Optimized liquid DEP droplet dispensing

Dielectrophoretic droplet dispensing on a substrate relies on initial actuation of the liquid to form a uniform, sessile rivulet along the length of coplanar electrodes, and subsequent capillary-driven instability to form the droplets after the voltage is removed. Periodic bumps disposed along the length of the structure promote droplet formation by establishing an initial, static perturbation from which the instability evolves. Droplet uniformity, essential for applications in the laboratory-on-a-chip, is promoted if the bumps are spaced close to the most unstable wavelength predicted by Rayleigh's cylindrical jet theory. Proper sizing of the bumps guarantees good coverage by the droplets without compromising reliable droplet formation. Finally, if one of the coplanar electrodes is divided into two electrically addressable sections, and then if the voltage application to the sections is sequenced to trap liquid along the active structure, precisely positioned droplet dispensing of very uniform volume results. The experiments reported in this paper demonstrate the effectiveness of these enhancements. To achieve further improvements, the analytical approaches taken in the present work will have to be supplemented or replaced by numerical simulations of the capillary-driven transient hydrodynamics.