AC Electrowetting and nanodrop ejection on Conducting Parallel Electrodes

The variation of contact angle for a drop of size $a$ on conducting parallel electrodes is shown via goniometry to be a strong function of the frequency of an applied electric field. While the contact line demonstrates the usual DC electrowetting behavior at low frequencies, no electrowetting was observed at frequencies higher than $\omega _c \sim \frac{D}{\lambda a}$, corresponding to the inverse \textit{RC} time scale for electrode screening. Below this screening frequency, the electric field is focused towards the contact line and leads to nanodrop ejection at a threshold voltage that is frequency dependent. Because of electrode screening, this threshold voltage for nanodrop ejection corresponds to a unique threshold electric field, which is captured with an asymptotic analysis.