Microfluidic chips have drawn great attention and interest due to their broad applications in chemical, biological and biomedical fields. These kinds of miniaturized devices offer many advantages over the traditional analysis instruments, such as reduced cost, shortened time, increased throughput, improved integration/automation/portability, etc. However, the concept of integrating multiple labs on a single chip to perform micro total analysis hasn’t been realized yet because of the lack of fundamental knowledge and systematic design of each component, especially for some particular applications. A thorough understanding and grasp of the basic physical phenomenon is the theoretical basis to develop functional devices to utilize them. In this study, we intend to investigate the electrokinetic fluid flow and coherent species transport processes in microchannels, and then try to effectively control them for designing related lab-on-a-chip devices. Rather than expensive experiments, numerical studies are performed to simulate the different processes involved in various electrokinetic chip applications. In the theoretical models, applied potential field, flow field and species concentration field are considered and corresponding governing equations with initial/boundary conditions are numerically solved by computational fluid dynamics techniques. The flow field is obtained by the developed SIMPLE algorithm and a slip-wall velocity boundary condition is applied in simulating electroosmotic flow. Grid independence tests and convergence studies are performed to ensure economic computation with adequate accuracy and stability. For every application with typical channel layout, parametric studies are performed to investigate different effects through the controlling parameters linked to them. For surface patterning or microfabrication using laminar flows, various operational parameters are investigated to explore the optimized configurations for multi-stream flow and mass transport control in cross-linked microchannels. Through a series of numerical simulations, it is found that applied potentials, electroosmotic mobilities of solutions and channel dimensions have significant effects on the flow and mass transport after converging in the intersection of channel network. Diffusion coefficient has less influence than the other parameters due to the presence of high Peclet number for such applications. For the microwashing with two different electrolyte solutions, a three-dimensional model is numerically solved to reveal the flow structure change. In a straight microchannel with a rectangle cross section, KCl solution and LaCl3 solution are mainly employed for tests. Displacement processes between two solutions in both orders are tested and analyzed. The observed flow structures such as back flow in channel center and distortion of plug-like velocity profile are noticed and discussed. The distortion of the flow field results from the induced pressure gradient, which is due to the non-uniformity of electroosmotic mobilities and electrical conductivities of two replaced solutions. The bigger difference two solutions have in chemical properties, the stronger effects on flow they have. Effect of applied potential field strength is also studied and the approximate linear influences are concluded. Finally, the unsteady on-chip sample injection and separation processes involved in microchip capillary electrophoresis are studied. Species’ electrophoretic migration effect is included and the theoretical model is non-dimensionalized in a unique manner with the key fundamental parameters: the Re Sci , species’ non-dimensional electrophoretic mobility and applied potentials. The species transport characteristics are revealed numerically and well understood for future effective control and innovative chip design. Species front movement during injection and sample plug development in separation are examined with diffusion effect; results include concentration profiles and contour plots over a range of injection and separation time. The influence of i Re Sc which characterizes the relative role of convection versus diffusion is examined over the commonly encountered range and the diffusion effect is found to have an essentially negligible effect. Through three species, the electrophoretic mobilities difference is demonstrated to be the reason for separation. Real-time monitoring of different species’ movements is performed for injection guidance.

