Date of Award

5-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Xuan, Xiangchun

Committee Member

Miller , Richard

Committee Member

Li , Gang

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 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 applied electric field, channel structure, channel depth and number of constrictions.

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