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This research project was part of the microfluidic program in the CRC for Microtechnology, Australia, during 2000 to 2003. The aim of this research was to investigate the feasibility of applying geometric variations in a microchannel to create effects other than pure molecular diffusion to enhance microfluidic mixing. Geometric variations included the shape of a microchannel, as well as the various obstacle structures inside the microchannel. Generally, before performing chemical or biological analysis, samples and reagents need to be mixed together thoroughly. This is particularly important in miniaturized Total Analysis Systems (�TAS), where mixing is critical for the detection stage. In scaling down dimensions of micro-devices, diffusion becomes an efficient method for achieving homogenous solutions when the characteristic length of the channels becomes sufficiently small. In the case of pressure driven flow, it is necessary to use wider microchannels to ensure fluids can be pumped through the channels and the volume of fluid can provide sufficient signal intensity for detection. However, a relatively wide microchannel makes mixing by virtue of pure molecular diffusion a very slow process in a confined volume of a microfluidic device. Therefore, mixing is a challenge and improved methods need to be found for microfluidic applications. In this research, passive mixing using geometric variations in microchannels was studied due to its advantages over active mixing in terms of simplicity and ease of fabrication. Because of the nature of laminar flow in a microchannel, the geometric variations were designed to improve lateral convection to increase cross-stream diffusion. Previous research using this approach was limited, and a detailed research program using computational fluid dynamic (CFD) solvers, various shapes, sizes and layouts of geometric structures was undertaken for the first time. Experimental measurements, published experimental data and analytical predictions were used to validate the simulations for selected samples. Mixing efficiency was evaluated by using mass fraction distributions. It was found that the overall performance of a micromixer should include the pressure drop in a microdevice, therefore, a mixing index criterion was formulated in this research to combine the effect of mixing efficiency and pressure drop. The mixing index was used to determine optimum parameters for enhanced mixing, as well as establish design guidelines for such devices. Three types of geometric variations were researched. First, partitioning in channels was used to divide fluids into mixing zones with different concentrations. Various designs were investigated, and while these provided many potential solutions to achieving good mixing, they were difficult to fabricate. Secondly, structures were used to create lateral convection, or secondary flows. Most of the work in this category used obstacles to disrupt the flow. It was found that symmetric layouts of obstacles in a channel had little effect on mixing, whereas, asymmetric arrangements created lateral convection to enhance crossstream diffusion and increase mixing. Finally, structures that could create complex 3D advections were investigated. At high Reynolds numbers (Re = 50), 3D ramping or obstacles generated strong lateral convection. Microchannels with 3D slanted grooves were also investigated. Mixers with grooved surfaces generated helicity at low Reynolds numbers (Re � 5) and provided a promising way to reduce the diffusion path in microchannels by stretching and folding of fluid streams. Deeper grooves resulted in better mixing efficiency. The 3D helical advection created by the patterned grooves in a microchannel was studied by using particle tracing algorithms developed in this research to generate streaklines and Poincare maps, which were used to evaluate the mixing performance. The results illustrated that all the types of mixers could provide solutions to microfluidic mixing when dimensional parameters were optimized.

A unique channel surface enhancement technique via diffusion-bonding a layer of conductive fine wire mesh onto the heating wall was developed and used to experimentally study flow boiling enhancement in parallel microchannels. Each channel was 1000 μm wide and 510 μm high. A dielectric working fluid, HFE 7000, was used during the study. Two fine meshes as well as two mesh materials were investigated and compared. According to the flow boiling curves for each channel, the amount of wall superheat was greatly reduced for all the mesh channels at four stream-wise locations; and the critical heat fluxes (CHF) for mesh channels were significantly higher than that for a bare channel in the low vapor quality region. According to the plots of local flow boiling heat transfer coefficient h versus vapor quality, a consistent increasing trend for h with vapor quality was observed for all the tested channels until the vapor quality reached approximately 0.4. However, the three mesh channels showed much higher values of h than the bare channel, with the 100 mesh copper performing the best. Visualization using a high-speed camera was performed thereafter to provide some insights to this enhancement mechanism. A significant increase in nucleation sites and bubble generation was observed, and departure rates inside the mesh channels were attributed to the flow boiling enhancement. A sudden increase of h for mesh channels can also be attributed to the characteristics of nucleate boiling and indicates that nucleate boiling was the dominant heat transfer mode. Another interesting point observed was that the 100 mesh bronze outperformed the 200 mesh bronze for most of the studies. This suggests that nucleations happened inside the mesh openings, instead of on the mesh openings. In addition, an optimal mesh size should exist for HFE 7000 flow boiling. / Graduation date: 2006

