Low voltage electrowetting-on-dielectric

TLDR

Reliable droplet pumping in microfluidic devices requires a large contact angle change, yet conventional electrowetting typically needs voltages above 100 V and is limited by dielectric breakdown and electrolysis. The study aims to lower the operating voltage required for electrowetting on dielectric (EWOD). Voltage is reduced by employing thinner, higher‑k dielectric layers and a 200 Å fluoropolymer coating to increase capacitance. Experiments confirm that thinner, higher‑k dielectrics lower the required voltage, achieving a 120°→80° contact angle change at only 15 V and enabling a microfluidic device to pump a 460 nL droplet with that voltage.

Abstract

This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°→80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric materials of similar thickness: SiO2 and barium strontium titanate. In all cases, the surface is coated with a very thin (200 Å) layer of amorphous fluoropolymer to provide initial hydrophobicity. Limiting factors such as the dielectric breakdown and electrolysis are also discussed. By using very thin (700 Å) and high dielectric constant (∼180) materials, a significant contact angle change (120°→80°) has been achieved with voltages as low as 15 V. Based on these results, a microfluidic device has been fabricated and tested, demonstrating successful transporting (pumping) of a 460 nL water droplet with only 15 V.