Biomedical Engineering Reference
In-Depth Information
Figure 3.69
AFM image of a gold surface treated with CF
4
-H
2
-He plasma discharges. The number
of discharges is (a) 0, (b) 3, (c) 7, and (d) 11. After 10 discharges, the surface is superhydrophobic
and the rugosities are still very small, of the order of 10 nm [37]. Reprinted with permission from
[37]. Copyright 2005 American Chemical Society.
hydrophobic character increases with the numbers of discharges. Note that in this
case, the increase in roughness of the surface is moderate. The rugosities created
by the plasma coating are only 10 nm (Figure 3.69). This method can be applied to
very different substrates, such as silicon, gold, or even cotton.
A liquid at rest on a Wenzel surface contacts the entire surface. It has been
found that the use of the Cassie law would be still more efficient to reinforce the
hydrophobic or hydrophilic property of a surface. The idea is to pattern the surface
with extremely pronounced rugosities, such as micropillars or grooves. Figure 3.70
shows an example of patterning a silicon surface with micropillars [38].
The roughness
r
of such surfaces is very large. If Wenzel's law is applicable, it
is expected that the hydrophilic/hydrophobic character will be very pronounced.
Thus, the question is: Can Wenzel's formula, taking into account a roughness based
on the shape of the microstructures, be used to derive the contact angle? The answer
is not that straightforward. It has been observed that the droplet does not always
contact the bottom plate and sometimes stays on top of the pillars, which is called
the
fakir effect
(Figure 3.71). In such a case, should not the Cassie law, based on a
juxtaposition of solid surface and air, have been used? Also, what is the limit be-
Figure 3.70
(a) Surface patterned with micropillars, and (b) surface patterned with microgrooves.
