| Manipulation of submerged, micron-scale objects via electric fields is fundamental to both established and emerging technologies, from commercial products such as e-ink displays to the high-throughput promise of microfluidic droplet reactors. Contact charge electrophoresis is one such manipulation technique which utilizes a static electric field to drive the continuous oscillation of a conductive particle immersed in an insulating liquid. Although it can be driven by no more than a household battery, the simplicity and efficiency of contact charge electrophoresis has been marginalized by the inability to produce anything except oscillatory motion in a straight line---a necessary first step and invaluable for basic study of the phenomenon, but lacking a demonstration of its potential usefulness.;Following a summary of the relevant literature, a notably simple addition to the contact charge electrophoresis system is presented which passively converts a portion of the back-and-forth oscillation into motion parallel to the electrode surface. Remarkably, the speed of the particle and design of these "ratchets" allows the particle's trajectory to be almost completely decoupled from the fluid flow. To illustrate the versatility of this concept, two applications in microfluidics are then demonstrated: particle separation from flowing suspensions, and liquid mixing in microfluidic channels. Finally, a new computational method is shown to provide efficient, accurate solutions to the electrostatic problem of a sphere between parallel planes, thereby enabling future design and evaluation of new particle geometries in silico..;This work constitutes a significant advancement of contact charge electrophoresis: freed from the confines of localized oscillation in a straight line, particle trajectories may now be engineered to follow channel bends, splits, and even loops. These capabilities, designed with simplicity and scalability in mind, are wellpositioned for widespread adoption and to drive continued innovation in the field. |
