Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 18

than simply ignoring the effective surface status and presenting the collected data as
referring to the 'real' material. An example of this mechanisms may be appreciated
in [162].
Apart of properly autophobic effects [193, 194] or reactive wetting problems
(indeed belonging to specific branches ex. high temperature issues) this kind of de-
pendence has been tested also for different applications by Searcy [195], Beruto et
al. [196], proposing a way based upon the concept of partial equilibrium to put in
correlation the macroscopic contact angle values with an overall energetic parame-
ter λ able to take in account all the solid-fluid interactions of the interface zone.
The above described facts determines some consequences:

(1) If cleaning procedures do not work in the proper fashion wetting test do not
evaluate the expected liquid-solid interactions but instead the adsorbed film-
liquid interactions.

(2) Wetting conditions have to be analyzed by carefully evaluating the dimensional
ratio between drops/films and solid surface defects.

(3) True equilibrium is a target to be achieved and not expected as automatically
given.

(4) Any kinetic effect due to deposition technique (sessile drop, ADSA, Wilhelmy
etc.) may false the real equilibrium.

(5) Macroscopic wetting test may hidden the real solid-liquid interactions that act
on smaller scale.

These facts therefore unwraps some problem with the correct application of YE to
real laboratory experiments. While thermodynamic, by simply evaluating the ener-
getic possibility that a reaction might take place or not [182] YE does not care about
time. Indeed real systems evolve, i.e., undergo to physical and/or chemical interac-
tions that may determine a transformation of the initial experimental conditions.
Moreover YE application faces the well known actual impossibility to get, as
experimental data, the numerical values of the solid-liquid ( γ sl ) and solid-vapor
( γ sv ) surface energies [7, 183-185]. Actually surface tension ( γ lv ), is commonly
experimentally accessible by different techniques as like Wilhelmy technique, drop
weight, drop shape, etc. [7, 139, 184, 197], while the solid surface energies are not
directly measurable. This fact makes YE an under-determined condition in which
one equation deals with two unknowns. This problem was originally empirically
fixed by considering the difference of the terms ( γ sv − γ sl ), i.e., the numerator of

the YE, as a single variable. By using this stratagem and assuming that in the most
of the cases the liquid-solid system reaches a quasi-static equilibrium close to a
true-equilibrium condition, it has been possible to make YE work but everybody
were aware of these fundamental theoretical weakness.