  Electrically actuated continuous motion of a water droplet over PDMS-coated
single active electrode is analyzed from detailed modeling and experiments. In
an experiment, continuous motion of the droplet is achieved when it is located
over an active electrode with a horizontal ground wire placed just above in an
open EWOD configuration. Using a CCD camera, the instantaneous centroid
position of the droplet is determined and its velocity is inferred by numerical
differentiation. The edge-detected image is also used to determine the
advancing and receding contact angles of the moving drop relative to the
substrate. Motion of 2, 6, and 10 microliter water droplets for voltages in the
range of 170-270 VDC is examined to investigate the effect of drop volume and
voltage on drop deformation and velocity. Simulations have been carried out
using COMSOL Multiphysics with full coupling between the electric field and
hydrodynamics. The motion of the droplet is initiated by Young-Lippmann
spreading at the three-phase contact line, followed by a nonuniform electric
force field distributed between the active electrode and the ground wire
localized at the droplet-air interface. The solver evaluates the Maxwell's
stress tensor and introduces it as a volumetric electrostatic force in the
Navier-Stokes equations. The fully coupled numerical solution shows a good
match with experimentally determined drop movement over a silicone oil-coated
PDMS layer for which contact line friction is absent. A contact angle model
with friction leads to close agreement between simulations and drop motion over
a bare PDMS layer. Over both surfaces, continuous motion of the water droplet
is seen to be achieved in three stages, namely, initial spreading,
acceleration, and attainment of constant speed. Numerical modeling that
includes electric field-fluid flow coupling is shown to yield data in
conformity with experiments.
