  The phenomenon of electrowetting, i.e., the dependence of the macroscopic
contact angle of a fluid on the electrostatic potential of the substrate, is
analyzed in terms of the density functional theory of wetting. It is shown that
electrowetting is not an electrocapillarity effect, i.e., it cannot be
consistently understood in terms of the variation of the substrate-fluid
interfacial tension with the electrostatic substrate potential, but it is
related to the depth of the effective interface potential. The key feature,
which has been overlooked so far and which occurs naturally in the density
functional approach is the structural change of a fluid if it is brought into
contact with another fluid. These structural changes occur in the present
context as the formation of finite films of one fluid phase in between the
substrate and the bulk of the other fluid phase. The non-vanishing Donnan
potentials (Galvani potential differences) across such film-bulk fluid
interfaces, which generically occur due to an unequal partitioning of ions as a
result of differences of solubility contrasts, lead to correction terms in the
electrowetting equation, which become relevant for sufficiently small substrate
potentials. Whereas the present density functional approach confirms the
commonly used electrocapillarity-based electrowetting equation as a good
approximation for the cases of metallic electrodes or electrodes coated with a
hydrophobic dielectric in contact with an electrolyte solution and an ion-free
oil, a significantly reduced tendency for electrowetting is predicted for
electrodes coated with a dielectric which is hydrophilic or which is in contact
with two immiscible electrolyte solutions.