The focus of this thesis is on electro-osmotically pumped flow of viscoelastic fluids through microchannels. Fluid transport in microscaled structures is typically laminar due to the low Reynolds numbers involved. However, it is known that viscoelastic polymeric liquids can display striking instabilities in low Reynolds number flows. The motion of polymer doped solutions electrically pumped through microchannels is studied at low Reynolds number. It is found that extensional instabilities can be excited in such microflows with standard electro-osmotic pumping (approximately mm/s flow rate regime), occurring at the viscoelastic instability threshold. The existence of these instabilities must inform design as microfluidic applications move beyond simple fluids towards using biological materials and other complex suspensions, many of which display elasticity. It is further found that discrete and persistent microgels are formed at sufficiently high current densities. Prior work has found up to orders of magnitude increase in mixing rates, however additional fluid deformation effects (notably shear) exist in other studies and high viscosity solvents are used. The flows here exclude shear, a ubiquitous feature in mechanically driven cavity flows, and low viscosity solvents typical in microfluidic applications are used. The device is also highly symmetric minimizing Lagrangian chaos deformation and mixing of fluids. It is demonstrated that viscoelastic instabilities reduce mixing relative to low viscosity polymer-free solutions. The decrease in mixing found is consistent with the understanding that viscoelastic flows progress towards Batchelor turbulence, and demonstrates that, in contrast to common expectations, viscoelastic flows are effectively diffusion limited. Electro-osmotic pumped devices are the ideal platform to study isolated viscoelasticity and elastic turbulence, where additional effects (such as shear, or Lagrangian deformation manipulations) can be introduced in a controlled manner allowing fundamental studies of viscoelasticity and mixing. Besides the viscoelastic experimental observations it is shown that (1) a recently discovered instability due to density fluctuation has an analogue in polymeric fluids corresponding to the viscoelastic instability threshold, (2) inspection of correlations in microparticle image velocimetry (micro-PIV) data in unstable polymer flows reveals the relaxation time of polymer solutions, and (3) poly(ether sulfone) polymer films can act as negative electron beam resist.

Herein is presented the collective experimental record of research performed in the Laboratory for Biological Surface Science. These investigations are generally classified under the category of bioanalytical surface science and include the following projects. Chapters III and IV describe the creation of a microfluidic device capable of generating fixed arrays of concentration gradients. Experimental results were matched with computational fluid dynamics simulations to predict analyte distributions in these systems. Chapters V and VI demonstrate the discovery and utility of photobleaching fluorophores for micropatterning applications. Bleached fluorophores were found to rapidly attach to electron rich surfaces and this property was used to pattern enzymes inside microfluidic channels in situ. Finally, Chapter VII exhibits a method by which solid supported lipid bilayers can be dried and preserved by specifically bound proteins. The intrinsic property of lateral lipid mobility was maintained during this process and a mechanism by which the protein protects the bilayer was suggested.

A simple and efficient technique for the concentration enrichment of charged species within a microfluidic device was developed. The functional component of the system is a hydrogel microplug photopolymerized inside the microfluidic channel. The fundamental properties of the nanoporous hydrogel microplug in modulating the electrokinetic transport during the concentration enrichment were investigated. The physicochemical properties of the hydrogel plug play a key role in determining the mode of concentration enrichment. A neutral hydrogel plug acts as a physical barrier to the electrophoretic transport of charged analytes resulting in size-based concentration enrichment. In contrast, an anionic hydrogel plug introduces concentration polarization effects, facilitating a size and charge-based concentration enrichment. The concentration polarization effects result in redistribution of the local electric field and subsequent lowering of the extent of concentration enrichment. In addition, an electroosmotic flow originating inside the pores of the anionic hydrogel manipulates the location of concentration enrichment. A theoretical model qualitatively consistent with the experimental observations is provided.

The next step in miniaturization of analytical devices involves the use of MEMS and Lab-on-a-Chip applications, where many biological or chemical reactions are carried out on the device in real time. Since detection mechanisms occur almost immediately after the reactions, inefficient mixing of reagents could cause a decrease in sensing capability, especially on micro- and nano-scaled devices. Thus a microfluidic mixer has become a crucial component in these applications. Here we propose a new design of a passive microfluidic mixer that utilizes the theories of chaotic advection to enhance mixing. The micro-channels for the mixer have dimensions with width ranging from 10µm to 40µm, depth 40µm, and a total length of 280µm. First the designs are simulated using CFD-ACE+ for computational analysis. After the device geometry has been decided, the actual devices are fabricated using traditional UV photolithography on silicon and bonded with pyrex glass by anodic bonding. To test the actual device mixing efficiency, we used a fluorescent dye rhodamine B solution to mix with DI water and put the devices under fluorescent microscope observations for real-time analysis. Images of fluorescent light intensities are taken at different flow rates during the analysis and are later used to study the experimental results calculated using a published mixing efficiency formula for comparison.

