Experiments were conducted on the investigation of the electroosmotic flow with five different electric field strength, four kinds of buffer solution concentration, six different pH values, and three kinds of microchannel geometry. Joule heating effects were also taken into consideration. Experiments were performed using a microparticle image velocimetry (MPIV) for full field velocity distributions and micro laser-induced fluorescent (£gLIF) for full field temperature distributions. It is found that the presence of Joule heating and flow area change could have a great impact on the microfluidic transportation, e.g. dispersion. Furthermore, data were presented and the relation between zeta potential and pH value were discussed in detail. It is found that, as pH > 7.5, all silanol sites are deprotonated.

Electroosmotic flow is a convenient mechanism for transporting polar fluid in a microfluidic device. The flow is generated through the application of an external electric field that acts on the free charges that exists in a thin Debye layer at the channel walls. The charge on the wall is due to the chemistry of the solid-fluid interface, and it can vary along the channel, e.g. due to modification of the wall. This investigation focuses on the simulation of the electroosmotic flow (EOF) profile in a cylindrical microchannel with step change in zeta potential. The modified Navier-Stoke equation governing the velocity field and a non-linear two-dimensional Poisson-Boltzmann equation governing the electrical double-layer (EDL) field distribution are solved numerically using finite control-volume method. Continuities of flow rate and electric current are enforced resulting in a non-uniform electrical field and pressure gradient distribution along the channel. The resulting parabolic velocity distribution at the junction of the step change in zeta potential, which is more typical of a pressure-driven velocity flow profile, is obtained. / Singapore-MIT Alliance (SMA)

Integrated microfluidic devices for biomedical analysis attract lots of interest in the MEMS (Micro-Electro-Mechanical-Systems) research field. However, the characteristic Reynolds number for liquids flowing in these microchannels is very small (typically less than 10). At such low Reynolds numbers, turbulent mixing does not occur and homogenization of the solutions occurs through diffusion processes alone. Hence, a satisfactory mixing performance generally requires the use of extended flow channels and takes longer to accomplish such that the practical benefits of such devices are somewhat limited. Consequently, accomplishing the goal of u¡VTAS requires the development of enhanced mixing techniques for microfluidic structures. This study first presents a microfluidic mixer utilizing alternatively switching electroosmotic flow and proposes two microchannel designs of T-form and double-T-form micromixer. Switching DC field is used to generate the electroosmotic force to drive the fluid and also used for mixing of the fluids simultaneously, such that moving parts in the microfluidic device and delicate external control system are not required for the mixing purpose. Furthermore, this study also proposed a novel pinched-switching mode in the T-form microfluidic mixer, which could be effectively increase the perturbation within the fluid to promote the mixing efficiency. In this study, computer simulation for the operation conditions is used to predict the mixing outcomes and the mixing performance is also confirmed experimentally. Result shows the mixing performance can be as larger as 95% within the mixing distance of 1 mm downstream the common boundary between the different sample fluids. The novel method proposed in this study can be used for solving the mixing problem in a simple way in the field of micro-total-analysis-systems. Furthermore, in order to demonstrate the proposed micromixer is feasible for on-line bio-reaction, this study designs a fully integrated device for demonstration of DNA/enzyme reaction within the microfluidic chip. The microchip device contains a pre-column concentrating region, a micro mixer for DNA-enzyme mixing, an adjustable temperature control system and a post-column concentration channel. The integrated microfluidic chip has been used to implement the DNA digestion and extraction. Successfully digestion of £f-DNA using EcoRI restriction enzyme in the proposed device is demonstrated utilizing large-scale gel electrophoresis scheme. Results show that the reaction speed doubled while using the microfluidic system. In addition, on-line DNA digestion and capillary electrophoresis detection is also successfully demonstrated using a standard DNA-enzyme system of $X-174 and Hae III. Finally, this reasearch also proposes a novel cell/microparticle manipulation platform by integrating an optical tweezer system and a micro flow cytometer. During operation, electrokinetically driven sheath flows are utilized to focus microparticles to flow in the center of the sample stream then pass through an optical manipulation area. An IR diode laser is focused to generate force gradient in the optical manipulation area to manipulate the microparticles in the microfluidic device. Moving the particles at a static condition is demonstrated to confirm the feasibility of the home-built optical tweezer. The trapping force of the optical tweezer is measured using a novel method of Stocks-drag equilibrium. The proposed system can continuously catch moving microparticles in the flowing stream or switch them to flow into another sample flow within the microchannel. Target particles can be separated from the sample particles with this high efficient approach. More importantly, the system demonstrates a continuously manipulation of microparticles using non-contact force gradient such that moving parts and delicate fabrication processes can be excluded. The proposed system is feasible of high-throughput catching, moving, manipulation and sorting specific microparticles/cells within a mixed sample and results in a simple solution for cell/microparticle manipulation in the field of micro-total-analysis-systems. In this thesis, low-cost soda-lime glass substrates are adopted for the microchip fabrication using a simple and reliable fabrication process. Three kinds of novel microfluidic devices including an electrokinetically-driven microfluidic mixer, a high throughput DNA/enzyme reactor and an optically cell manipulation platform are successfully demonstrated. It is the author¡¦s believes that the results of this study will give important contributions in the development of micro-total-analysis-systems in the future. With the success of this study, we have a further step approaching to the dream of lab-on-a-chip system for bio-analytical applications.

