Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 254

Figure 11. Static contact angle, θ , as a function of the applied AC voltage (sine wave, 700 Hz), V RMS ,
for a droplet of a bmim.BF4-water mixture immersed in hexadecane on an electrode insulated with
Teflon AF1600. The solid lines are the best fit of the Young-Lippmann equation. The dashed line
indicates the saturation contact angle, θ S , obtained with DC voltage. Adapted with permission from
Paneru et al. [49]. Copyright 2010 American Chemical Society.

The dynamics of spreading and retraction of the ionic liquid is well illustrated by
the time dependence of the base area of the droplet, A ( r 2 ), shown in Fig. 12. =π

After switching on the DC voltage (at t 0), the area A increases, initially very

rapidly, then more gradually, and finally arrives asymptotically (not shown in the
figure) at the static value prescribed by the electrowetting curve shown in Fig. 6.
When the voltage is switched off the ionic liquid droplet quickly retracts back to its
original shape.
As the applied DC voltage is increased one and the same droplet of ionic liquid
spreads further (Fig. 12a). Positive and negative voltages of the same magnitude
produce identical effects (open and filled symbols in Fig. 12). At

=

, a lim-
iting spreading curve is obtained and a subtle difference is found—positive voltage
is slightly less efficient than negative (open and filled triangles in Fig. 12a). During
the retraction of the droplet (Fig. 12b, when voltage is switched off), polarity is ir-
relevant and the droplet contracts to one and the same position (and therefore base
area) which is essentially identical to the position assumed before the electrowetting
experiment had started.
The spreading kinetics for a series of Rmim.BF 4 ionic liquids is shown in Fig. 13.
The behaviour is exponential as discussed above and the rate of spreading decreases
as the length of the alkyl chain increases from 2 to 10 carbon atoms. The spreading
of bmim.BF 4 -water mixtures (not shown) is similar, though at very low viscosity
(high water content) the sudden application of a DC voltage disrupts the shape of
the conductive droplet and the A(t) curve becomes non-monotonic [49].

| V | | V S |