Manipulation
of a conductive droplet by electrowetting has been
a popular topic in microfluidics whereby wettability of the droplet
on a solid surface is increased by applying a voltage between the
conductive droplet and the insulated surface. However, the opposite
phenomenon, e.g., decreasing the wettability of a nonconductive droplet
and increasing its contact angle (CA) by the reversed electrowetting
(REW) effect, has been scarcely reported. Such a process involves
not only the transient dynamics of droplet dewetting but also a critical
condition for droplet detachment from the adhesive surface. In this
work, actuation of a nonconductive droplet in an aqueous surrounding
fluid by REW is studied experimentally. Silicone oil is used for the
actuated droplet, and filtered water is used as the surrounding fluid.
The solid substrate is made of a glass substrate coated with an indium
tin oxide (ITO) film and then deposited by a dielectric layer of Parylene
C. Potential difference is applied between the substrate and the surrounding
fluid, eliminating the disturbance from the top needle on the motion
of the droplet. Three different regimes are identified in the full
range of operation. An underactuated regime occurs at low applied
voltages, in which the CA of the droplet shows a monotonic increase
with the increase of voltage (V). The friction coefficient
of the contact line decreases with V before the CA
saturation (V
s) but shows little change
when V > V
s. At high
voltages, the contact line of the sessile droplet is contracted excessively
by REW. The droplet shows oscillation, and it refers to the overactuated
regime. A combined time scale is proposed, and it verifies that the
viscous dissipation of the contact line and liquid inertia show comparable
contributions in the droplet dynamics. At sufficiently high voltages,
the droplet is rejected completely from the surface. A critical equation
for the threshold voltage of droplet detachment is built, and its
validity is confirmed by experimental results.
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