Polydimethylsiloxane (PDMS) microfluidic channels, fabricated using low cost and simple soft lithography methods, conventionally have rectangular cross-sections. Despite being often used for organs-on-a-chip and cardiovascular research, these devices do not mimic the circular cross-sections of blood vessels in the human body, creating potential inaccuracies in observed flow conditions and cell behaviours. The purpose of this thesis is to (i) compare and optimize fabrication techniques for microchannels with circular cross-sections, (ii) assess biocompatibility of different surface functionalization approaches for Human Umbilical Vein Endothelial Cell (HUVEC) adhesion and growth, (iii) culture HUVECs within circular microchannels to mimic blood vessel features, and (iv) compare gene expression of HUVECs cultured in 3D circular microchannels to those cultured on 2D surfaces. We show that wire molding is superior to the gas stream technique for producing circular cross-section microchannels with high aspect ratios, circularity, and channel geometry precision. Fibronectin (FN) and polydopamine (PD) surface coatings on PDMS, as well as alternative collagen substrates, were tested for biocompatibility with HUVECs in 2D cultures; fibronectin coated PDMS (PDMS-FN) substrates facilitated cell attachment, spreading and growth. We demonstrate the capability of growing HUVECs on the inner surface of circular PDMS microchannels created using the wire-mold method and treated with fibronectin. A syringe pump was used to induce shear stress on the HUVECs grown in circular microchannels. Relative to static growth conditions, longer cell culture growth periods were more feasible under flow and altered cell morphology was observed. Finally, Microarray analysis revealed significantly different gene expression profiles for HUVECs cultured within PDMS-FN circular cross-section microchannels as compared to HUVECs cultured on PDMS-FN in a 2D environment, thereby highlighting the critical importance of in vitro conditions for mimicking the in vivo reality.

Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto. / The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

Master’s thesis deals with the optimalization of low-temperature cofired ceramic lamination process that affects the final quality of 3D structures (eg., channels, cavities, membranes, etc.). The paper evaluates the influence of lamination parameters (pressure, temperature, time) on the formation of defects in the structure, which were detected by appropriate methods. The last parts describes designed and simulation cooling water system using LTCC technology.

Microchannels and micro pin fins have been employed for almost four decades in the cooling of microelectronic devices and research is still being done in this field to improve the thermal performance of these micro heat sinks. In this research, the constructal design and computational fluid dynamics code was used with a goal-driven optimisation tool to numerically investigate the thermal performance of a novel design of combining microchannels and micro pin fins for microelectronic cooling applications. Existing designs of microchannels were first optimised and thereafter, three to seven rows of micro pin fins were inserted into the microchannels to investigate whether there was further improvement in thermal performance. The microchannels and micro pin fins were both embedded in a highly conductive solid substrate. three-dimensional geometric structure of the combined micro heat sink was optimised to achieve the objective of maximised thermal conductance, which is also minimised thermal resistance under various design conditions. The micro heat sinks investigated in the study were the single microchannel, two-layered microchannels with parallel and counter flow configurations, three-layered microchannels with parallel and counter flow configurations, the single microchannel with circular-, square- and hexagonal-shaped micro pin-fin inserts and the two-layered microchannels with circular-shaped micro pin-fin inserts. A numerical computational fluid dynamics (CFD) package with a goal-driven optimisation tool, which employs the finite-volume method, was used to analyse the fluid flow and heat transfer in the micro heat sinks investigated in this work. The thermal performances of all the micro heat sinks were compared for different application scenarios. Furthermore, the temperature variation on the heated base of the solid substrate was studied for the different micro heat sinks to investigate which of the heat sink designs minimised the temperature rise on the heated base best. This is very important in microelectronic cooling applications because temperature rise affects the reliability of the device. The heat sink design that best maximised thermal conductance and minimised temperature rise on the heated base was chosen as the best for microelectronic cooling. For all the cases considered, fixed volume constraints and manufacturing constraints were applied to ensure real-life applicability. It was concluded that optimal heat sink design for different application scenarios could be obtained speedily when a CFD package which had an optimisation tool was used. / Thesis (PhD)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / PhD / Unrestricted

