Dispensing Picoliter Droplets Using Dep Micro-actuation

Non-uniform, AC electric fields created by coplanar electrodes patterned on a 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. Simple, co-planar electrode strips are used to divide microlitersized, sessile water droplets into large numbers of droplets down to ~40 picoliters. The dispensing system uses the electrodes to draw a long finger or rivulet of liquid from the parent microliter droplet. When the voltage is removed, the rivulet breaks up into numbers of droplets as a result of the familiar capillary instability. We propose and provide data that supports a very simple power law dependence of the finger length upon time: Z(t ) ∝ t , which governs the time required to fill a structure. A capillary instability, very similar to the case of the cylindrical jet, leads to droplet formation when voltage is removed. 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.