Experiments were investigated on passive mixing enhancements in different geometric microchannels with roughened surfaces and flow was driven by electroosmotic flow (0.027 ≤ Re ≤ 0.081). Experiments were perform using micro particle image velocimetry (MPIV) technology for velocity measurements and relative analysis. Iodine and DI water mixing experiments were captured by common optical microscope for flow visualization, and rhodamine B and buffers mixing experiments were measured by micro laser-induced fluorescence (µLIF) technology for concentration field measurements and analysis. The experimental results showed that the Twr and Tcdr micromixers can generate chaotic flow and enhance the mixing performance in the short channel length. Finally, the mixing length was developed in terms of within accuracy between the experimental data and prediction data.

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Electrokinetic phenomena in liquid foams is at a junction between two areas. On one side is the investigation of liquid foam drainage, and on the other side is electrokinetics of surface driven flow on solid-liquid interfaces. However, the electrokinetic phenomena in liquid foam films significantly lack understanding. Therefore, the novelty of the thesis is to address the mentioned gap in three stages. The outcome has potential applications in a novel separation approaches of biological molecules such as proteins and DNA. In the first stage, the electrokinetic flow of a sufficiently thick (180 μm) free liquid film was investigated using cationic and anionic surfactants by confocal micron-resolution particle image velocimetry (μ-PIV). The reverse of the surface charge resulted in a shift in charge of the electrical double layer at the free liquid film interface, which caused the direction of the electroosmotic velocity to reverse. In each surfactant type used, the fluid velocity profiles were measured at different depths of the free liquid film (different z-planes). It was found how the fluid velocity varied with depth. Numerical simulations of the electroosmotic flow in the same system were also performed using Finite Element Method to understand the flow dynamics. A reasonably good agreement was found between the numerical simulations and the experimental results validating the model. In the second stage, instead of flow visualisation particles, rhodamine B (RB) and fluorescein isocyanate (FICT) dye were added to the free liquid film. Under the initial conditions of pH 7.2, RB is a neutral dye, and FICT has a -2 charge. Under an imposed electric field pH variations were detected and an interesting flow profile was observed. The CFD model developed earlier (stage one) was modified to include the local pH variation. The behaviour of the simulated pH had a good agreement with the behaviour of the FICT. Further confirmation of local pH variation was undertaken using extra new experiments which also showed a good agreed with the simulation. In the third stage, a liquid foam electrokinetic separation chamber was designed to extend the study to include practical applications. The first challenge was to achieve a stable foam under external electric field. A polymer-surfactant mixture can solve the stability problem. However, the mixture of polymers required an alkaline pH (>9) condition for the polymer mixture to be soluble in the aqueous system. Lectin and tetramethylrhodamine goat anti-rabbit (IgG) protein mixture with different molecular mass to charge ratio (50 kDa and 150 kDa) were injected near the anode. The system was monitored in three location: (a) in a vicinity of the injection region, (b) between the two electrodes and (c) in a vicinity of the cathode. In the region (a), a decay of the luminescence intensity of the fluorescein of the two proteins was noted with varying rate. In region (b), an increase followed by a decrease in fluorescein intensity of the proteins was observed again at a varying rate. In region (c), an increase of the dye concentration was observed and again at a different rate. The observed difference was caused by difference of the electrophoretic velocity of the two proteins. The setup proved that proteins could be separated based on their electrophoretic mobility inside a liquid foam. The findings from the thesis show the ability to manipulate fluid flow within a free liquid film, and inside a liquid foam system by an external DC electric field, is not only interesting academically but has potential application in a novel separation approach of biological molecules and beyond. The result show, with the correct surfactant formulation, it possible to make a stable foam under an electric field which can be set up for separation of proteins using foam electrokinetics.