This thesis describes a study of various aspects of chemical reactions conducted in microfluidic reactors. (i) In the first project, we proposed the application of the 'internal trigger' approach to multi-step microfluidic polymerization reactions conducted in droplets, namely, polyaddition and polycondensation. We hypothesized and experimentally established that heat generated in the exothermic free radical polymerization of an acrylate monomer triggers the polycondensation of the urethane oligomer. As a result, we synthesized monodispersed poly(acrylate/urethane) microparticles with an interpenetrating polymer network structure. (ii) In the second project, we developed a multiple modular microfluidic reactor with the purpose of increasing productivity in microfluidic synthesis. Compared to the productivity of the single microfluidic reactor < 1g/hr, we synthesized poly(N-isopropylacrylamide) particles at a productivity of approximately 50g/hr with a CV < 5%. We analyzed and addressed several challenges of this process, such as the fidelity in the fabrication of microfluidic reactors, crosstalk between individual reactors sharing a common liquid supply, and coalescence of droplets. (iii) We developed an integrated microfluidic reactor comprising four parallel individual reactors to study the effect of geometry and surface energy of the microchannels on the emulsification process. We spontaneously generated droplets with different volumes by integrating individual droplet generators in parallel with varying geometry. This approach is important in studies of the effect of droplet surface and volume on chemical reactions, and in the studies of diffusion-controlled processes. (iv) We conducted a microfluidic study of the reversible binding of CO2 to secondary amines in the process that mediates solvent polarity switch. We studied reaction rates and CO2 uptake by generating plugs of gaseous a CO2 and monitoring the change in their dimensions. We also demonstrated fast screening of reaction conditions, as well as the ability to reverse the reaction in situ.

Microfluidics was introduced in the early 1990’s and was posited to usher in a new age of integrated analysis systems in the form of labs-on-a-chip. To date, numerous embodiments of microfluidic technologies including fully integrated analysis systems have been described for various applications. Microfluidics can be sub-divided into two paradigms based on fluid manipulation in streams or droplets. In the former, streams of fluids flow through micron-dimension channels, and typical volumes manipulated are in the pico-liter to nano-litre range. These devices are mainly employed for rapid, high efficiency chemical separations, among other applications. In the latter, droplets are manipulated on a dielectric-coated array of microelectrodes in a process called digital microfluidics (DMF). In DMF each droplet is individually addressable, giving superior spatial control over fluid droplets with volumes in the pico-liter to micro-litre range. Independently addressable droplets make DMF amenable to carrying out sequential reactions. This thesis presents methods towards the integration of these two microfluidic paradigms into “hybrid microfluidic” platforms. Hybrid devices contain a DMF array for sample preparation and a microfluidic channel network for on-line analysis by chemical separation. Sample transfer between the platforms is made by way of a digital-channel interface, which has been fabricated in two geometries: side-on and vertical. Chemical separations on hybrid devices are performed in various open-channel and chromatographic modes. In open-channel methods analytes are separated by microchannel zone electrophoresis (MZE) or micellar electrokinetic chromatography (MEKC). In chromatographic separations porous polymer monolithic (PPM) columns were created in situ by UV-initiated polymerization of acrylate monomers. Prior to integration into hybrid microfluidic devices PPMs were optimized for use in gradient elution microchannel electrochromatography (MEC) of peptides. It is anticipated that hybrid microfluidic devices will bridge a large bottleneck for myriad analyses by combining sample preparation with on-line analysis by chemical separation.

The physical mechanism responsible for droplet manipulation in electrowetting on dielectric (EWOD) devices is not yet fully understood. This investigation will examine the role of capillary forces on droplet manipulation to further the physical understanding of these devices. An analytical model for the capillary force acting on a confined droplet at equilibrium is developed here. Model predictions were validated using optical measurements of the droplet interface in the vertical plane. It was found that the capillary force and interface shape predicted by the equilibrium model were over an order of magnitude more accurate than predictions from the model commonly used in EWOD investigations. The equilibrium model was adapted to droplets with arbitrary shapes to predict droplet dynamics in EWOD devices. It was found that droplet motion could be described using the driving capillary force and frictional forces from wall shear, the contact line, and contact angle hysteresis. Comparison with experimental data shows that this model accurately predicts the effects of applied voltage and droplet aspect ratio on the transient position and velocity of droplets. This model can be used to design EWOD devices and predict the simultaneous manipulation of droplets required to meet the high throughput demands of practical applications. A robust system for droplet monitoring must be automated before EWOD devices can be used reliably in practical applications. Although capacitance measurements have been used to automate droplet detection in EWOD devices, manual optical measurements are generally used to monitor droplet mixing. This may not be possible in high throughput applications with multiple droplets and limited optical access. Here, capacitance measurements are shown to be an accurate and repeatable means of monitoring droplet composition and real time mixing. Experiments were performed with this technique to show that mixing efficiency is better characterized by the number of translations required for full mixing, not mixing time.