Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 12

(A) (B) (C)

Figure 6. Most diffuse (A and C) deviations observable at microscopic level of the liquid-solid profiles
over a technical surface.

teractions [182]. In term of YE, liquids spread the best on materials that show a
solid-surface energy higher than their liquid-vapor surface tension. This feature is
a common guide that indeed works as a rule of thumb (the chance that a liquid
effectively spreads over a solid surface depends also on polar/nonpolar features,
chemisorption phenomena, line tension effects, roughness, etc.). It must be re-
marked that normally metals heavily interact with the external environment. Apart
of the obvious case of iron and its rust, more technological metals, such as inox
steels, titanium alloys or even aluminum, have the tendency to passivate, i.e., to
autonomously create a thin native oxide layer on their surface, that in brief may be
thought closer to a ceramic-like phase. Unfortunately, this coating cannot be con-
sidered as uniformly distributed due to surface cracking phenomena or differential
corrosion events. By the cooperative action of these facts it can be stated that, from
the microscopic wetting point of view, the presence of surface heterogeneities, and
consequently different surface energy values, gives rise to different micro-wetting
conditions over the same sample . It can be therefore expected that, by following the
YE, liquids assume at microscopic size over technical substrates different shapes
depending on the local energy level. This occurrence, even if negligible in a macro-
scopic observation, assumes a fundamental importance in the microscopic scale.
Figure 6 shows a scheme of how may look, on the micro scale, the very end of dif-
ferent liquid-solid interfaces. While the liquid profile of Fig. 6B in its microscopic
final end just follows the profile that already presents in its macroscopic behavior,
this does not happen for profiles A and C.
In nearly all of the cases the microscopic profile of drops do not ends with the
same tendency that showed on a macroscopical level.
Profile A shows a condition in which the liquid-solid system finally appears to
be more liophobic while on the contrary profile B is typical of a more liophilic
behavior. These kind of situations, specially the B one, are quite diffuse when the
wetting analysis is shifted on a lower dimensional case. It has been come in use
to talk about pseudo-partial wetting conditions [10, 11]. Figure 7 illustrates the
situation.
The chemical-physical and morphological conditions (capillarity) of the surface
give rise to a sort of dynamic defects , in the sense that a slow but evident move-
ment of the liquid profile generates what is known as precursor film and lately as
protruding foot , that looks like a thin crown around the drop. This phenomenon is