L’électrophorèse est une technique puissante permettant de séparer des biomarqueurs présents dans les liquides biologiques.L’électrophorèse de zone libre transporte des molécules en milieu liquide sous l’influence de deux contributions : le flux électrophorétique et le flux électroosmotique (EOF). C’est ce dernier flux EOF qui permet d’optimiser la résolution analytique de la séparation et donc de simplifier le mélange avant sa détection. Notre équipe a développé un contrôle en temps réel de l’ EOF en intégrant une interface polarisable diélectrique dans un dispositif microfluidique. Le carbone amorphe azoté (CNx avec x=15%) a été choisi comme ce matériau.Comme le CNx ne peut pas être déposé directement sur un substrat de verre à cause de sa faible adhérence, deux matériaux différents ont été proposés comme couche d’accroche : le carbure de silicium (SiC) et le platine (Pt). Nous avons tout d’abord optimisé l’adhésion entre le film CNx et la couche d’accroche SiC par différentes procédures de fabrication. Cependant, en raison d’une faible adhérence, le film CNx s’est rapidement décollé en électrolyte liquide. Par contre, nous avons prouvé que certaines architectures hybrides incluant du Pt dans la couche d’accroche sont incroyablement robustes. Même après deux mois dans une solution millimolaire de KCl, le CNx adhérait toujours au verre sans aucune trace de délamination. Ce dispositif a fourni aussi une grande fenêtre de polarisabilité (de -1V à +1V). Nous avons enfin développé une architecture hybride « couche d’accroche isolée/couche électriquement polarisable/électrodes de grille enterrées/ polymère » afin d’éviter toute perte faradique dans l’électrolyte liquide ou vers les circuits conducteurs du dispositif. A l’issue de ces travaux, nous pensons être en mesure de proposer un composant fluidique complexe et robuste qui permet une modulation en temps réel de l’ EOF lors de migrations électrophorétiques. / Electrophoresis is currently an efficient way to separate precious biomarkers from complex mixtures. It takes place to transport molecules under two contributions: the electrophoretic flow and the electroosmotic flow (EOF). The latter allows to optimize the analytical resolution of the separation.Our team has developed a real-time dynamic control of the EOF by integrating a dielectric polarizable interface in the microfluidic device.Amorphous carbon nitride (CNx with x=15%) has dielectric properties and was chosen to be the polarizable interface. Since it cannot be deposited directly onto glass substrate, we have proposed and studied two different materials as the sticking underlayer: silicon carbide (SiC) and platinum (Pt).In the case of SiC, we have optimized the adhesion between CNx film and SiC underlayer through different fabrication procedures.However, due to poor adhesion, CNx film delaminated into liquid electrolyte quickly.Compared to SiC, Pt is a good sticking underlayerfor CNx. It was found out that even after two months in KCl solution, CNx still stuck robustly toPt. Meanwhile, the device provided a large windowof polarizability (from -1V to +1V). Finally, toavoid any faradic loss in the liquid electrolyte ortowards the conductive circuitry of the device, we have developed a sticking underlayer/electrically polarizable/polymeric hybrid architecture. This architecture appears to be the most robust existing polarizable interface for strong and long-term adhesion onto glass substrates.

The goal of this project is to eliminate electroosmotic flow (EOF) in a microfluidic chip. EOF is a naturally occurring phenomenon at the fluid-surface interface in microfluidic chips when an electric field is applied across the fluid. When isoelectric focusing (IEF) is carried out to separate proteins based on their surface charge, the analytes must remain in the separation chamber, and not migrate to adjacent features in the microfluidic chip, which happens with EOF. For this project, a microfluidic chip was designed and commissioned to be photolithographically transferred onto a Si wafer. A PDMS component was then casted on the Si wafer and plasma bonded to a glass substrate. This chip was initially designed to carry out continuous IEF, and the focus of the project was shifted to the analysis of eliminating EOF in a microfluidic chamber. Per previous research test methods, methylcellulose will be used to analyze the phenomenon of electroosmotic flow in the chamber. A COMSOL model is used a theoretical basis of comparison when analyzing the flow velocities of the treated versus untreated microfluidic chips. The purpose of this project is to use the research performed in on this chip as a precursor to future analyses of continuous IEF on microfluidic chips in the Cal Poly Microfluidics group.