Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 256

forces), Fowkes approach [15] can be used to relate the interfacial tension, γ 12 ,to
the surface tension of the two separate materials, γ 1 and γ 2 ,as:

2 (γ 1 γ 2 ) 1 / 2 . γ 12 = γ 1 + γ 2 − (10)

With the surface tensions of hexadecane (27.6 mJ/m 2 ) and Teflon AF1600 (12.4 mJ/
m 2 ) we estimate the interfacial tension of the AF1600/HD interface at 3.0 mJ/m 2 .
By using Young equation with θ 0 = 21 . 7mJ/m 2 , we estimate the

interfacial tension of the AF1600/bmim.BF 4 to be 21.8 mJ/m 2 . Because γ AF1600 / HD
and γ IL / HD are very close, the static contact angle at zero voltage is very large.
In solid-liquid-vapour systems, such large contact angles are seen only on super-
hydrophobic surfaces [71], where—due to the combination of hydrophobicity and
surface roughness— γ SV and the effective γ SL are very similar (the liquid droplet is
supported largely on a 'cushion' of air).
Another key feature of solid-ionic liquid-alkane electrowetting is the very low
contact angle hysteresis—approximately 2 ◦ for both DC and AC voltages. This
hysteresis is substantially lower than the lowest one found in low-hysteresis elec-
trowetting systems: Teflon-aqueous salt-vapour ( θ A −

150 ◦ and γ IL / HD =

θ R < 10 ◦ ) [2, 18]. Again,
such low (effectively negligible) hysteresis is also observed in superhydrophobic
systems [71]. Contact angle hysteresis is a macroscopic manifestation of the imper-
fections of the solid surface and can be very large [15]. One of the best behaved
electrowetting curves for a solid-liquid-vapour system was reported by Verhei-
jen and Prins [25]. The hysteresis on Teflon AF1600 surface was within 2 ◦ but,
crucially, the dry solid surface was impregnated with silicone oil before carrying
out the electrowetting experiment. Unusually low hysteresis during electrowetting
in solid-liquid-liquid systems was found by Berge and Peseux [6] and they at-
tributed the fact to the presence of a residual thin oil film trapped under the droplet.
Janocha et al. [31] reported large contact angle changes and small hysteresis in
solid-liquid-liquid electrowetting. Maillard et al. [72] also reported hysteresis of 2 ◦
or less in various solid-liquid-liquid systems exhibiting large static contact angles
in the absence of applied voltage. Static and transient capacitance measurements
convincingly showed the presence of an oil film intercalated between the conductive
droplet and the solid surface [14]. Replacing air with oil in microfluidic electrowet-
ting experiments significantly reduces evaporation, contamination, and biofouling,
and enhances droplet movement [14, 34]. These are considerable practical advan-
tages when designing devices (e.g., valves or actuators) where a maximum change
in the capillary force is required.
The electrowetting curves obtained in this study were fully reversible when volt-
age was cycled. This was observed even in runs where the voltage exceeded V S .
Therefore charge injection into the insulator, which is widely considered as a major
mechanism for electrowetting saturation [25], was not significant in our experi-
ments. This may be tentatively related to the presence of a thin wetting film of
hexadecane separating the ionic liquid from the insulator.