Degree Candidacy: Doctor of Philosophy in Mechanical Engineering
Date: Tuesday, May 13, 2014
Time: 10:00 AM
Location: 132 EIB
Advisor: Dr. Xiangchun Xuan
Committee Members: Dr. John R. Saylor, Dr. Richard Miller, and Dr. Rui Qiao
Title: Microfluidic particle and cell manipulation using reservoir-based dielectrophoresis
The ability to manipulate synthetic particles and biological cells in a complex mixture is important to a wide range of applications in biology, environmental monitoring, and the pharmaceutical industry etc. In the past two decades microfluidics has evolved to be a very useful tool for particle and cell manipulations in miniaturized devices. A variety of force fields have been demonstrated to control particle and cell motions in microfluidic devices, among which electrokinetic technique is most often used; however, to date, studies of electrokinetic transport phenomena have been primarily confined within the area of the microchannels. Very few works have addressed the electrokinetic particle motion which occurs at the reservoir-microchannel junction. This region acts as the interface between the macro (i.e., reservoir) and the micro (i.e., microchannel) worlds in real microfluidic devices. This dissertation is dedicated to the study of electrokinetic transport and manipulation of particles and cells at the reservoir-microchannel junction of a microfluidic device using a combined experimental, theoretical, and numerical analysis.
First, we performed a fundamental study of particles undergoing electrokinetic motion at the reservoir-microchannel junction. The effects of AC electric field, DC electric field, and particle size on the electrokinetic motion of particles passing through the junction were studied. A two-dimensional numerical model using COMSOL 3.5a was developed to investigate and understand the particle motion through the junction. It was found that particles can be continuously focused and even trapped at the reservoir-microchannel junction due to the effect of reservoir-based dielectrophoresis (rDEP). The electrokinetic particle focusing increases with the increase in AC electric field and particle size but decreases with the increase in DC electric field. It was also found that the larger particles can be trapped at lower electric fields compared to their smaller counterparts.
Next, we utilized rDEP to continuously separate particles with different sizes at the reservoir-microchannel junction. The separation process utilized the inherent electric field gradients formed at the junction due to the size difference between the reservoir and the microchannel. It was observed that the separation efficiency was reduced by inter-particle interactions when particles with small size differences were separated. The effect of enhanced electrokinetic flow on the separation efficiency was investigated experimentally and was observed to have a favorable effect. We also utilized the rDEP approach to separate particles based on surface charge. Same sized particles with differences in surface charge were separated inside the microfluidic reservoir. The streaming particles interacted with the trapped particles and reduced the separation efficiency. The influences from the undesired particle trapping have been found through experiments to decrease with a reduced AC field frequency.
Then, we demonstrated a continuous microfluidic separation of live yeast cells from dead cells using rDEP. Because the membrane of a cell gets distorted when it loses its viability, a higher exchange of ions results from. The increased membrane conductivity of dead cells leads to a different Claussius-Mossoti factor from that of live cells, which enables their selective trapping and continuous separation based on cell viability. A two-shell numerical model was developed to account for the varying conductivities of different cell layers, the results of which agree reasonably with the experimental observations. We also used rDEP to implement a continuous concentration and separation of particles/cells in a stacked microfluidic device. This device has multiple layers and multiple microchannels on each layer so that the throughput can be significantly increased as compared to a single channel/single layer device.
Finally, we compared the two-dimensional and three-dimensional particle focusing and trapping at the reservoir-microchannel junction using rDEP. We observed that the inherent electric field gradients in both the horizontal and vertical planes of the junction can be utilized if the reservoir is created right at the reservoir-microchannel junction. Three-dimensional rDEP utilizes the additional electric field gradient in the depth wise direction and thus can produce three-dimensional focusing. The electric field required to trap particles is also considerably lower in three-dimensional rDEP as compared to the two-dimensional rDEP, which considerably reduces the non-desired effects of Joule heating. A three-dimensional numerical model which accounted for the entire microfluidic device was also developed to predict particle trajectories.