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JOULE HEATING EFFECTS on ELECTROKINETIC TRANSPORT in CONSTRICTION MICROCHANNELS Sriram Sridharan Clemson University, [email protected]

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JOULE HEATING EFFECTS on ELECTROKINETIC TRANSPORT in CONSTRICTION MICROCHANNELS Sriram Sridharan Clemson University, Srirams@G.Clemson.Edu Clemson University TigerPrints All Theses Theses 5-2011 JOULE HEATING EFFECTS ON ELECTROKINETIC TRANSPORT IN CONSTRICTION MICROCHANNELS Sriram Sridharan Clemson University, [email protected] Follow this and additional works at: https://tigerprints.clemson.edu/all_theses Part of the Mechanical Engineering Commons Recommended Citation Sridharan, Sriram, "JOULE HEATING EFFECTS ON ELECTROKINETIC TRANSPORT IN CONSTRICTION MICROCHANNELS" (2011). All Theses. 1061. https://tigerprints.clemson.edu/all_theses/1061 This Thesis is brought to you for free and open access by the Theses at TigerPrints. It has been accepted for inclusion in All Theses by an authorized administrator of TigerPrints. For more information, please contact [email protected]. JOULE HEATING EFFECTS ON ELECTROKINETIC TRANSPORT IN CONSTRICTION MICROCHANNELS A Thesis Presented to The Graduate School of Clemson University In Partial Fulfillment of the Requirements for the Degree Master of Science Mechanical Engineering by Sriram Sridharan May 2011 Accepted by: Dr. Xiangchun Xuan, Committee Chair Dr. Richard Miller Dr. Gang Li ABSTRACT Microfluidic technology involving multidisciplinary studies including MEMS, chemistry, physics, fluids and heat transfer has been developed into a promising research field in the recent decade. If offers many advantages over conventional laboratory techniques like reduced reagent consumption, faster analysis, easy fabrication and low chemical waste. Microfluidic lab-on-a-chip devices have been used to manipulate cells and particles like sorting, separating, trapping, mixing and lysing. Microfluidic manipulation can be achieved through many methods and insulator based dielectrophoresis (iDEP) is one of the highly used method in the recent years. In iDEP, both DC and AC voltages can be applied to the remote electrodes positioned in end-channel reservoirs for transporting and manipulating particles. The electric field gradients are caused by the blockage of electric current due to in-channel hurdles, posts, and ridges. However, iDEP devices suffer from the issue of Joule heating due to locally amplified electric field around the insulators. A parametric study of Joule heating effects on electroosmotic fluid flow in iDEP is studied under various electric fields. It was determined that depending upon the magnitude of DC voltage, a pair of counter rotating vortices fluid circulations can occur at either downstream end or each end of the channel constriction. Moreover, pair at the downstream end appears larger in size than the upstream end due to DC electroosmotic flow. A numerical model is ii developed to simulate the fluid circulations occurred due to the action of electric field on Joule heating induced fluid inhomogeneities in the constriction region. Focusing particles or cells into a single stream is usually a necessary step prior to counting and separating them in microfluidic devices such as flow cytometers and cell sorters. A systematic study of Joule heating effects on electrokinetic particle transport in constriction microchannels under DC and DC biased AC electric fields is presented in this work. A numerical model is developed to capture the particle trace observed in the experiments. It was determined that particle transport is greatly affected by electrothermal effects where Joule heating is high. At very low DC magnitude where the electrothermal effects dominate the electrokinetic flow, particles in the shallow depth channel are being trapped and particles in deep channels are transported to the downstream reservoir from the constriction in a single streamline. Electrothermal flow circulations should be taken into account in the design and operation of iDEP devices, especially when highly conductive solutions and large electric fields must be employed. They may potentially be harnessed to enhance microfluidic mixing and immunoassay for lab-on-a-chip applications. A numerical study of Joule heating effects on the sample mixing performance in constriction microchannels is presented in this work. It was determined that Joule heating induced electrothermal force enhanced the sample mixing by generating circulations at the ends of the constriction under DC biased AC electric fields. Furthermore, mixing performance was also studied for various parameters like iii applied electric field, channel structure, channel depth and number of constrictions. iv ACKNOWLEDGEMENTS I would like express my sincere gratitude to my advisor Dr. Xiangchun Xuan for his continuous support and valuable guidance throughout my graduate studies. His thoughtful guidance has helped me