Studies Of Some Key Issues On Transport Phenomena In Electrical Field-Driven Microfluidic Chip

The microfluidic chip, one of the Micro-Electro-Mechanical Systems (MEMS) or Miniaturized Total Analysis System (μ-TAS), can integrate many functions of a conventional lab (including sample-getting, diluting, reaction, sample separation, as well as detection etc.) into a chip with an area of only several square centimeters. It has the advantages of automated, fast, and low reagent consumption. This kind of chip has a great potential in the processes of Polymer-enzyme Chain Reaction (PCR), nucleic acid analysis, gene mutation detection, analysis of DNA and amino acid, as well as the analysis and detection of single cell.Although the final goal for microfluidic chips are to be used as practical products, however, because of the high manufacture cost, the studies on transport phenomena (such as fluid flow, particles'separation, and heat transfer) in these microfluidic chips play very important roles in enhancing experimental efficiency, decreasing the experimental cost, as well as optimizing the chip structure. At present, microfluidic chips, which are driven by electrical field and based on the theory of electrohydrodynamics, are mainly used. There are two types of electrical field-driven methods in microfluidic chips: DC electrical field-driven technique and AC electrical field-driven technique. Both of these techniques involve the fluid driven mechanism of electroosmosis, and the particles'driven mechanism of electrophoresis. Also, because of the imposed electrical field, the Joule heating effect occurs in the flow field that led to non-isothermal temperature distribution. Based on the principles of electrohydrodynamics and transport principles, and by using numerical simulation and experimental methods, some studies of key issues on transport phenonema in electrical field-driven microfluidic chips have been carried out and several novel designs for the fluid control in microfuidic chips, such as electrothermal flow enhanced micromixer and electrothermal flow pump, have been developed in this thesis. Finally, the microfluidic chips for the dielectrophoresis experiment are also made through the micro-machine technique, and the dielectrophoresis experiments are performed by using these chips, which also validated the simulation results for the dielectrophoresis spectrum analysis.The main topics studied in this thesis are as follows:Some key problems based on the DC electrokinetics are studied. These include: (i) DC Electroosmosis Flow: Develop a new method to control the DC electroomosis field. The effect of induced zeta potential, which is caused by adding polarizable mental membrane on the microchannel wall, on the electroosmosis field is analyzed, and the effects.are compared with the effect caused by changing the zeta potential through changing the wall's surface characteristics. It is found the induced zeta potential is not constant and the induced charge electroosmosis flow has a more significant effect on disturbing the DC electroosmotic flow flied. (ii)DC Electrophoresis: A numerical model, which can be used to predict the sample stacking process in a microfluidic chip with a double T microchannel, is developed. The impacts factors, such as the charge, concentration ratio of the buffer, as well as the initial length of the sample plug, on the stacking processes are analyzed. It is found that a better stacking effect can be obtained from a higher buffer concentration ratio and longer initial sample plug length. (iii)Joule Heating Effect Caused by the DC Electrical Field: The temperature distribution in PDMS microfluidic chip is computed, and its impact such as the buffer concentration, chip material and thickness, on the temperature distribution are also analyzed. Joule heating effects on the sample band transport velocity and electrophoresis separation efficiency in PDMS microfluidic chips are inferred from the results in capillary. The methods to decrease the Joule heating effect in PDMS microfluidic chips are also discussed. It is found that lower buffer concentration, smaller chip thickness, and chip materials with larger thermal conductivity can decrease the Joule heating effect.The key problems based on the AC electrokinetics studied in this thesis are: (i) AC Electroosmosis Flow: A model for AC electroosmosis (ACEO) pump is developed to compute the potential and AC electroosmotic velocity. The impacts of frequency and electrolyte concentration on the ACEO pump velocity are studied. It is found that the frequency at which the optimal ACEO velocity appears depends on the charge relaxation time of the electrolyte. A RC circuit is developed by analyzing the capacitance characteristic of electrical double layer and resistance characteristic of electrolyte solution, and the impedance spectra of the ACEO pump is analyzed with different electrodes widths, channel heights and electrolyte concentrations. It is found that the smaller impedance value occurs at the condition of larger electrode widths, larger channel heights and larger electrolyte concentration. (ii)Dielectrophoresis: The dielectrophoresis model for the polystyrene (PS) particles and cells are developed, and the dielectrophoresis spectrums for these particles are analyzed. The levitation heights for the PS particles in nDEP process in microfluidic chip with interdigitated electrodes is computed by considering the dielectrophoretic forec, net gravity force, as well as the Stokes drag force caused by the electrothermal flow. It is found that the Stokes drag force is the dominant factor for the particles'levitation height for small particles, especially at high electrical field. (iii) Joule Heating Effect Caused by the AC Electrical Field (Electrothermal Flow):A novel micromixer based on the vortex caused by the electrothermal flow near the microelectrodes is proposed, the impact of electrode arrangement style, magnitude and frequency of the AC potential on the mixing efficiency are also studied. It is found that this kind of micromixer has a good mixing efficiency. A new kind of electrothermal flow micropump, based on the electrothermal flow in microchannel with asymmetrical interdigitated microelectrodes is proposed, and the impacts of the electrodes width ratio, electrolyte concentration and chip materials on the velocity of this pump are studied. It is found that the electrothermal flow pump can work in a wider frequency than the ACEO pump. Chinese patents for the electrothermal flow enhanced micromixer and the electrothermal pump have been applied.The experimental and numerical simulation work on the dielectrophoresis in microfluidic chip include: (i) Microfluidic chips with different electrode structures are fabricated for the DEP experiments by using micromachine technique. Both positive and negative dielectrophoresis experiments for PS particles are performed, and the dielectrophoresis spectrum analysis results are also validated by experimental data; and (ii) The dielectrophoresis microfluidic chip with 3D columns on the planar interdigitated microelectrodes are developed and manufactured, and a better capability on attracting particles in pDEP are obtained, which agrees with the prediction well.This study covers some key issues on transport phenomena in electrical field-driven microfluidic chips, they are interdisciplinary in nature, which also involve multi-scale and multi-physics fields. Both numerical simulation and experimental investigation have been carried out for these problems. The models and results in this thesis are useful to enhance the present understanding of transport phenomena and complement the lack of model development and theory analysis in electrical field-driven microfluidic chips, and are helpful for the design and optimization of the microfluidic chip. The noval fluid control methods proposed in this thesis are also helpful for the continuing work about microfluidic chips.