Dispensing picoliter droplets using dielectrophoretic (DEP) microactuation

Nonuniform AC electric fields created by coplanar electrode strips patterned on an insulating substrate are used to move and manipulate aqueous liquid masses and to dispense very small droplets. This liquid dielectrophoretic microactuation scheme has potential applications for microfluidic systems in the laboratory on a chip. This dispensing system uses the electrodes to draw a long finger or rivulet of liquid from the parent microliter droplet. We propose and provide data that supports a very simple power law dependence of the finger length upon time: Z(t) proportional to roott, which governs the time required to fill a structure. Microliter-sized, sessile water droplets are divided into large numbers of droplets down to similar to40 picoliters when the voltage is removed. The breakup of the rivulet into droplets is a result of the familiar capillary instability. The hydrodynamic instability features a critical wavelength, below which instability is not possible, and a most unstable wavelength, which controls the volume and spacing of the droplets formed.