Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 251

Figure 6. Static contact angle, θ , as a function of the applied DC voltage, V , for a droplet of bmim.BF 4
immersed in hexadecane on an electrode insulated with Teflon AF1600. The solid line is the best fit
of the Young-Lippmann. The lower graph shows contact angle hysteresis, H = θ A − θ R , vs. applied
voltage, V . Reprinted with permission from Paneru et al. [50]. Copyright 2010 American Chemical
Society.

voltage polarity, i.e., around V 0, and closely obeys the Young-Lippmann equa-

tion (shown with a solid line) all the way up to the saturation point ( V S =±

=

120 V).
Beyond that voltage the contact angle remains effectively constant. The advancing
and receding static contact angles (shown with open and filled symbols), obtained
when increasing or decreasing the voltage so that the ionic liquid advances or re-
cedes over the solid surface, practically coincide. In other words, the contact angle
hysteresis, H ( θ R ), is very small.

The electrowetting effect is very large and the value of the static contact angle
can be quickly and easily varied between 145 ◦

= θ A −

0) and 48 ◦ ).

When a droplet of ionic liquid was repeatedly electrowet on the same location of
the electrode, the electrowetting effect was robust and reproducible. Variations in
the voltage increment used (5-100 V) did not affect the curve shown in Fig. 6.
Electrowetting in the same solid-liquid-liquid system was even more effective
when using AC voltage—Fig. 7. This result was obtained with a square wave at a
frequency of 500 Hz. Small variations in the shape or the frequency (100-1000 Hz)
of the AC signal did not affect the outcome significantly. The electrowetting curve
obtained with AC voltage is also reversible, robust, and covers an even wider range
of static contact angles: from 145 ◦ down to about 15 ◦ (Fig. 7).
The electrowetting curves obtained with DC voltage for three different tetraflu-
oroborate ionic liquids are shown in Fig. 8. Another three 1-butyl-3-methyl-
imidazolium ionic liquids with different anions were also tested and the results are
plotted in Fig. 9. In all cases the electrowetting curves are symmetric, follow the
Young-Lippmann equation (shown with a solid line), and saturate at contact angles
of about 50 ◦ . The contact angles at zero voltage are very similar for all the BF 4 and

(at V = (at | V | | V S |