Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 16

Figure 11. YE hypothesis vs. real surfaces.

signal that the substrate has to be in equilibrium with the saturated vapor pressure
( P 0 ) of the external environment, i.e., that a thin adsorbed film of pressure π 0 ther-
modynamically exists on the surface. Obviously the higher the solid surface energy
the more probable the existence of adsorbed films. This fact it was described [2,
155-158] by the equilibrium spreading pressure as expressed by (17)

π = γ s − γ sv . (17)

Surface adsorption phenomena may happen either by chemical or by physical path.
Chemi-adsorbed films are generally thin, stable and not as easy to remove as like
physi-adsorbed ones. This issue depends upon the fact that, briefly speaking, chemi-
adsorbed films rely on chemical bonds between the 'pollutant' molecule and the
solid surface substrate. On the other hand physisorbed films are generally due to
molecules temporarily trapped, generally by Van der Waals forces, on the solid sup-
port, able therefore to be more easily removed with usual cleaning procedures. The
striking difference between these two conditions may be appreciated, for example,
with the BET Adsorption Isotherms Technique [185, 188, 189]. By working with
powdered samples usually presenting a convenient surface area it is possible, by
vacuum and heating treatments, to remove any gas or surface pollutant and replac-
ing it with a known gas like nitrogen or water vapor. Even if this technique is usually
adopted to study surface porosity problems [162] the experimental results may pro-
vide an interesting profile of the solid surface energy conditions by the evaluation
of the adsorbing curves. Apart of the adsorbed molecular quantity it isimportant
to stress how a monolayer-Langmuir-like profile, equivalent to a chemisorption,
differs from a multilayer system that, indeed, may be approximated to a thick ph-
ysisorption phenomenon. When dealing with wetting issues the eventual presence
of adsorbed mono or multilayer of chemical compounds over the solid surfaces
could dramatically change the experimental data even at macroscopic dimensions
[163-166].
The work of Gee et al. [165] may provide an interesting example of this ten-
dency. By BET and Ellipsometry techniques they measured the contact angle values