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pointing out that any other claimed external influence (gravity, mass, etc.) should
not interfere within the core of the wetting problem. This formula in fact explicitly
requires that:
The solid-liquid system has to be in true equilibrium.
The solid surface has to be rigid, flat and chemically homogeneous.
Both liquid and solid have to be 'perfect' and their physical and chemical prop-
erties must be accurately determined before any wetting experience.
The liquid and the solid phases do not undergo any transformation during the
experiment.
No chemical interactions between liquid, solid and environment take place.
No external dynamic effects capable of altering the reached equilibrium posi-
tion are allowed.
The fulfilling of all these requirements is considered very difficult to be achieved
on a practical experimental scenario. At a first glance, by following the classical
wetting statements, it appears intuitive that natural, polycrystalline and multiphase
materials would be quite difficult to be evaluated in a wetting experience [79, 81]
while engineered materials usually are described as much more predictable in their
wetting behavior. As indicated in Fig. 1, YE states that the contact angle may be de-
duced by the configuration of three vectors originating from the same point placed
on the border among the three insisting phases of the drop. YE seems therefore to
be satisfactorily applicable when a pure mechanical equilibrium is achieved by the
fluid-liquid-solid system as depicted by Eq. (2):
n
n
F i =
0 ,
M i =
0 .
(2)
i
=
1
i
=
1
This requirement is yet necessary but indeed not sufficient by itself to fulfill the need
of the true equilibrium as prescribed by the YE that, as many Authors emphasized
[179-181], strictly requires the coexistence of mechanical and thermodynamic
equilibrium of the experimental set up, i.e., the equivalence of all the chemical
potentials of all the substances involved in the experiments as (3):
μ i 1 +
μ i 2 +
μ i n +
kT log X 1 =
kT log X 2 =···=
kT log X n =
μ
(3)
1 , 2 , 3 ...,
where μ i X is the energy contribution that accounts for the specific molecular in-
teractions of a region of the system, T is the temperature, and X is the molecular
concentration in the region. The term k log X n is usually known as the entropy of
mixing , so that finally μ gives the total free energy per molecule. This definition
includes in the concept of true equilibrium also the absence of molecular and ther-
mal fluxes and assures that no further internal causes may determine spontaneous
=
constant for all states / substances n =
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