Contact Charge Electrophoresis

The manipulation of small particles - that is, colloids, droplets, capsules, macromolecules, or living cells - by electric fields underlies many important technologies for particle separations, electronic displays, cell sorting, and DNA sequencing to name just a few. Most commonly, charged particles are actuated by spatially uniform electric fields through electrophoresis; electrically polarizable particles are actuated by electric field gradients via dielectrophoresis. Recently, a new type of electrophoretic transport has been observed - here, termed contact charge electrophoresis (CCEP) - in which a conductive particle or droplet is first charged by contact with an electrode surface in the presence of an electric field and then actuated by that field via electrophoresis. Each time the particle contacts an electrode, its charge changes sign and its velocity changes direction. In contrast to traditional EP and DEP, contact charge electrophoresis allows for rapid, sustained particle motion (>10 cm/s for a micron-scale colloid) driven by low power DC voltages (~1 nW). These attributes make CCEP an ideal mechanism for powering the various active components needed for mobile (i.e., small, battery powered) microfluidic technologies. We describe our recent efforts (i) to understand CCEP fundamentals (e.g., of contact charge transfer and complex particle motions), and (ii) to apply CCEP principles to power active colloidal systems (e.g., for rapid particle transport and microfluidic mixing).