Microfluidic chips have drawn great attention and interest due to their broad applications in chemical, biological and biomedical fields. These kinds of miniaturized devices offer many advantages over the traditional analysis instruments, such as reduced cost, shortened time, increased throughput, improved integration/automation/portability, etc. However, the concept of integrating multiple labs on a single chip to perform micro total analysis hasn’t been realized yet because of the lack of fundamental knowledge and systematic design of each component, especially for some particular applications. A thorough understanding and grasp of the basic physical phenomenon is the theoretical basis to develop functional devices to utilize them. In this study, we intend to investigate the electrokinetic fluid flow and coherent species transport processes in microchannels, and then try to effectively control them for designing related lab-on-a-chip devices. Rather than expensive experiments, numerical studies are performed to simulate the different processes involved in various electrokinetic chip applications. In the theoretical models, applied potential field, flow field and species concentration field are considered and corresponding governing equations with initial/boundary conditions are numerically solved by computational fluid dynamics techniques. The flow field is obtained by the developed SIMPLE algorithm and a slip-wall velocity boundary condition is applied in simulating electroosmotic flow. Grid independence tests and convergence studies are performed to ensure economic computation with adequate accuracy and stability. For every application with typical channel layout, parametric studies are performed to investigate different effects through the controlling parameters linked to them. For surface patterning or microfabrication using laminar flows, various operational parameters are investigated to explore the optimized configurations for multi-stream flow and mass transport control in cross-linked microchannels. Through a series of numerical simulations, it is found that applied potentials, electroosmotic mobilities of solutions and channel dimensions have significant effects on the flow and mass transport after converging in the intersection of channel network. Diffusion coefficient has less influence than the other parameters due to the presence of high Peclet number for such applications. For the microwashing with two different electrolyte solutions, a three-dimensional model is numerically solved to reveal the flow structure change. In a straight microchannel with a rectangle cross section, KCl solution and LaCl3 solution are mainly employed for tests. Displacement processes between two solutions in both orders are tested and analyzed. The observed flow structures such as back flow in channel center and distortion of plug-like velocity profile are noticed and discussed. The distortion of the flow field results from the induced pressure gradient, which is due to the non-uniformity of electroosmotic mobilities and electrical conductivities of two replaced solutions. The bigger difference two solutions have in chemical properties, the stronger effects on flow they have. Effect of applied potential field strength is also studied and the approximate linear influences are concluded. Finally, the unsteady on-chip sample injection and separation processes involved in microchip capillary electrophoresis are studied. Species’ electrophoretic migration effect is included and the theoretical model is non-dimensionalized in a unique manner with the key fundamental parameters: the Re Sci , species’ non-dimensional electrophoretic mobility and applied potentials. The species transport characteristics are revealed numerically and well understood for future effective control and innovative chip design. Species front movement during injection and sample plug development in separation are examined with diffusion effect; results include concentration profiles and contour plots over a range of injection and separation time. The influence of i Re Sc which characterizes the relative role of convection versus diffusion is examined over the commonly encountered range and the diffusion effect is found to have an essentially negligible effect. Through three species, the electrophoretic mobilities difference is demonstrated to be the reason for separation. Real-time monitoring of different species’ movements is performed for injection guidance.

This study developed a microfluidic flow sensor for the detections of velocity and viscosity, especially for ultra-low viscosity detection. An etched optic fiber with the diameter of 9 £gm is embedded in a microfluidic chip to couple green laser light into the microfluidic channel. The flow induced vibration causes periodic flapping motion of the optic fiber cantilever because of the pressure difference from two sides of fiber cantilever. Through the frequency analysis, the fluidic properties including the flow rate and the viscosity can be detected and identified. Results show that this developed sensor is capable of sensing liquid samples with the flow rates from 0.17 m/s to 68.81 m/s and the viscosities from 0.306 cP to 1.200 cP. In addition, air samples (0.0183 cP) with various flow rates can also be detected using the developed sensor. Although the detectable range for flow rate sensing is not wide, the sensitivity is high of up to around 3.667 mm/(s¡EHz) in test liquid in DI water, and when detecting air the sensitivity is 6.190 mm/(s¡EHz). The developed flow sensor provides a simple and straight forward method for sensing flow characteristics in a microfluidic channel.

