Contact Angle Measurements over Macro, Micro and Nano Dimensioned Systems - Drops and Bubbles in Contact with Solid Surfaces - page 24

(A) (B)

Figure 16. (A) SPFM non-contact electrostatic mode. (B) Geometrical approximation.

[202-210] when working at nanoscopic dimension, as like vaporizing-condensing
effects or blown-jet removing techniques.
Independently by the evaluated solid-liquid system it is important to finally re-
mark that while at microscopic dimensions experimental techniques as like ESEM
may provide a net and clean image of the observed liquid drop, at nanoscopic di-
mensions this task becomes much difficult.
Noncontact Probe Microscope techniques as like SPFM, further applied in
Kelvin Probe double feedback control, are indeed able to give an extremely high
scanning resolution, in the order of Angstroms or at least as a function of the best
between the tip scanning radius or the scanning distance. This precision is unfor-
tunately much higher over the drop apex while decays over the border of the drop,
right the place where one may like it the much. Figure 16A shows the tip-drop
working scheme. To overcome the problem and evaluating the total margin of error
eventually committing by this procedure Salmeron and co-workers, that promoted
this technique, suggested to adopt a geometrical approximation, as described in
their papers [203, 206, 207] and briefly reported in Fig. 16B, to get the final contact
angle value. This procedure has been found efficient due to the very low contact
angle values that normally one has to manage with these tests. At nanodimen-
sion, as in the example briefly described, it is quite common to deal with drops
lying/coexisting with thin film of their own substance. This situation may be ac-
cepted only if one admits that the fluid of the film assumes a 'consistence' different
by the one of the normal massive liquid.
This tendency is already known and indicated as autophobic behaviour [193,
194]. A fluid, due to particular surface conditions, becomes liophobic to itself. By
accepting this fact it can be accepted that a liquid drop stands over a fluid film with-
out loosing its shape. YE holds if one admits that an energetic difference between
the film and the drop exists and the substrate layer is considered as a support-
ing sample for the drop. Figure 17 provides an example of this double-structure
recorded by the Author.