Date of Award
8-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
Committee Chair/Advisor
Dr. Xiangchun Xuan
Committee Member
Dr. Rodrigo Martinez-Duarte
Committee Member
Dr. Joshua Bostwick
Committee Member
Dr. Phanindra Tallapragada
Abstract
Classical electrophoretic theory describes the linear response of particles suspended in Newtonian fluids under weak electric fields. When the electric field exceeds this weak limit, a nonlinear response arises due to non-uniform surface conduction over the curved particles. This nonlinear behavior introduces dependencies on fluid and particle properties not considered in the classical theory, offering potential advantages for microfluidic particle manipulation. While this phenomenon has been extensively studied theoretically and numerically in Newtonian fluids, experimental investigations have been relatively limited.
Fluid rheology can significantly alter particle dynamics under applied electric fields. Rheological properties like shear-thinning and elasticity which are not considered in classical electrophoretic theory, play crucial roles. Shear-thinning behavior, for instance, can be intuitively predicted to exhibit a nonlinear response to electric fields, affecting both fluid and particle behavior. Theoretical predictions suggest that non-Newtonian fluids can demonstrate nonlinearity even within the weak electric field limit and can deviate from the typical plug-like velocity profile seen in Newtonian electroosmotic flows. Investigating this behavior is essential due to the prevalence of non-Newtonian fluids in various applications, especially in analytical chemistry and microbiology. As with high electric field-induced nonlinear electrophoresis, despite substantial theoretical and numerical research, experimental studies in this area have been lacking. Addressing this gap in experimental research is crucial for fully understanding and leveraging the potential of nonlinear electrophoretic phenomena in practical applications.
This dissertation aims to address the limited experimental focus on nonlinear electrokinetics arising from strong electric fields or fluid rheology. By exploring these areas experimentally, the findings will enhance the understanding of fluid and particle interactions, leading to more accurate theoretical and numerical predictions. Ultimately, this research seeks to bridge the gap between theories and practical applications by providing valuable insights for advancing microfluidic particle manipulation techniques.
The study begins with a systematic experimental investigation into the isolated effects of buffer concentration, particle size, and particle zeta potential on the electrophoretic velocity of polystyrene particles in a straight rectangular microchannel under a wide range of electric fields. The findings highlight that particle zeta potential significantly influences nonlinearity. It is well-established that in Newtonian fluids, the classical electrophoretic velocity of a charged particle is independent of its size under the thin-Debye-layer limit. However, our observations, consistent with theoretical predictions, indicate that particle size affects behavior in the nonlinear regime. This discovery led to further investigation into the effect of particle geometry, which is particularly relevant for studying microorganisms or silica particles in various media. Existing theories on nonlinear electrophoresis primarily focus on spherical particle models, with no reported experiments on the influence of particle shape. To address this gap, we conducted an experimental study comparing the nonlinear electrophoretic velocities of rigid peanut- and pear-shaped particles to spherical particles with similar diameter and surface charge in a rectangular microchannel. We observed that as particle slenderness increases, the nonlinear electrophoretic velocity decreases, while the degree of nonlinearity decreases within the nonlinear regime.
The study explored the effects of rheology on nonlinearity under weak electric fields by experimentally verifying nonlinear electrophoresis in shear-thinning Xanthan Gum (XG) solutions. Addition of polymer to a Newtonian buffer solution shifted the electric field dependence from linear to super-linear for electroosmotic, electrokinetic, and electrophoretic velocities. Increasing polymer concentration, which enhances shear-thinning, and adjusting buffer concentration, affecting the Debye length, were investigated. Findings suggested that the impact of slight increases in shear-thinning on nonlinearity was of lesser significance compared to the effects of polymer depletion, which is associated with a thinner Debye length. The study further examined the effect of particle size in XG solutions using three distinct particle sizes with identical zeta potentials to ensure similar classical electrophoretic behavior. Across various polymer concentrations, results demonstrated size dependence, with electrophoretic velocities increasing with particle size. This was attributed to extensional electrolyte activities outside the electric double layer (EDL) in purely shear-thinning fluids like XG solutions. Prompted by these findings, the study also investigated particle size dependence in viscoelastic Polyethylene Oxide (PEO) solutions. This focused on the effects of polymer length and concentration which influence the elasticity of the solutions. A similar but relatively stronger size dependence was observed, linked to activities within the EDL by elastic polymer strands.
Recommended Citation
Bentor, Joseph Armakan, "Fundamental Study of Nonlinear Electrophoresis" (2024). All Dissertations. 3693.
https://open.clemson.edu/all_dissertations/3693
Author ORCID Identifier
0009-0002-6198-347X
Included in
Biochemical and Biomolecular Engineering Commons, Complex Fluids Commons, Other Mechanical Engineering Commons, Polymer Science Commons