Date of Award

8-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Dr. Xiangchun Xuan, Committee Chair

Committee Member

Dr. Xin Zhao

Committee Member

Dr. Ethan Kung

Abstract

Microfluidic devices have been increasingly used for diverse particle manipulations in various chemical and biological applications. Fields such as water quality control, environmental monitoring and food safety require the continuous trapping and concentration of particles (either bio- or non-bio) for enhanced detection and analysis. To achieve this, various microfluidic techniques have been developed using electric field as well as other fields including magnetic, optical, acoustic, hydrodynamic, gravitational and inertial. Among these methods, electrokinetic manipulation of particles is the most often used due to its advantages over other methods such as simple operation and easy integration etc. It transports fluids and controls the motion of the suspended particles via electroosmosis, electrophoresis and dielectrophoresis. However, there is an inevitable phenomenon accompanying electrokinetic devices, i.e., Joule heating due to the passage of electric current through the conductive suspending medium. Previous studies indicate a negative impact of Joule heating on the trapping and concentration of micron-sized particles in insulator-based dielectrophoretic microdevices. We demonstrate in this thesis that the Joule heating-induced electrothermal flow can actually enhance the electrokinetic manipulation, leading to the otherwise impossible trapping and concentration of submicron particles in ratchet microchannels. We fabricated ratchet microchannels with polydimethylsiloxane and used them to study the transport and control of submicron particles in a moderately conductive phosphate buffer solution. Our research group did the experiments previously. We developed a numerical multiphysics depth average model, which can predict the observed particle trapping in the ratchet region. The numerical model consists of coupled electric current, fluid flow, heat transfer and mass transport equation. A depth average analysis of these governing equations was done to develop a 2D model on the horizontal plane of the microchannel, which gives us numerical results that are as good as a full-scale 3D model developed previously, but with much less computational resources. Numerical analysis of the developed model predicts the formation of two counter rotating electrothermal vortices at the ratchet tips. Moreover, particles can be seen trapped inside these vortices and the concentration of particles trapped in electrothermal vortices can be observed to increase with time. Further, on doing the parametric study we found out that with increase in voltage the size of these vortices increases. We also changed the shape of the ratchet, but that does not seem to affect particle trapping in a significant manner. These obtained numerically predicted results are found to be in good agreement with our experimental observations, which further validates our numerical modelling.

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