Liquid dielectrophoresis on the microscale

Polarizable liquids respond to a non-uniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis (DEP) is a well-known example of the ponderomotive force, with the field acting on individual (molecular) dipoles in the liquid. When electrode dimensions are scaled down to ∼10 to 100 μm, liquid DEP makes possible a new type of high-speed, microfluidic liquid actuator. Of particular interest is that these devices can operate successfully with water-based media which are conductive. DEP microactuation is very fast; nanoliter droplets form in ∼0.1 s and transient velocities of liquid menisci exceed 5 cm/s. Joule heating is a problem, though proper electrode design and an operational mode where voltage is applied for very short time periods effectively controls temperature rise. The transient flows that occur when voltage is applied are not well understood. Capillarity strongly influences the hydrostatics and dynamics, and surface wetting, which is affected by rf discharges on the dielectric surface, is also important.