  Even though electrowetting-on-dielectric (EWOD) is a useful strategy in a
wide array of biological and engineering processes with numerous
droplet-manipulation applications, there is still a lack of complete
theoretical interpretation on the dynamics of electrowetting. In this paper, we
present an effective theoretical model and use Onsager variational principle to
successfully derive the governing equations of non-equilibrium electrowetting
dynamics for a droplet in both overdamped and underdamped regimes. It is found
that the spreading and retraction dynamics of a droplet on EWOD substrates can
be fairly well captured in the overdamped regime. By varying liquid viscosity,
droplet size, and applied voltage, we confirm that the transient dynamics of EW
can be characterized by a timescale independent of liquid viscosity, droplet
size and applied voltage. Our model provides a complete fundamental explanation
of EW-driven spreading dynamics, which is important for a wide range of
applications, from self-cleaning to novel optical and digital microfluidic
devices.