Monitoring and controlling pH is of utmost importance in bioprocessing as it directly affects product yield and quality. Multiplexed experiments can be performed in nanobioreactors for optimisation of yield and cell heterogeneity in a relatively quick and inexpensive manner. In this thesis, a pH holographic sensor (holosensor) is miniaturised to 3.11 nL in volume and integrated into a PDMS-glass microfluidic chip for monitoring the growth of Lactobacillus casei Shirota. Although other established methods for monitoring cell cultures can be utilised, miniaturised holosensors enable real-time and non-consumptive monitoring of the bacterial cell culture growth medium. The 2-hydroxyethylmethacrylate (HEMA)-co-2-(trifluoromethyl) propenoic acid (TFMPA) holosensor was fabricated using an adapted technique from photolithography, coupled with the use of a polymerisation inhibitor to control the gel polymerisation with diameters not exceeding a standard deviation of 0.067. The hologram brightness was optimised to 1.05 ms integration time with 36X magnification using a low power (0.290 mW) 532 nm green continuous wave (CW) laser with a devised beam-offset technique. The holosensor was characterised with ionic strength balanced (9.50 mS/cm) McIIvaine pH buffers and a calibration curve plotted together with measured ionic strength, optical density at 600 nm (OD600) and pH. Correspondingly, RGB-xyY transformed values were plotted in the CIE 1931 chromaticity diagram. Later, a miniaturised 0.4φ HEMA-co-TFMPA holosensor and array was also demonstrated. Together with the 3.0φ holosensor, an accuracy parameter for the 0.4φ spot and array holosensors were calculated to be 99.08%, 99.38% and 97.77% respectively. Further work involved studying the issues associated with fabricating gels with unusually flat gel profiles. Other preliminary results suggested the alternative of utilising polymers as a holosensor substrate, together with a dye-free method for hologram fabrication, outlined the prospective possibility of a miniaturised holosensor integrated into a polymer microfluidic chip with the flexibility of hologram colour customisation for cell culture monitoring.

This research presents a detection scheme for analyzing the temperature distribution produced by the Joule heating effect nearby the channel wall in a microfluidic chip utilizing a temperature-dependent fluorescence dye. An advanced optical microscope system¡Xtotal internal reflection fluorescence microscope (TIRFM) is used for measuring the temperature distribution on the inner channel wall at the point of electroosmotic flow in an electrokinetically driven microfluidic chip. In order to meet the short working distance of the objective-type TIRFM, microscope cover glass are used to fabricate the microfluidic chips. The short fluorescence excitation depth from a TIRFM makes the intensity information obtained is not sensitive to the channel depth variation which ususally biases the measured results while using conventional epi-fluorescence microscope (Epi-FM). Therefore, a TIRFM can precisely describe the temperature profile of the distance within hundreds of nanometer of the channel wall where consists of the Stern layer and the diffusion layer for an electrokinetic microfluidic system. In order to investigate the temperature distribution produced by the Joule heating effect for electrokinetically driven microchips, this study not only measures the temperature on the microchannel wall by the proposed TIRFM but also measures the temperature inside the microchannel by an Epi-FM. In addition, this research presents a method to reduce the Joule heating effect and enhance the separation efficiency of DNA biosamples in a chip-based capillary electrophoresis (CE) system utilizing pulse DC electric fields. Since the average power consumption is reduced by the pulse electric fields, the Joule heating effect can be significantly reduced. Results indicate the proposed TIRFM method provides higher measurement sensitivity over the Epi-FM method. Significant temperature difference along the channel depth measured by TIRFM and Epi-FM is experimentally observed. In addition, the measured wall temperature distributions can be the boundary conditions for numerical investigation into the Joule heating effect. The proposed method gives a precise temperature profile of microfluidic channels and shows the substantial impact on developing a simulation model for precisely predicting the Joule heating effect in microfluidic chips. Moreover, in the research of reducing the Joule heating effect and enhancing the separation efficiency in a chip-based CE system utilizing pulse electric fields, the experimental and numerical investigations commence by separating a mixed sample comprising two fluoresceins with virtually identical physical properties. The separation level is approximately 2.1 times higher than that achieved using a conventional DC electric field. The performance of the proposed method is further evaluated by separating a DNA sample of Hae III digested £XX¡V174 ladder. Results indicate the separation level of the two neighboring peaks of 5a (271 bp) and 5b (281 bp) in the DNA ladder is as high as 120% which is difficult to be achieved using a conventional CE scheme. The improved separation performance is attributed to a lower Joule heating effect as a result of a lower average power input and the opportunity for heat dissipation during the zero-voltage stage of the pulse cycle. Overall, the results demonstrate a simple and low-cost technique for achieving a high separation performance in CE microchips.

