Date of Award
8-2025
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Mechanical Engineering
Committee Chair/Advisor
Dr. Xiangchun Xuan
Committee Member
Dr. Yuhao Xu
Committee Member
Dr. Zhen Li
Abstract
Electrokinetic phenomena have been widely used for fluid pumping and particle manipulation in micro/nanofluidic systems. They refer to the motion of fluids or particles in response to an electric field because of the spontaneous formation of an electric double layer at the fluid-solid interface. Understanding electrokinetic phenomena in micro/nanochannels is critical for a wide range of applications across various engineering fields, such as disease diagnostics, contaminant treatment and cell analysis. To date, numerous studies have been reported on electrokinetic phenomena in Newtonian fluids, where the classical Helmholtz-Smoluchowski theory indicates a linear relationship between fluid electroosmosis/particle electrophoresis and electric field. However, in practice, most biological and chemical fluids exhibit a shear-thinning behavior, whose effect on electrokinetic phenomena has not been fully understood. This thesis presents a comprehensive computational study of nonlinear electrokinetic phenomena in shear-thinning fluids and compares the modeling results with experimental measurements in xanthan gum polymer solutions through a straight rectangular microchannel.
In the first part of this thesis, we focused on the nonlinear electrokinetic motion of a particle. An axisymmetric numerical model was developed using a fluid–particle interaction approach to simulate the motion of a microparticle in a cylindrical channel filled with shear-thinning fluids. This model incorporated a polymer depletion layer (PDL) near the channel wall to account for the polymer-induced drag reduction effect, which has been reported in previous studies to cause a significant underprediction of electroosmotic velocity. It predicts that the electroosmotic, electrokinetic, and electrophoretic velocities in a shear-thinning fluid each exhibit a nonlinear dependence on the imposed electric field. Among these three velocities, electrophoresis has the highest nonlinear index with respect to the electric field due to the curvature of the particle surface. Furthermore, increasing polymer concentration enhances the fluid shear-thinning effect and hence, the nonlinearity of each electrokinetic phenomenon. All these predicted trends are found to be consistent with our experimental data on polystyrene microparticles in xanthan gum solutions through a straight rectangular microchannel. However, the predicted electric field dependences of electrophoresis and electroosmosis are both noticeably weaker than the experimental measurements.
The second part of this thesis focused on revising the PDL-based numerical model in the first part to improve further the prediction of the nonlinear electric field dependence of electroosmotic flow in shear-thinning fluids. We observed that the presence of PDL, which contains a Newtonian fluid, tends to diminish the shear-thinning effect and underestimate the nonlinearity of the electric field. To address this issue, we proposed a new model that incorporates a Navier slip condition on the walls of a straight rectangular microchannel. This model considers the polymer-induced drag reduction by the use of a slip length on the walls while treating the entire fluid as shear thinning. It offers a better prediction of both the experimentally obtained electroosmotic velocity and its electric field nonlinearity than the PDL model.
Recommended Citation
Leung, Wai Yuen, "Computational Study of Nonlinear Electrokinetic Phenomena in Shear-thinning Fluids" (2025). All Theses. 4567.
https://open.clemson.edu/all_theses/4567