Dynamics of droplets under saturated electrowetting effect.

2021 
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 show that the initial contact line velocity and the resulting capillary waves on the droplet surface are not limited by CAS. We establish the relation between the initial contact-line velocity and the applied voltage. We also characterise 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 for utilisation of the EWOD effect beyond CAS and have strong bearing on emerging applications such as digital microfluidic and ink-jet printing.
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