Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 10

Figure 3. Metastable equilibrium status over a technical solid surface.
originated by the adopted polishing powders were visible. A further cerium oxide
powder refining step provided the flattest possible surface but, even if appearing as a
mirror on a macroscopic examination, when explored by contact AFM it was found
looking like Fig. 2C, where, as usual in AFM practice, X - Y lengths are expressed
in microns and Z height is in Angstroms. Regular parallel scratches are carved on
the surface while also several polishing particles may be detectable enclosed in the
metal surface. By Fig. 2 it may be argued that on such a kind of irregular surfaces
the triple line of micro sized drops stands on different places along its path, so ap-
pearing indented and irregular (Fig. 3). As a consequence of it the determination of
the effective contact angle value on microscopic case may be doubtful due to the
presence of various and different liquid-solid profiles.
This condition is described in Fig. 3 that shows how metastable equilibrium has
to be thought, on technical substrates, as a default condition if no special attention is
reserved to surface preparation or depositing techniques. It is in fact sufficient that
liquid drops or layers have dimensions of the same order of the average solid surface
defects to determine a situation favorable to multiple equilibrium conditions.
Real materials indeed have bumps, holes and pores so real surfaces, when evalu-
ated from the microscopic point of view that fits with wetting science need, usually
are much rougher than a regular succession of channels.
The presence of this typology of defects may obviously deviate the profile of
the drop from its natural energetic path but also the opposite situation may hap-
pen. Figure 4 shows an alumina sample surface, observed at optical microscope,
in which a wetting gap causes a discontinuity in the structure of a thin paraffin oil
film placed on it. This effect is generated by chemical heterogeneities, that are usu-
ally not as easy to detect as physical discontinuities. Real surfaces present a mixed
condition of both these defects. As an extra disturbing issue, one has also to be con-
cerned with the environmental air (endemically liophobic) bubbles or films [160,
161] eventually trapped between the solid surface and the liquid profile. In such a
case the liquid surface does not entirely match the solid substrate profile. This lat-