Academic literature on the topic 'Microchannels'

The ability to control the motion of particles and cells in microchannels has been a center of fascination since the advent of microfluidics. Entire fields have been created in order to accomplish separation, volume reduction and overall positioning of particles and cells within microfluidic devices in the fastest and most accurate manner possible. While most of these technologies rely on low Reynolds number operation, one technique entitled inertial focusing takes advantage of the inertia of the surrounding fluid and the interaction between a particle and the channel itself which cause the lateral migration of particles across streamlines to equilibrium positions within a flow. The major advantage of inertial microfluidics in biomedical and microfluidic applications is that it is inherently high throughput being dependent on inertia whereas most microfluidic concepts are dependent on low Reynolds number operation.<br>Engineering and Applied Sciences

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.

Heat transfer and flow characteristics of phase change material slurry flow in microchannels with constant heat flux at the base were investigated. The phase change process was included in the energy equation using the effective specific heat method. A parametric study was conducted numerically by varying the base fluid type, particle concentration, particle size, channel dimensions, inlet temperature, base heat flux and melting range of PCM. The particle distribution inside the microchannels was simulated using the diffusive flux model and its effect on the overall thermal performance of microchannels was investigated. Experimental investigation was conducted in microchannels of 101 [micro]m width and 533 [micro]m height with water as base fluid and n-Octadecane as PCM to validate the key conclusions of the numerical model. Since the flow is not fully developed in case of microchannels (specifically manifold microchannels, which are the key focus of the present study), thermal performance is not as obtained in conventional channels where the length of the channel is large (compared to length of microchannels). It was found that the thermal conductivity of the base fluid plays an important role in determining the thermal performance of slurry. The effect of particle distribution can be neglected in the numerical model under some cases. The performance of slurry depends on the heat flux, purity of PCM, inlet temperature of the fluid, and base fluid thermal conductivity. Hence, there is an application dependent optimum condition of these parameters that is required to obtain the maximum thermal performance of PCM slurry flows in microchannels.<br>Ph.D.<br>Department of Mechanical, Materials and Aerospace Engineering;<br>Engineering and Computer Science<br>Mechanical Engineering PhD

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 119-123).<br>The main body of this work considers the design and development of a microfluidic, continuous electrochemical sensor capable of measuring accurate potential differences. The key challenge in creating such a device is the implementation of a miniaturized reference electrode and salt bridge. The purpose of a salt bridge is to allow ionic conduction between the reference and working electrodes while maintaining a physical separation between the two systems. Macro reference electrode and salt bridge techniques are difficult to implement on a micro scale. Instead of attempting to conform one of these techniques to function in a micro system, new methods were developed that take advantage of the conditions in a continuous microfluidic device. In particular, laminar flow and slow relative diffusion times allow for a reference electrode that does not require a physical salt bridge. Ionic conduction is maintained between neighboring reference and analyte streams while slow mixing effectively separates the two systems. Several different device designs were investigated focusing on the prevention of reference electrode contamination. If the reference electrode is chemically contaminated it will no longer behave as expected and can not be used as a reference point. Contamination at the reference electrode was evaluated while varying flow rates and the geometry of the microfluidic device. Mathematical models were simulated in order to understand the mass transport in each device design. Based on these simulations, dimensionless groups were found that defined the dominant physics in each system. These dimensionless numbers were then validated experimentally and numerically over a range of device parameters. Subsequently, operation criteria were developed to ensure that the reference electrode remains stable and uncontaminated. By creating a stable reference electrode on chip, any homogeneous electrochemical system that was previously studied on the macro scale can now be studied continuously in a microfluidic device. A secondary portion of this work investigates the role of surface charge with respect to electrodynamics in a microchannel. As the surface area to volume ratio increases, the concentration of charge at a channel wall may begin to approach the electrolyte concentration in the bulk solution. This phenomenon is studied numerically, with and without convection, in particular as it relates to a possible mechanism for overlimiting current. Additionally, a potential de-ionization device is theorized based on this mechanism along with scaling arguments that can be used to aid device design.<br>by EthelMae Victoria Dydek.<br>Ph.D.

