Fundamentals Of Fluids Transport In Microfluidics And Nanofluidics Systems

Mircofluidics and nanofluidics have emerged as powerful technologies with many established and relevant applications (e.g. chemistry, biology and clinical diagnosis). However, the complexity in different applications induces many challenges for microfluidics and nanofluidics. Since the mass transport in micrometer and nanometer scales determines the chips design and applications, manipulating such mass transport in micrometer and nanometer scales becomes the bottleneck in developing the microfluidics and nanofluidics. Especially, when the scale of chips is decreased to nanometer scales, interesting phenomena could appear which are not existed in macroscale systems. Therefore, exploring and understanding of these phenomena would helpful to the development of microfluidics and nanofluidics. The present thesis focuses on hybrid-chip electrophoresis, flow injection analysis mininaturation and theory of bienzyme coupling reaction.1. Study on the Influence of Cross-Sectional Area and Zeta Potential on Separation for Hybrid-Chip-Based Capillary Electrophoresis Using 3D SimulationsHybrid chips combing microchips with capillaries have displayed particular advantages in achieving UV-vis and mass spectroscopic detection. In this work, systematic 3-D numerical simulations have been carried out to explore the influence of junction interface cross-sectional area and ζ-potential distribution on sample band broadening in hybrid-chip electrophoresis separation. In this case, the ratio of cross-sectional area of chip to capillary channel (Sratio) is used as the parameter of the variation in junction interface cross-sectional area. Theoretical simulations demonstrated that decrease of the Sratio would increase the separation efficiency in the hybrid-chip based capillary electrophoresis (CE) with uniform ζ-potential distribution. For the problem of ζ-potential distribution along the axial direction of the channel also affect mass transport in hybrid-chip based CE. Therefore, the effect of ζ-potential distribution has been considered in the 3D simulation. Theoretical simulation results reveal that ζ-potential distribution rather than the interface cross-sectional area variation (Sratio) controls the sample band broadening and manipulates sample separation efficiency in the hybrid-chip based capillary electrophoresis (CE) with non-uniform ζ-potential distribution. Both the theoretical simulations and experimental results show that optimal hybrid-chip CE separation efficiency can be achieved at Sratio=1.2. Modeling study of the diffusion and reaction kinetics on a micro flow injection analysis systemA finite element method was developed for micro-flow analysis system (FIA) based on the gravity driving force. For reducing the computation burden, the numerical model built on the manifold of micro-FIA was meshed in different scale. The simulation results revealed that diffusion played an important role in mixing on a micro-FIA system and also demonstrated that the Taylor dispersion predominated in the micro-FIA system. For simplicity, the pseudo-first-order chemical reaction kinetics was used to modeling the contribution of chemical reaction kinetics to the dispersion under nonequilibrium conditions. With the rate coefficient increasing, the concentration profiles of product in pseudo-first-order reaction decreased. In addition, the effect of velocity and injection volume was simulated in the numerical model. The results are good agreement with the reported results. At the same time, the simulation results demonstrated Taylor-Aris diffusion induced nonlinear phenomena for the pseudo-first-order reaction at low velocity, which has been verified by the experimental results. The FEM model can facilitate understanding the mechanism in micro-FIA and optimize the manifold of micro-FIA system.3. Modeling study of kinetics in dual-enzyme coupling catalytic reactionsEnzymes play central roles in biochemical processes, they catalyze hundreds of reactions that degrade the nutrient molecules, conserve and transform the chemical energy. However, almost all reported investigations for multi-enzymes coupling catalytic reactions are only treated as integrated systems. In this part, dual-enzyme coupling catalytic reaction has been investigated from different procedures of reaction by using the numerical model based on experiment results. At first, the coupling catalytic reaction with acetylcholinesterase (AchE) and choline oxidase (ChO) is investigated by using scanning electrochemical microscopy (SECM) combined with a dual-electrode probe. Numerical model simulated the process of coupling catalytic reaction based on the experiment results and obtained the kinetic parameter of AchE (KmAchE=l.lmM). The simulated results are in agreement with experiment ones. In addition, we further developed a numerical model for investigating the coupling reaction in microchannels. The optimal gap distance for dual-enzyme is obtained from the simulated results.4. Primary study for transport phenomena of fluids in circle nanochannelsThe finite element method (FEM) has been applied to study the behavior of electrical double layer (EDL) with different buffer concentration (1-80 Mm) in 20 and 200 nm circle nanochannels. The new mesh technique is used in simulation for obtaining high resolution results. The simulated results show that continuum theory can still be applied reasonably well in nanochannel with diameter larger than 20 nm. The results of theory formulation for EDL thickness in circle nanochannels are in good agreement with simulated results. It demonstrated that approximate treatment in theory has no influence on the results. Based on the solutions of Possion-Boltzmann equation, electroosmotic flow is calculated in the numerical model. By comparing electroosmotic flow velocities in two nanochannels, a conclusion can be derived that the electroosmotic flow velocity profile is influenced remarkably by EDL thickness in nanochannels.