Wetting at High Temperature - Drops and Bubbles in Contact with Solid Surfaces - page 327

Figure 21. Microscopic contact angle of Au on a ZrB 2 single grain.
etc.) are present, by measuring their partial pressure or concentration, and flux
rate.
3. The surface roughness has to be quantified: substrates with a roughness param-
eter R a < 100 nm must be used in order to achieve an acceptable accuracy on
θ (a few degrees). In particular, the area on which this measurement has been
done should be clearly stated, because measurements along a short, unique, line
are not reliable enough.
4. When the substrates are ionocovalent oxides, a heat treatment in high vacuum
at temperatures higher than 1000 ◦ C prior to wetting allows any adsorbed OH
group to be removed from the surface.
5. After the tests a check must be carried out on whether the solid surface re-
mained plane or not. If dissolution has occurred, the measured angle cannot
be assigned as the true contact angle of that particular system, but labeled as
'apparent contact angle'.
6. With non-oxide but oxidizable substrates the characterization of their surface
chemistry before and after the wetting experiment is often necessary in order to
identify clearly the meaning of the measured contact angle.
7. Although, as a general rule, advancing contact angles are involved in wetting
experiments, only the simultaneous inspection of θ(t) , R(t) and h(t) curves
allows to confirm this feature. By doing this, evaporation and infiltration phe-
nomena leading to variations in the drop volume can be easily detected.
8. When using automatic techniques based on image-analysis softwares, the con-
tact angle should be derived from the solution of the Laplace equation or, if a
portion of the drop close to the triple line is used, taking care that the num-
ber of coordinate points used is large enough to warrant a sufficient statistical
accuracy.
9. Evidence should be offered, at least with a check on the solidified drop, that an
axisymmetric profile has been retained during the test: asymmetric spreading