Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 244

A.2. Electrowetting Saturation
The Young-Lippmann equation describes adequately most experimentally obtained
electrowetting curves (contact angle vs. voltage), provided that the voltage does
not exceed a threshold value, V S (saturation potential). At voltages beyond V S ,
the macroscopic contact angle deviates from the Young-Lippmann equation and
becomes effectively independent of the applied voltage. Contact angle saturation
limits strongly the practical usefulness of electrowetting.
The physical mechanism of saturation is a matter of debate. Several mechanisms
explaining contact angle saturation have been proposed: charge trapping at the solid
surface [25], ionization of the ambient fluid close to the contact line [26], defects in
the insulating layer [27], non-zero liquid resistance [28], and dielectric breakdown
[29, 30]. We argue that the validity of the Young-Lippmann equation is limited
from below by γ SL = V S . This condition provides the

following prediction [2] for the saturation contact angle, θ S :
cos θ S =

0 which is achieved at V =

γ S
γ ,

(5)

where γ S is the surface tension of the solid. According to this equation, θ S is com-
pletely determined by the material properties of the system (namely γ SV and γ ).
Thus the saturation contact angle is not expected to be zero and electrowetting sat-
uration is not a defective phenomenon but imposed by the limit of validity of the
thermodynamic model. Equation (5) provides a reasonable estimate for fluoropoly-
mer surfaces and several other cases (using the critical surface tension of wetting,
γ C , as an approximation for γ S ) [2]. More recently, electrowetting measurements in
the presence of surfactants [22, 23] have lent further support to the validity of the
zero-interfacial-tension hypothesis. It should be noted that equation (5) estimates
the point of deviation from the Young-Lippmann equation rather than the lowest
contact angle achievable during electrowetting.
A.3. Solid-Liquid-Liquid Electrowetting
A major portion of the research on electrowetting has been carried out in solid-
liquid-air systems, usually a drop of electrolyte in ambient air [3]. Replacing air
with an immiscible oil, however, offers some advantages [14] (no evaporation,
lesser contamination, small contact angle hysteresis and therefore easier actuation,
and improved liquid transport) and solid-liquid-liquid systems are getting more
and more popular. Janocha et al. [31] attempted electrowetting of a decane droplet
immersed in water on several polymer surfaces with variable success. Berge and
Peseux [6] used organic liquid droplets immersed in an aqueous solution of sodium
sulphate. Quilliet and Berge [32] estimated theoretically that, under equilibrium
conditions, a thin film of ambient oil ( 20 nm, stabilized by van der Waals forces)

could be present under the water droplet. This film of oil effectively lubricates the
water droplet and hence is responsible for the very low contact angle hysteresis
found in solid-liquid-liquid systems. This idea is encountered in microfluidic stud-
ies of a water droplet moving through an immiscible oil [33] or physiological fluids

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