Concentration polarization electroosmosis: Theory and applications

Abstract

Electrokinetics plays a significant role in microfluidic and Lab on Chip systems. Over the last 25 years, the understanding of electrokinetic phenomena has matured considerably, leading to new ways of manipulating fluids and particles on the micro-scale efficiently. This thesis describes experimental characterization and theoretical modelling of a new AC electrokinetic effect termed Concentration-Polarization Electroosmosis, or CPEO. This phenomenon describes steady-state electroosmotic flows that occur around charged insulating micro-structures in the presence of a low-frequency AC electric field. CPEO arises from concentration polarization (CP) and is driven by surface conductance, meaning that the characteristic time is governed by diffusion, t~a^2/D (a is the length scale and D the diffusion constant of the electrolyte ions). As a result, CPEO is observed at low frequencies, e.g. around 300 Hz and below for a 1 μm-sized object immersed in a low-conductivity KCl electrolyte. In this thesis CPEO flows are studied around features fabricated within microfluidic channels, such as pillars and constrictions. A theoretical framework is presented based on recent analytical approaches that provided macroscale boundary conditions for the electric and hydrodynamic problem; these are extended for the case of AC fields. A linear expansion for small Dukhin numbers gives an analytical description for the flows, which is found to be in excellent agreement with experimental observations. Further description for arbitrary Dukhin numbers (large surface conductance) and small electric field magnitudes predicts CPEO flows around charged microparticles. New image analysis methods allow experimental measurements of CPEO on a smaller scale, matching theoretical predictions. It is known that during electrophoresis, particles are repelled from the channel walls. This behaviour is magnified at low frequencies and low conductivities, something that has been reported in the literature but remains unexplained. For these experimental conditions, CPEO flows around suspended particles in microchannels predict hydrodynamic wall-particle repulsive interactions. Experiments confirm that the observed repulsion is caused by quadrupolar fluid flows around the particles, and a detailed characterization has demonstrated that the phenomenon is due to CPEO. Given the known dependence of these flows upon the particle size and surface charge (zeta-potential), CPEO was exploited to fractionate different polystyrene particles. The thesis also describes novel procedures and techniques developed to deliver the main results of the project. A simplified method was discovered for measuring the zeta potential of a microfluidic channel with improved precision. Also, a user-friendly software has been developed for rapid particle detection called Particle Finder. This tool performs automated image analysis of large numbers of images allowing rapid analysis of particles moving in a continuous flow. Finally, the thesis describes how the ideas of CPEO can be used to explain the low-frequency behaviour of an electrokinetic biased Deterministic Lateral Displacement (DLD) device, where the CPEO flows were initially observed. Numerical simulations of particle trajectories were made considering the wall repulsion from the pillars in the DLD array. These show agreement with previous experiments, providing a full quantitative understanding of the DLD separation system for all frequencies and conductivities.

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Identifiers

ORCID for Raul Fernandez-Mateo: orcid.org/0000-0003-1413-8046

ORCID for Hywel Morgan: orcid.org/0000-0003-4850-5676

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