| Electrokinetics plays an important role in facilitating fluid transport and particle manipulation in microfluidic devices. The first part of the dissertation studies the forces that act on particles and macromolecules suspended in electrolyte solution in the presence of an alternating electric field. The works were inspired by the experiments of Arsenault et al. (2007), who observed that actin filaments stiffen in the presence of an electric field. The standard model consisting of the Poisson-Nernst-Planck equations is used to study the electric double layer polarization of charged dielectric particles, which plays a major role in the particles' migration. The cases of a long, cylindrical particle in transverse electric field, a cylindrical particle in an axial electric field, and a spherical particle are modeled, in particular, when the double layer thickness is large relative to the particle's radius. The theoretical predictions were compared and favorably agree with experimental observations. The study provides insights to the behavior of nanoparticles and biomolecules in response to electric fields. In addition, I study induced-charge electro-osmosis around a conducting cylindrical particle near a dielectric wall. An analytical solution for the net force acting on the particle is derived. The resulting force tends to repulse the particle away from the wall. This effect might adversely affect PIV near-wall measurements and biomolecules' binding to surfaces.;The second part of the dissertation focuses on electrokinetically-induced flows. A novel, active chaotic stirrer is proposed to enhance mixing in microdevices. The stirrer consists of a chamber which contains a conducting cylinder surrounded with two, alternately actuated pairs of electrodes. When one pair of electrodes is active, the polarizing cylinder induces a flow pattern. By alternating between these two flow patterns, we induce chaotic advection that provides efficient mixing. In another application, I use electrokinetics to induce transverse, secondary flows in a chromatographic column. I demonstrate that the secondary flows can greatly reduce dispersion. The implementation of the above ideas can improve the efficiency of liquid chromatography devices. |