What role do high and low wall shear stresses play in the deterioration of arteriole and capillary walls? Plaque buildup is common around bifurcations in arterioles, indicating that low wall shear stress may play a role in the weakening of the walls. This thesis investigates the creation of blood analog fluid used in a Polydimethylsiloxane (PDMS) curved channel to explore the fluid properties and characteristics near bifurcations. Major results in the experiments showed the viscosity and surface tension trends of a blood analog fluid composed of xanthan gum, glycerin, and distilled water with the addition of Silver Coated Hollow Glass Spheres in varying volume fractions. All experiments were conducted at room temperature with varying flow rates between 0.1-2 µL/second. The velocity profile was characterized at each flow rate. Important results that will be discussed will include the variation of flow near bifurcations and at different flow rates and RBC concentration. Full parabolic velocity profiles formed in the straight region of the channels as expected. After the bifurcation, the velocity profile was skewed to the outer wall. At lower flow rates there were fewer particles flowing near the wall of the channel.

No description available.

This work describes the results of experimental study of the flow of soft objects (microgels) through microchannels. This work was carried with the intention of building a fundamental biophysical model for the flow of neutrophil cells in microcirculatory system. In Chapter 1 we give a summary of the literature describing the flow of cells and “model cells” in microchannels. Paramount to this we developed methods to modify microchannels fabricated in poly(dimethyl siloxane) (PDMS). Originally, these microchannels could not be used to mimic biological microenvironments because they are hydrophobic and have rectangular cross-sections. We designed a method to create durable protein coatings in PDMS microchannels, as outlined in Chapter 3. Surface modification of the channels was accomplished by a two-step approach which included (i) the site-specific photografting of a layer of poly(acrylamide) (PAAm) to the PDMS surface and (ii) the bioconjugation of PAAm with the desired protein. This method is compatible with different channel geometries and it exhibits excellent longevity under shear stresses up to 1 dyn/cm. The modification was proven to be successful for various proteins of various molecular weights and does not affect protein activity. The microchannels were further modified by modifying the cross-sections in order to replicate cardiovascular flow conditions. In our work, we transformed the rectangular cross-sections into circular corss-sections. Microchannels were modified by polymerizing a liquid silicone oligomer around a gas stream coaxially introduced into the channel, as outlined in Chapter 3. We demonstrated the ability to control the diameter of circular cross-sections of microchannels. The flow behaviour of microgels in microchannels was studied in a series of experiments aimed at studying microgel flow (i) under electrostatic interactions (Chapter 4), (ii) binding of proteins attached to the microgel and the microchannel (Chapter 5) and (iii) under the conditions of varying channel geometry (Chapter 6). This work overall present’s new methods to study the flow of soft objects such as cells, in the confined geometries of microchannels. Using these methods, variables can be independently probed and analyzed.

