Date of Award

Fall 12-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical & Aerospace Engineering

Program/Concentration

Mechanical Engineering

Committee Director

Xiaoyu Zhang

Committee Member

Venkat Maruthamuthu

Committee Member

Tianbing Xu

Committee Member

Yan Peng

Abstract

Electroosmotic flow (EOF) has been widely used in various biochemical microfluidic applications, many of which often involve the use of viscoelastic non-Newtonian fluids. Due to the existence of the elastic effect, the viscoelastic EOF develops into chaotic flow under extremely low Reynolds numbers, which is known as elastic turbulence. The mechanism of elastic turbulence in electroosmotic flow remains unclear. Numerical simulation plays an important role in understanding the mechanisms of elastic turbulence. This dissertation is aimed to study the EOF of viscoelastic fluids in constriction microchannels under various direct current (DC) and alternating current (AC) electric fields. First, the EOF of viscoelastic fluid in a straight contraction microchannel is investigated. The influences of the polymer concentration and the applied DC electric field on the elastic instabilities are analyzed. The flow fluctuations and secondary upstream vortices before the entrance of the microchannel are found to be related to the induced elastic stress within the microchannel. The polymer concentration shows a more significant influence on the elastic instability. A flow map in polymer concentration and electric field domain is formed as guidance for further studies.

Then, the study is extended to the viscoelastic EOF in a microchannel with 90◦ bends under the combination of DC and AC electric fields. The elastic turbulence is identified from the fluctuation of the velocity field and upstream vortices. The energy spectra of the velocity fluctuation show power-law decay over a wide range of frequencies, which is a typical characteristic of elastic turbulence. The 90◦ bends show influence on the dye concentration profile in cross sections of the microchannel. A more even dye concentration distribution is obtained with an increasing number of 90◦ bends. Moreover, the opening angle of the particle trace at the exit of the contraction microchannel show dependency on the frequency of the AC electric field, which is related to the characteristic frequency of the viscoelastic EOF.

The study is then focused on the influence of the frequency of the AC electric field on the viscoelastic EOF. Short contraction microchannels are adopted for the frequency study. The peak in the energy spectra of the velocity fluctuation under DC electric field indicates the characteristic frequency of the viscoelastic EOF. Under AC electric field, the highest amplitude of the energy spectra is obtained when the frequency of AC electric field is close to the characteristic frequency. The same trend is also observed in the statistical results of the average velocity. However, when the frequency is relatively high, both the amplitude of the energy spectra and the average velocity decrease to a level even lower than under a DC electric field, which indicates the existence of an optimal frequency of the AC electric field in order to achieve the highest flow rate.

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Copyright, 2022, by Jianyu Ji, All Right Reserved.

DOI

10.25777/htx1-vb63

ISBN

9798371976048

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