Electrowetting of Ionic Liquids in Solid–Liquid–Liquid Systems - Drops and Bubbles in Contact with Solid Surfaces - page 243

liquid-vapour interfaces. Wettability is directly related to adhesion, usually ex-
pressed as work of adhesion, W A [15, 16]:

W A = γ + γ SV − γ SL = γ( 1 + cos θ 0 ). (2)

The enhanced adhesion achieved under electrowetting conditions can be formally
split into 'chemical' work of adhesion and 'electrical' work of adhesion (in analogy
to the chemical and electric components of the electrochemical potential):

1

2 CV 2 .

W A = W 0 + W EL = W 0 + (3)

The first term is given by Young-Dupré's equation (2), while the second one is
the electrostatic energy (per unit area) stored in the capacitor. From equations (2)
and (3) one obtains the Young-Lippmann equation [1-3] which relates the contact
angle, θ , as to the applied voltage, V :

εε 0

2 γh V 2 .

cos θ = cos θ 0 + (4)

The dielectric constant of the insulating material is ε,ε 0 is the permittivity of vac-
uum, γ is the interfacial tension of the liquid/fluid interface, h is the thickness of
the insulating layer, and θ 0 is the contact angle at zero voltage ( V 0).

The Young-Lippmann equation is a thermodynamic equation and therefore
strictly applies only under equilibrium conditions [17]. Significant contact angle
hysteresis is often observed in experiments and one should interpret such results
carefully. This chapter is devoted to electrowetting in solid-liquid-liquid systems,
where contact angle hysteresis is minimal and therefore should not be of major
concern. It is assumed that the applied voltage affects the solid-liquid interface only
and this seems to be a good approximation [2, 17, 18]. The electrowetting effect (the
second term in equation (4)) is predicted to be identical for positive and negative
voltages. This could be different if strong specific adsorption is present but such
cases are rare. For instance, in solid-liquid-vapour electrowetting, we observed
a systematic deviation from the Young-Lippmann equation at positive potentials.
Due to the influence of ionic strength, pH and oxygen content in the fluoropolymer
insulating coating the effect was attributed to specific adsorption of hydroxyl ions
[18]. The electrowetting effect is larger for insulating layers with higher dielectric
constant and smaller thickness [19-21]. Finally, the effect in solid-liquid-liquid
systems should be more significant as the interfacial tension of a pure liquid is
lower than its surface tension. Surfactants adsorbing at the liquid-fluid interface can
be very effective in reducing the voltage needed to obtain a certain electrowetting
effect [22-24].
The Young-Lippmann equation has been criticized but, although alternative or
improved approaches should be explored, we see no reason to reject the simple and
clear physical basis it provides [17].

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