Dissipadores de calor baseados em microcanais são apresentados como solução para a remoção de fluxos de calor elevados em espaços restritos, pois proporcionam elevados coeficientes de transferência de calor quando comparados a canais convencionais. Tais trocadores também proporcionam elevadas razões entre a área superficial em contato com o refrigerante por unidade de volume do dissipador. Além dos microcanais, a utilização de nanofluidos também se apresenta como tecnologia com potencial de incremento do coeficiente de transferência de calor. Os nanofluidos consistem na adição de nanopartículas a um fluido base visando alterar suas propriedades de transporte termodinâmicas. Neste contexto, o objetivo do presente estudo é avaliar o coeficiente de transferência de calor para escoamentos monofásicos e ebulição convectiva de nanofluidos aquosos no interior de microcanais. Para isto, foram realizados experimentos em canais com diâmetro de 1,1 mm e comprimento de 200 mm para água deionizada, nanofluidos de alumina com diâmetros de 20-30 e 40-80 nm, nanofluidos de dióxido de silício com diâmetros de 15 e 80 nm, e nanofluidos de cobre com diâmetro de 25 nm. Estas soluções foram ensaiadas para concentrações volumétricas de nanopartículas de 0,001, 0,01 e 0,1, velocidades mássicas de 200, 400 e 600 kg/m2s e fluxos de calor de 20 a 350 kW/m2. A análise dos resultados revelou que a adição de nanopartículas a água deionizada proporciona o incremento do número de Nusselt para escoamentos monofásicos, principalmente na região inicial do tubo. Concluiu-se que os efeitos da adição de nanopartículas a um fluido base no coeficiente de transferência de calor durante a ebulição convectiva estão relacionados ao recobrimento da superfície com uma camada porosa. A deposição de nanopartículas com diâmetro inferior a 30 nm resultou na redução do coeficiente de transferência de calor e das instabilidades térmicas do escoamento em relação a água deionizada. O coeficiente de transferência de calor e as instabilidades térmicas não apresentaram variações significativas da deposição de nanopartículas com diâmetro superior a 40 nm. Por meio da análise da textura das superfícies recobertas e do critério de nucleação proposto por Kandlikar et al. (1997) concluiu-se que tal comportamento encontra-se associado aos efeitos do acabamento superficial na densidade de cavidades de nucleação ativas. / Microchannels based heat exchangers were introduced as a solution to high heat flux removal in restrict spaces due to their high heat transfer coefficients compared to heat exchangers based on conventional channels. The high ratio of surface are per volume is an additional advantage to microchannels in relation to conventional channels. Beside the microchannels technology, the nanofluids also present itself as a technique with potential to increase the heat transfer coefficient. Nanofluids consist of a solution containing nanoparticles dispersed in a base fluid with the goal to improve its thermodynamic and transport properties. In this context, the objective of the present study is to evaluate the heat transfer coefficient for single-phase flow and convective boiling of aqueous nanofluids inside microchannels. Experiments were performed for channels with internal diameter of 1.1mm and 200 mm long for DI-water, nanofluids containing alumina- (nanoparticles diameters of 20-30 and 40-80 nm), silicon dioxide (nanoparticles diameters of 15 and 80 nm), and copper (nanoparticles diameter of 25 nm). These solutions were evaluated for volumetric concentrations of 0.001, 0.01 and 0.1%, mass velocities of 200, 400 and 600 kg/m2s and heat fluxes from 20 to 350 kW/m2. The analysis of the results revealed that the addition of nanoparticles to DI-water causes an increment in the Nusselt number for single phase flows, especially at the inlet of the tube. The results for flow boiling indicated that the effects of adding nanoparticles to the base fluid are related to the deposition on the heating surface of a nanoparticles porous layer due to the boiling process. The deposition of nanoparticles smaller than 30 nm promoted a reduction of the heat transfer coefficient compared to DI-water on a clean surface, and thermal instabilities were minimized. For the deposition of nanoparticles larger than 40 nm these parameters did not presented significant variations in comparison to DI-water. A combined analysis of the surfaces finishing and the criterion of Kandlikar et al. (1997) for bubble nucleation revealed that such behaviors are correlated to the effects of the surface texture associated to the boiling process on the density of active nucleation cavities.