Microfluidics deals with the behavior, control and manipulation of fluids which are confined at micrometer length scale. It has important application in lab-on-a chip technology, micro-propulsion, additive manufacturing, and micro-thermal technologies. Microfluidics has been widely used in detection, separation, transportation, and mixing of fluids and particles. The work carried out for the thesis to study the fluid-structure interaction in micro-channel involves an experimental part and a simulation part. In the experimental part the characterization of biofluid (RBC in BSA) is carried out based on the power law of fluid and flow behavior is studied. Also the dependence of fluid concentration on the viscosity in the channel is studied. The results are analyzed. Transition of fluid behavior from non-Newtonian shear thickening to non-Newtonian shear thinning is observed when the RBC concentration varies from 5.5×106 to 5.5×107 cells/ml in the channel. From the viscosity measurements of the biofluid it is observed that the average viscosity in the channel increases on increasing concentration of the fluid for shear thickening fluids. In the simulation part, interaction behavior of biomolecule DNA is studied in the channel containing biofluid which is characterized in the experimental part. Cell free DNAs are common problem in infectious disease detection. Based on the assumptions of the WLC model, DNA strand is assumed as a one dimensional elastic member with its one end fixed at the channel wall and the other end free to move in the fluid. Bent and straight DNAs are considered for the study. Multiple scales are involved in this problem which is not fully understood. DNA strands in the channel are exposed to different forces in the channel which are mainly due to the pressure and viscous effects. Numerical simulations are carried out for the multiphysics problem of DNA in the fluid using a coupled multiphysics finite element scheme and the results are obtained. Same procedure is carried out considering smaller channels and also for PBS solution as background fluid to obtain consistent results. It is found that when the channel width increases the tip displacement of DNA decreases. It was observed that DNA tip displacement is more in the channel when its end-to-end length is approximately half the width of the channel. Potential application of these modeling and simulation are in molecular screening processes to improve the performance of microfluidic DNA chips, and in design of micro-channel structures of microfluidic devices.

This work deals with fabrication and test of microfluidic chip for separating substances. For separation of substances is used electrophoresis and detection is by integrated electrodes.

The diploma thesis deals with cellular membrane model preparation on microfluidic devices. It summarizes means of microfluidic device fabrication, phospholipid bilayer formation mechanisms, optimization techniques and characterization methods of those systems. It focuses on free-standing planar lipid bilayers which are easily accessible by a number of different characterization methods and at the same time exhibit good stability and variability. The aim of this work is to design and prepare a microfluidic chip on which a planar lipid bilayer can be prepared. It therefore presents microfluidic device prepared by soft lithography of PDMS adapted for model membrane formation by self-assembly of phospholipids at the interface of aqueous and organic phases created by the architecture of the microfluidic device. Formation of the model membrane was visualized by optical microscopy and fluorescence-lifetime imaging microscopy.

The goal of this study is to characterize the mixing that occurs in a microfluidic chip. To characterize the mixing, the minimum length to complete mixing and evolution of mixing will be investigated. There are two types of mixing that occur within a microfluidic channel, diffusion and advection. In the beginning of the microfluidic chip, diffusion is the dominant form of mixing, and in the later portion of the microfluidic chip advection is the dominant form of mixing. The type of design used for this experiment was a zig zag geometry microfluidic chip with channel dimensions of 60 µm X 500 µm X 522 mm. The minimum length for complete mixing was 361 ± 3.475 µm at a flow rate of 25 mL/hr. The mixing was measured using optical light microscopy. For all flow rates less than 20 mL/hr the flow rate was too low to mix the two fluids. The pressure produced by the 30 mL/hr flow rate caused the microfluidic chip to fail.

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