Biomedical Engineering Reference
In-Depth Information
ization techniques, make a geometrical and energetic uniform coating layer able to
deeply change the usual wetting features of the bulk material upon which they are
placed. This feature may be either directly detected, for example with devices as
like contact/noncontact AFM, able to provide the morphological map of the sub-
strate, either indirectly by submitting coated samples to wetting tests.
Figure 13A clearly shows that the obtained surface is energetically and mor-
phologically uniform, causing the liquid to be displaced as a myriad of drops,
dimensionally equal, all over the solid substrate. By the point of view of the YE
it can be therefore stated that this surface has effectively no evident chemical or
physical defects and it is featured by a unique determined surface energy value, be-
ing therefore very close to the specifications of an ideal support as needed by YE
itself or Neumann's ESA. The sensibility of this visual evaluation procedure may
be yet appreciated by considering the straight line of drops evident in the picture.
This effect is a consequence of a slight planar misalignment of the atomic planes.
This kind of defects are practically completely hidden over usual technical surfaces
by the presence of much more important disturbs. Over planar, near perfect, sur-
faces they may yet be detected by AFM contact analysis that unfortunately has a
very local operational area, greatly reducing its efficacy when dealing with huge
surfaces evaluations.
Figure 13B shows the same paraffin oil over a freshly cleaved muscovite mica
substrate. This material is known to be the flattest available substrate and therefore
it is often adopted to perform high sensibility AFM measurements [207]. Its sen-
sibility to the environmental humidity threshold causes a sharp change of the own
electrical conductivity behavior due to moisture-activated ion mobility. This feature
may create some extra effects when the substrate is wetted by a polar fluid but does
not interfere with oil wetting tests as here reported.
It can be clearly seen that even if perfect rounded drops are lying on the surface
their dimensions range in a wider interval if compared to a silanized surface. This
behavior can be assumed as an example of a solid surface that even if being totally
flat does not completely fit, by the energetic uniformity, with the requirements of the
pure YE as illustrated in Fig. 5. The sharp fracture line visible over the sample sub-
strate is an endemic feature of this material that suffers of delamination problems,
causing fluid infiltration inside the material and providing the appearance of 'inter-
nal stains' clearly appreciable in their multicolor fashion at optical microscope.
Apart of the different drop dimension existing over the observed samples it has
to be here highlighted that no apparent liquid film appears to be lying over the
solid substrates. This fact may provide further support to the concept that both of
these surfaces are very close to the YE statements, showing a behavior that fits
with the threshold of contact angle existence, i.e., showing a wetting behavior fully
deducible by YE statements.
This is not the case of Fig. 13C, where the same oil is displaced on a silicon
wafer substrate (i.e., extremely flat) over which a 300 (nm) thick molybdenum layer
was previously sputtered. This solid surface, initially explored as clean and dry by
Search WWH ::
Custom Search