Investigations of induced charge electrokinetic phenomena
Date of Issue2016-05-27
School of Mechanical and Aerospace Engineering
Induced charge electrokinetic phenomena are receiving increasing attention recently due to their promising potential applications for particle and fluid manipulations in micro/nanofluidics. Unlike linear electrokinetics, induced charge electrokinetic phenomena are caused by interactions between applied electric field and the self-induced polarisation surface charges on conducting or polarisable surfaces. The corresponding electric double layer formed on the surfaces possesses a zeta potential linearly proportional to the applied electric field strength. Therefore, the resulting flow velocity is a quadratic function of the applied electric field strength. This characteristic not only enables induced charge electrokinetic phenomena to provide stronger fluid flows, but also generate net fluid flows in AC electric fields. Pair interactions in induced charge electrophoresis of conducting cylinders are analytically characterised based on the limits of thin electric double layer and weak applied electric field assumptions. A uniform electric field is applied normal to the axis of cylinders in two basic ways: perpendicular and parallel to the connecting line of the two cylinder centres. The study reveals that microvortices are generated on the cylinder surfaces due to the nonlinearity of induced zeta potentials, which is useful in fluid mixing in micro/nanofluidics. Furthermore, due to the nonuniformity of the surrounding electric field and the asymmetry of the surrounding fluid flow, the cylinders are driven into motion: translating away from each other under the perpendicular applied electric field and migrating towards each other under the parallel applied electric field. The electrostatic force generated by the nonuniform surrounding electric field is much smaller than the induced charge electrophoretic force due to asymmetric surrounding fluid flow, and diminishes to zero significantly as the distance between the two cylinders increases. Thus, the cylinder motion can be accurately described by the induced charge electrophoretic velocities. Moreover, pair interactions between a conducting and a non-conducting cylinder in uniform electric fields are also analytically investigated with electric fields imposed perpendicular and parallel to the connecting line of the two cylinder centres. The results show that electroosmosis and induced charge electroosmosis are generated on the non-conducting and the conducting cylinders, respectively. The nonuniform local electric field and the corresponding asymmetric fluid flow drive the cylinders into motion: both translating and rotating under the perpendicular applied electric field, while solely migrating along the $x$-axis under the parallel applied electric field. The velocity component due to electrostatic force is negligible compared to that due to electrophoresis or induced charge electrophoresis. Since the pair interactions reduce as distance increases, the cylinder velocities decrease accordingly. Furthermore, efficient mixing is of significant importance in numerous chemical and biomedical applications but difficult to realise rapidly in microgeometries due to the lack of turbulence. Hence, mixing enhancement by introducing Lagrangian chaos through electroosmosis or induced charge electroosmosis in an eccentric annulus is proposed. The analysis reveals that the created Lagrangian chaos can achieve homogeneous mixing much more rapidly than either pure electroosmosis or pure induced charge electroosmosis. The systematic investigations on the key parameters, ranging from the eccentricity, the alternating time period, the number of flow patterns in one time period, to the specific flow patterns utilised for the Lagrangian chaos creation, show that the Lagrangian chaos is considerably robust. The system can obtain good mixing effect with wide ranges of the eccentricity, the alternating time period, and the specific flow patterns utilised for the Lagrangian chaos creation, so long as there are two flow patterns in one time period. When the applied electric field is large, the Lagrangian chaos created by induced charge electroosmosis can achieve homogenous mixing much more rapidly than that of electroosmosis. Lastly, induced charge electroosmosis around a conducting cylinder in AC electric fields is experimentally investigated through micro particle image velocimetry ($\mu$PIV). The captured velocity vector fields show that four vortices are generated around the cylinder. The fluid velocity is linearly proportional to the square of the electric field strength, obtains a peak value as the electric field frequency increases, and increases as the NaCl concentration increases.
DRNTU::Engineering::Mechanical engineering::Fluid mechanics