Biomedical Engineering Reference
In-Depth Information
contact AFM analysis, showed a uniform roughness with an average peak value
equivalent to 30 (nm) along the
Z
-axis. Sputtering technique permit to get a metal
surface able to be studied by nanoscopic wetting point of view. As it can be seen,
the liquid morphology is now completely different by the previous illustrated cases.
The visible big rounded drop coexist with some liquid unshaped structures on its
right and all of the visible objects lie over a substrate that looks like a granular roof.
This visible diffused substrate is indeed formed by the same oil that, initially, did
not make any drop but created a film all over the surface. The presence of this film
was proofed by performing multiple AFM force-distance curves during the various
steps of this fluid deposition since the original bare substrate.
The observed effect is a direct signal of both solid surface energy and morphol-
ogy importance in wetting tests. In this latter case in fact the solid substrate presents
a high surface energy coupled with a planarity well less than perfect, as on the con-
trary was postulated for the case A and B. The overall condition, in terms of Fig. 5,
may be described as non-flat, non-energetically neither chemically uniform status,
due to the unavoidable presence in these sputtered films of some traces of pollutants
(as like titanium or nickel coming from the sputtering chamber). Cracking effects
of the metallic layer due to autotensions developed after the sputtering deposition
step finally determined the conditions for a geometrical highly disturbed surface.
Figure 14A, B and C shows how the metal sputtered surface was looking at the
Scanning Polarization Force Microscopy analysis at
t
0 and after 10 and 30 sec
of oil vapor exposure, while microscopic Fig. 13C has to be intended after 120 sec
of the same treatment. The paraffin oil, a low surface tension fluid, initially covered
with a very thin and adherent film all the substrate surface. This fact fits with YE
statements. The huge energetic difference between the solid and the liquid made the
fluid totally spread over the surface.
In a second step indeed, dependent by the amount of vapor oil condensed over
the sample, some very early drops begun to grew up starting from the more ev-
ident geometric defects. The complete path, from the nanoscopic level up to the
microscopic level, is described in [210].
Apart the obvious difference among the wetting behavior of these three unlike
samples the striking importance of the solid surface status may be further appreci-
ated in Fig. 15 in which it is reported the completely different behavior of the same
oil, deposited by the same condensing technique, over a simply mechanically pol-
ished bulk molybdenum surface. As it can be appreciated oil lays over the artificial
roughness that shows the same dimensional features of the liquid film becoming a
sort of preferential way for driving the fluid displacement far by its natural ener-
getic displacement. This condition is able to create artificial metastable equilibrium
states.
Fluid displacing technique
assumes therefore a great importance when perform-
ing wetting analysis.
Over macroscopic and microscopic dimensions a huge variety of deposition
systems, like sessile drop, ADSA or piezo dispensers are available. It has to be
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