  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.
