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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Membrane-bound proteins and charged lipids are separated based on their charge-to-size ratio by electrophoretic-electroosmotic focusing (EEF) method on supported lipid bilayers (SLBs). EEF uses opposing electrophoretic and electroosmotic forces to focus and separate proteins and lipids into narrow bands from an initially homogeneous mixture. Membrane-associated species were focused into specific positions within the SLB in a highly repeatable fashion. The steady-state focusing positions of the proteins could be predicted and controlled by tuning experimental conditions, such as buffer pH, ionic strength, electric field and temperature. Careful tuning of the variables should enable one to separate mixtures of membrane proteins with only subtle differences. The EEF technique was found to be an effective way to separate protein mixtures with low initial concentrations and it overcame diffusive peak broadening problem. A "SLB differentiation" post-separation SLB treatment method was also developed by using magnetic particles to rapidly slice the whole SLB into many small patches after electrophoretic separation, while keeping the majority of materials on surface and avoiding the use of chemical reactions. Label-free detection techniques were also developed based on EEF on SLBs. First, a new separation based label-free detection method was developed based on the change of focusing position of fluorescently labeled ligands. This technique is capable of simultaneous detecting multiple protein competitive binding on the same ligand on SLBs. Low concentration protein can be detected in the presence of interfering proteins and high concentration of BSA. The fluorescent ligands were moved to different focusing positions in a charged SLB patch by different binding proteins. Both free ligand and protein bound ligand concentrations were obtained. Therefore, both protein identity and quantity information were obtained simultaneously. Second, the focusing position of fluorescent biomarkers on SLB was used to monitor the phospholipase D catalyzed hydrolysis of phosphatidylcholine (PC) to form phosphatidic acid (PA), which is involved with the change of charge on the phospholipids. The focusing position of fluorescent membrane-bound biomarker in the EEF experiment is directly determined by the negative charge density on SLB. Other enzyme reactions involved with the change of phospholipids charge can be monitored in a label-free fashion in a similar way.

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.

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