Dielectrophoretic liquid actuation and nanodroplet formation

Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis ~DEP! makes possible a variety of microelectromechanical liquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including ‘‘wall-less’’ flow structures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ;7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ;60 kHz. A simple RC circuit model explains this frequency-dependent behavior. DEP actuation of small water volumes is very fast. We observe droplet formation in less than 0.1 s and transient, voltage-driven movement of water fingers at speeds exceeding 5 cm/s. Such speed suggests that actuation can be accomplished using preprogrammed, short applications of the rf voltage to minimize Joule heating. © 2001 American Institute of Physics. @DOI: 10.1063/1.1332799#