and even reduced water density locally in absence of dissolved air. Slip of water on hydrophobic surfaces has been demonstrated to get amplified by high degree of roughness and patterning. Trapping of air in the surface asperities has been cited as the possible reason. The present work focuses on the study of effect of surface chemistry and roughness on flow behavior close to solid surfaces. Superhydrophobic surfaces have been generated by novel methods and wet-etching has been used to generate well-defined patterns on silicon surfaces. For flow characterisation, a micrometre resolution Particle Image Velocimetry (micro-PIV) facility has been developed and flow measurements have been carried out with a spatial resolution of less than 4 µm. It has been found from the experiments that flow of water on smooth surfaces, with or without chemical modification, conforms to the no-slip within the resolution limits of experiments. Deviation is observed in case of rough and patterned hydrophobic surfaces, possibly because of trapped air in asperities. Total Internal Reflection experiments, used to visualise the air pockets, confirmed the trapping of air at asperities. Diffusion of air out of the crevices seems to be the limiting factor for the utility of these surfaces in under-water applications.