Dynamics of droplets under electrowetting effect with voltages exceeding the contact angle saturation threshold

TL;DR

This study investigates droplet behavior under electrowetting with voltages exceeding the contact angle saturation limit, revealing that initial contact-line dynamics follow classical laws and capillary wave behavior can be modeled beyond saturation.

Contribution

It introduces a framework for understanding and modeling droplet spreading and surface waves in electrowetting at voltages above the saturation threshold, supported by experimental validation.

Findings

Contact-line velocity still follows Young-Lippmann law beyond CAS voltage.

Capillary wave amplitude increases with applied voltage.

A viscous wave profile model is experimentally verified.

Abstract

Electrowetting-on-dielectric (EWOD) is a powerful tool in many droplet-manipulation applications with a notorious weakness caused by contact-angle saturation (CAS), a phenomenon limiting the equilibrium contact angle of an EWOD-actuated droplet at high applied voltage. In this paper, we study the spreading behaviours of droplets on EWOD substrates with the range of applied voltage exceeding the saturation limit. We experimentally find that at the initial stage of spreading, the driving force at the contact line still follows the Young-Lippmann law even if the applied voltage is higher than the CAS voltage. We then theoretically establish the relation between the initial contact-line velocity and the applied voltage using the force balance at the contact line. We also find that the amplitude of capillary waves on the droplet surface generated by the contact-line's initial motion increases with the applied voltage. We provide a working framework utilising EWOD with voltages beyond CAS by characterising the capillary waves formed on the droplet surface and their self-similar behaviours. We finally propose a theoretical model of the wave profiles taking into account the viscous effects and verify this model experimentally. Our results provide avenues to utilise the EWOD effect with voltages beyond CAS threshold and have strong bearing on emerging applications such as digital microfluidic and ink-jet printing.