Enhanced particle transport in nonpolar solvents driven by electric fields: Elucidating the roles of electrophoresis and electro-osmosis through numerical simulation.

By introducing appropriate surfactants to nonpolar solvents, charged inverse micelles can be incorporated as charge carriers, facilitating stable particle suspensions via electrostatic interactions. The presence of these charge carriers enables electric-field-induced transport phenomena, notably electrophoresis and electro-osmosis, to occur in these systems. As a consequence, these nonpolar-solvent systems are used in a wide range of applications, such as electronic paper displays and smart windows. In previously reported experimental work, we found that, under the right circumstances, electrophoresis and electro-osmosis act synergistically to transport particles unexpectedly fast. This work aims to uncover the underlying physics of experimentally observed particle velocity fields and trajectories driven by an applied electric field in a nonpolar solvent. Our approach involves a comprehensive numerical model to analyze particle motion in nonpolar solvents. By comparing simulation results of particle velocity fields and trajectories with experimental data obtained through astigmatism microparticle tracking velocimetry, we find that both electrophoresis and electro-osmosis contribute to particle motion. By quantifying the contributions of electrophoresis and electro-osmosis based on average particle velocities, we further confirm that electro-osmosis contributes significantly to particle transport. Two modes of electro-osmosis are considered, one that is caused by the electrical double layer near the glass surfaces and the other that is caused by the induced space charge in the vicinity of the driving electrodes. Additionally, enhanced particle velocities are found mainly in the center of the cell and result from the superposition of electrophoresis and electro-osmosis. Finally, we propose a scheme that explains how particle trajectories emerge as a result of the interplay between electrophoresis and electro-osmotic flows generated near the glass surface and in the vicinity of the driving electrodes. This study contributes to the fundamental understanding of the interplay between electrophoresis and electro-osmosis in nonpolar solvents and offers insights for advancing the design of enhanced electrokinetic displays.