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Title: Dynamics of droplets under electrowetting effect with voltages exceeding the contact angle saturation threshold
Authors: Vo, Quoc
Tran, Tuan
Keywords: Engineering::Mechanical engineering
Issue Date: 2021
Source: Vo, Q. & Tran, T. (2021). Dynamics of droplets under electrowetting effect with voltages exceeding the contact angle saturation threshold. Journal of Fluid Mechanics, 925.
Project: MOE2018-T2-2-113
Journal: Journal of Fluid Mechanics
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.
ISSN: 0022-1120
DOI: 10.1017/jfm.2021.677
Schools: School of Mechanical and Aerospace Engineering 
Rights: © 2021 The Author(s). Published by Cambridge University Press. All rights reserved.
Fulltext Permission: none
Fulltext Availability: No Fulltext
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