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 m anifestation of dielectrophoresis (DEP) makes possible a variety of microel ectromechanical liquid actuation schemes. In particular, we demonstrate a n ew class of high-speed DEP actuators, including "wall-less" flow structures , siphons, and nanodroplet dispensers that operate with water. Liquid in th ese microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on t hese substrates from a micropipette, are manipulated, transported, and subd ivided into droplets as small as similar to7 nl by sequences of voltage app lication and appropriate changes of electrode connections. The finite condu ctivity of the water and the capacitance of the dielectric layer covering t he electrodes necessitate use of rf voltage above similar to 60 kHz. A simp le RC circuit model explains this frequency-dependent behavior. DEP actuati on of small water volumes is very fast. We observe droplet formation in les s than 0.1 s and transient, voltage-driven movement of water fingers at spe eds exceeding 5 cm/s. Such speed suggests that actuation can be accomplishe d using preprogrammed, short applications of the rf voltage to minimize Jou le heating. (C) 2001 American Institute of Physics.