Betalaínas são alcalóides coloridos que atuam como mediadores químicos e sensores em processos de interesse tecnológico. Esta Tese de Doutorado investiga o comportamento de betalaínas naturais, betanina e indicaxantina, em três sistemas químicos diferentes. O primeiro capítulo discute a estabilidade de emulsões água-óleo-água contendo betanina. As emulsões foram preparadas em um sistema de microcanais empregando-se óleo de soja e tensoativos grau alimentício. Betanina pura tende a estabilizar a emulsão em relação ao controle sem o pigmento, enquanto suco de beterraba liofilizado e betanina comercial diluída com maltodextrina não mostram a mesma atividade. No capítulo seguinte, a interação entre indicaxantina e cloreto de 7-metil-4-metóxiflavílio revela a ocorrência de transferência de energia e dados de espectroscopia de 1H-RMN indicam a formação de um complexo no estado fundamental. Finalmente, a interação entre betanina e paraquat na presença e ausência de luz e oxigênio é investigada. Dados de espectroscopia de 1H-RMN sugerem a formação de um complexo entre as espécies e o estudo cinético sugere que betanina interfere na regeneração da forma oxidada do paraquat por oxigênio. / Betalains are colored alkaloids that act as chemical mediators and sensors in processes of technological interest. This doctoral thesis investigates the behavior of natural betalains, betanin and indicaxanthin, in three different chemical systems. The first chapter discusses the stability of water-oil-water emulsions containing betanin. The emulsions were prepared in a microchannel system using soybean oil and food grade surfactants. Pure betanin tends to stabilize the emulsion relative to the control without the pigment, while freeze-dried beet juice and commercial betanin diluted with dextrin do not show the same activity. In the following chapter, the interaction between indicaxanthin and 7-methyl-4-methoxyflavylium chloride reveals the occurrence of energy transfer and 1H-NMR spectroscopy data suggests the formation of a complex in the ground state. Finally, the interaction between betanin and paraquat in the presence and absence of light and oxygen is investigated. 1H-NMR spectroscopy data suggest the formation of a complex between species and the kinetic study suggests that betanin interferes in the regeneration of the oxidized form of paraquat by oxygen

Betalaínas são alcalóides coloridos que atuam como mediadores químicos e sensores em processos de interesse tecnológico. Esta Tese de Doutorado investiga o comportamento de betalaínas naturais, betanina e indicaxantina, em três sistemas químicos diferentes. O primeiro capítulo discute a estabilidade de emulsões água-óleo-água contendo betanina. As emulsões foram preparadas em um sistema de microcanais empregando-se óleo de soja e tensoativos grau alimentício. Betanina pura tende a estabilizar a emulsão em relação ao controle sem o pigmento, enquanto suco de beterraba liofilizado e betanina comercial diluída com maltodextrina não mostram a mesma atividade. No capítulo seguinte, a interação entre indicaxantina e cloreto de 7-metil-4-metóxiflavílio revela a ocorrência de transferência de energia e dados de espectroscopia de 1H-RMN indicam a formação de um complexo no estado fundamental. Finalmente, a interação entre betanina e paraquat na presença e ausência de luz e oxigênio é investigada. Dados de espectroscopia de 1H-RMN sugerem a formação de um complexo entre as espécies e o estudo cinético sugere que betanina interfere na regeneração da forma oxidada do paraquat por oxigênio. / Betalains are colored alkaloids that act as chemical mediators and sensors in processes of technological interest. This doctoral thesis investigates the behavior of natural betalains, betanin and indicaxanthin, in three different chemical systems. The first chapter discusses the stability of water-oil-water emulsions containing betanin. The emulsions were prepared in a microchannel system using soybean oil and food grade surfactants. Pure betanin tends to stabilize the emulsion relative to the control without the pigment, while freeze-dried beet juice and commercial betanin diluted with dextrin do not show the same activity. In the following chapter, the interaction between indicaxanthin and 7-methyl-4-methoxyflavylium chloride reveals the occurrence of energy transfer and 1H-NMR spectroscopy data suggests the formation of a complex in the ground state. Finally, the interaction between betanin and paraquat in the presence and absence of light and oxygen is investigated. 1H-NMR spectroscopy data suggest the formation of a complex between species and the kinetic study suggests that betanin interferes in the regeneration of the oxidized form of paraquat by oxygen