Many technological and biomedical applications ranging from water filtration and oil extraction to arteriosclerosis and vein thrombosis rely upon the transport of solids in liquids. Particulate matter suspended in liquid flowing through channels that are often microscopic or millimeters in size which leads to clogging. This dissertation examines the clogging behavior of microscopic channels by microscopic particles suspended in liquid. We physically model clogging in microchannels by flowing microparticles through microfluidic channels. Unlike previous studies, we choose non-uniform microchannels; specifically, we study clogging in microchannels whose width narrows over the length of the channel. Converging channels are inspired by the pore size variations in real porous media like membrane filters and sandstone. Initially we study the clogging behavior of microparticles in arrays of parallel microchannels as we vary the microchannel entrance (mouth) width and microchannel length. We measure the time until each channel clogs and we calculate the number of particles that pass prior to clogging. Contrary to expectation, we show that the number of particles passing through a pore increases exponentially with increasing mouth width but decreases linearly as the channel length increases. Changing the dimensions of the channels changes the particulate suspension’s flow rate which in turn changes the shear stresses that particles experience near the channel wall. When particles experience higher near-wall shear stress, the particles are less likely to adhere to channel walls and engender clogging. We confirm the effect of flow rate on channel clogging by demonstrating that the number of particles needed to clog a tapered channel increases as the pressure applied to the particulate suspension increases. The connection between flow rate and clogging highlights the interplay between hydrodynamic forces and intermolecular forces that govern particle attachment and ultimately clogging. We further explore this relationship by modulating the interaction between the particle and channel wall in a single tapered channel. While observing single channels clogging, we also resolve individual particles gradually building up on channel walls and forming clogs. Interestingly, particles also cluster on upstream channel walls only to later detach and clog at the downstream constriction. At low pressures, the channel clogs when particles accumulate individually near the constriction. At high pressures, the channel clogs when particle clusters detach from channel walls upstream and flow into the constriction. Finally, we compare the clogging behavior of particles with long, electrosteric stabilizing molecules on the surface to the clogging behavior of particles with shorter electrostatic stabilizing molecules on the surface. We also compare the clogging behavior of both particle types in the presence of varying concentrations of a monovalent salt. We show that clogging is mitigated when Debye length is comparable to the length of the stabilizing molecule on the particle’s surface.<br>Engineering and Applied Sciences - Applied Physics

A study of two the sequential displacement of gas and liquid phases in microchannels for eventual application in temperature swing adsorption (TSA) methane purification systems was performed. A model for bulk fluid displacement in 200 m channels was developed and validated using data from an air-water flow visualization study performed on glass microchannel test sections with a hydraulic diameter of 203 m. High-speed video recording was used to observe displacement samples at two separate channel locations for both the displacement of gas by liquid and liquid by gas, and for driving pressure gradients ranging from 19 to 450 kPa m-1. Interface velocities, void fractions, and film thicknesses were determined using image analysis software for each of the 63 sample videos obtained. Coupled 2-D heat and mass transfer models were developed to simulate a TSA gas separation process in which impurities in the gas supply were removed through adsorption into adsorbent coated microchannel walls. These models were used to evaluate the impact of residual liquid films on system mass transfer during the adsorption process. It was determined that for a TSA methane purification system to be effective, it is necessary to purge liquid from the adsorbent channel. This intermediate purge phase will benefit the mass transfer performance of the adsorption system by removing significant amounts of residual liquid from the channel and by causing the onset of rivulet flow in the channel. The existence of the remaining dry wall area, which is characteristic of the rivulet flow regime, improves system mass transfer performance in the presence of residual liquid. The commercial viability of microchannel TSA gas separation systems depends strongly on the ability to mitigate the presence and effects of residual liquid in the adsorbent channels. While the use of liquid heat transfer fluids in the microchannel structure provides rapid heating and cooling of the adsorbent mass, the management of residual liquid remains a significant hurdle. In addition, such systems will require reliable prevention of interaction between the adsorbent and the liquid heat transfer fluid, whether through the development and fabrication of highly selective polymer matrix materials or the use of non-interacting large-molecule liquid heat transfer fluids. If these hurdles can be successfully addressed, microchannel TSA systems may have the potential to become a competitive technology in large-scale gas separation.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO<br>CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO<br>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.<br>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.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003.<br>Includes bibliographical references (leaves 59-63). Also available in electronic version. Access restricted to campus users.