in the research and writing this thesis. I am thankful for the grading assistantships he provided to fund my living expenses. I am also thankful to my committee members Dr. Richard Miller and Dr. Gang Li for their valuable inputs. Besides my advisor, I would like to thank Junjie Zhu for his valuable information he gave me about the experiments and helping me in learning the COMSOL multiphysics software tool. I also thank my fellow mates in the research group: Litao Liang, Bhaskar Ramesh, Saurin Patel, Amit Srivatsva and Srijeeth Sathyamoorthy who have been supportive in my work. Finally, I would like to thank my family for their continual love and support during my graduate career. v TABLE OF CONTENTS TITLE PAGE……………………………………………………………………………...i ABSTRACT………………………………………………………………………………ii ACKNOWLEDGEMENTS………………………………………………………………v LIST OF TABLES……………………………………………………………………..…x LIST OF FIGURES…………………………………………………………………..…xi NOMENCLATURE…………………………………………………………………….xiv CHAPTER 1 INTRODUCTION………..…………………………………………………………....1 1.1 Objective and motivation…………………………………………………………..1 1.2 Electrokinetic phenomena..………………………………………………………..3 1.2.1 Electrical double layer……………………………………………………4 1.2.2 Electroosmosis……………………………………………………………5 vi 1.2.3 Electrophoresis……………………………………………………………6 1.2.4 Dielectrophoresis…………………………………………………………7 1.3 Concept of Joule heating…………………………………………………………..8 1.4 Electrothermal body force………………………………………………………….9 2 JOULE HEATING EFFECTS ON ELECTROOSMOTIC FLUID FLOW IN CONSTRICTION MICROCHANNELS............................................. 11 2.1 Introduction……………………………………………………………………….. 11 2.2 Experimental setup………………………………………………………………..13 2.3 Theory………………………………………………………………………………14 2.3.1 Electrical equation……………………………………………………... 15 2.3.2 Energy equation…………………………………………………………16 2.3.3 Flow field equation ……………………………………………………..17 2.4 Numerical method and boundary conditions…………………………………...18 2.4.1 Conductive media DC…………………………………………………..19 2.4.2 Convection and Conduction…………………………………………...19 2.4.3 Incompressible Navier-Stokes…………………………………………20 2.4.4 Temperature dependent properties…………………………………...20 2.5 Results and discussion…………………………………………………………...23 2.5.1 Effect of DC biased AC voltages………………………………………24 2.5.2 Effect of DC voltages…………………………………………………...29 2.5.3 Effect of constriction size……………………………………………….32 vii 2.6 Summary…………………………………………………………………………...36 3 JOULE HEATING EFFECTS ON ELECTROKINETIC PARTICLE TRANSPORT IN CONSTRICTION MICROCHANNELS……………..………... 37 3.1 Introduction……………………………………………………………………….. 37 3.2 Experimental setup………………………………………………………………. 39 3.3 Theory………………………………………………………………………………40 3.3.1 Dielectrophoretic force………………………………………………… 40 3.4 Numerical modeling……………………………………………………………… 42 3.5 Results and discussion………………………………………………………..… 44 3.5.1 Particle motion at various DC voltages……………………………….44 3.5.2 Particle motion at various DC biased AC voltages……………….…48 3.6 Summary…………………………………………………………………...54 4 JOULE HEATING EFFECTS ON ELECTROKINETIC SAMPLE MIXING IN CONSTRICTION MICROCHANNELS………………………….…...53 4.1 Introduction………………………………………………………………………...53 4.2 Theory…………………………………………………………………………...... 55 4.2.1 Convection diffusion equation………………………………………... 56 4.3 Numerical modeling and boundary conditions………………………………... 57 4.3.1 Conductive media DC…………………………………………………..58 4.3.2 Convection and convection…………………………………………….59 4.3.3 Incompressible Navier-Stokes...……………………………………... 59 viii 4.3.4 Convection and diffusion …………………………………………….. 60 4.3.5 Temperature dependent properties…………………………………...60 4.4 Results and discussion………………………………………………………….. 60 4.4.1 Effect of channel depth……………………………………………….. 64 4.4.2 Effect of channel structure……………………………………………. 66 4.4.3 Effect of constriction numbers…………………………………………68 4.4.4 Effect of electric field……………………………………………………69 4.5 Summary…………………………………………………………………………...72 5 CONCLUSIONS AND FUTURE WORK…………………………………………..73 REFERENCES…………………………………………………………………………77 ix LIST OF TABLES Tables Page Table 2-1: List of constants used in the numerical simulation of joule heating effects on electroosmotic fluid flow………………..... 21 Table 3-1: List of constants used in the numerical simulation of joule heating effects on electrokinetic particle transport……...….. 43 Table 4-1: List of constants used in the numerical simulation of joule heating effects on electrokinetic sample mixing……………. 60 x LIST OF FIGURES Figures Page Fig. 1-1: Electrokinetic mechanisms ……………………….…………………………4 Fig. 1-2: Illustrates the formation of electrical double layer ………………………..5 Fig. 1-3: Electroosmotic flow and the velocity profile in a slit microchannel when an electric field is applied ……………….……... 6 Fig. 2-1: Picture of the constriction microchannel with dimensions labeled in the inset…………………………………………...14 Fig. 2-2: Zoom-in top view of the meshed constriction region of the microchannel…………………………..…………………… 19 Fig. 2-3: Comparison of simulation and experimental results
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