Date of Award

8-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Xiangchun Xuan, Committee Chair

Committee Member

Phanindra Tallapragada

Committee Member

Gang Li

Abstract

Microfluidic devices have become a powerful tool for chemical and biological applications in the past two decades due to their several advantages over the conventional benchtop counterparts such as smaller size, reduced reagent consumption and higher efficiency. Manipulations of samples and electrolytes, for instance, sorting, trapping, and mixing etc., are important in these applications. Electrokinetics is an efficient method for performing these manipulations by the application of electric field. It is the preferred method of sample transport over the traditional pressure driven flow due to the essentially plug-like velocity profile that has smaller dispersion effects. However, the majority of the previous works on electrokinetic phenomena is limited to inside the microchannel. Very few studies have been reported on electroosmotic fluid entry from reservoir to microchannel. Due to the mismatch in sizes of the macro-scale reservoir and the micro-scale channel, several electrokinetic phenomena such as dielectrophoresis (DEP), Joule heating effects and induced charge electroosmosis (ICEO) are prominent at their interface. This thesis tries to provide a fundamental insight on the effects of Joule heating and ICEO on electroosmotic entry flows through experimental and numerical studies.

Joule heating is inevitable in an electrokinetic system especially when the solution’s electrical conductivity is high and/or a low-thermal-conductivity material such as polydimethylsiloxane (PDMS) is used for manufacturing the device. In such cases the electrolyte temperature increases which in turn affects the temperature dependent fluid properties. This leads to the formation of electrothermal flows which under certain conditions can disrupt the flow. We performed an experimental and numerical study of Joule heating effects on electroosmotic flow at the entrance of the microchannel. We observed that two counter rotating circulations can be formed under appropriate conditions. We also did a parametric study and found that the vortex becomes stronger with increasing AC to DC voltage ratio. Moreover we developed a two dimensional depth-averaged numerical model which is able to predict the Joule heating and its effects on electrokinetic phenomenon with reasonable accuracy. This model does not require the high memory or processing requirement as a three dimensional model, nor does it have any unrealistic assumptions as the previous two dimensional model.

ICEO is due to the leakage of electric field through the substrate around the microchannel. It is strong in low-conductivity solutions wherein Joule heating effects can be neglected. We conducted a combined experimental and numerical study of ICEO effects on electroosmotic entry flow in such a situation. We found that, similar to electrothermal flows, ICEO has two counter rotating circulations but are rotating in opposite directions. We also pointed out that a regular two dimensional model cannot accurately predict the flow field in this phenomenon. We therefore developed a depth averaged two-dimensional model, which predicted the experimentally obtained flow field with a good agreement. This numerical model was used to explain the formation of induced zeta potential on the channel surface due to electric field leakage. The effects of ICEO on fluid flow and particle motion at the reservoir-microchannel junction are discussed. Moreover, parametric study is done to further understand the effects by changing the AC to DC ratio, relative permittivity of polymer substrate and the channel width.

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