Biomedical Engineering Reference
In-Depth Information
Figure 3.73  Plot of the Wenzel and Cassie laws for a sessile droplet sitting on a surface textured
with micropillars.
to two straight lines, the first one with a slope r , and the second one with a slope f .
The two lines intersect, because
S
S
top
total
horizontal
r
=
> =
f
S
S
horizontal
The two lines intersect at a Young contact angle q i defined by q C = q W , so that
f
r
-
1
cos i
θ
=
(3.93)
-
f
In the diagram of Figure 3.72, for a given Young angle, there are two contact
angles. Which one is the real one? From energy considerations—for example, by
using Laplace's law—it can be deduced that the real contact angle is the smaller one,
so that when the Young contact angle is not very hydrophobic ( q < q i ), the contact
corresponds to a Wenzel regime and the drop wets the whole surface. When the
Young contact angle is more hydrophobic ( q > q i ), the drop is in a Cassie regime
and sits on top of the pillars.
Note that the situation we have just described does not correspond always to
the reality. It happens that a droplet is not always in its lowest energy level and that
they are sometimes in metastable regimes. One example was given by Bico et al.
[39-41]. A drop deposited by a pipette on a pillared surface, even if it should be in
a Wenzel regime, does not necessarily penetrate between the pillars; it may stay on
top of the pillars. An impulse—mechanic, electric or acoustic—is necessary for the
drop to regain the expected Wenzel regime.
A surface is said to be superhydrophobic when the contact angle of aqueous
liquid is close to 180°. In nature, some tree leaves in wet regions of the globe have
 
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