Automated on-chip droplet dispensing with volume control by electro-wetting actuation and capacitance metering

In this work, a method is presented for controlling on-chip droplet dispensing by electro-wetting actuation in conjunction with capacitance feedback. The method exploits the built-in capacitance of an electro-wetting device to meter the droplet volume and control the dispensing process. A self-contained system is built to provide continuous-flow loading, capacitance measurement, and electro-wetting chip control. Automated droplet generation at rates up to 120 droplets/min is demonstrated for droplets of 0.1 M KCl aqueous solution dispensed onto 1 mm pitch buried electrodes and a 500 m channel gap. The reproducibility of the droplet volumes is tested against dispensing parameters including production rate, fluid viscosity, channel aspect ratio, dispensing needle position and dispensing volume. The overall reproducibility is ±5% for production rates varying from 8 to 45 droplets/min and ±6% for fluidic viscosity ranging from 1 to 58 Cst, which implies the application of the method to wide ranges of solutions and pressures. Feedback metering compares well with droplet dispensing without capacitance feedback, which gives a reproducibility of ±13% for on-chip dispensing from a reservoir and ±12% for constant pressure-assisted dispensing. © 2003 Elsevier B.V. All rights reserved.