Biomedical Engineering Reference
In-Depth Information
superhydrophobic surfaces in order to force water droplets to roll off the leaves,
preventing rotting of the leaves [45].
It can be shown that the best situation for superhydrophobicity for a geometri-
cally textured surface is having
f
as small as possible and
r
as large as possible
(Figure 3.74).
Hydrophilic Surface
In the preceding section, we discussed nonwetting textured surfaces. Here we ex-
amine the case of a hydrophilic (wetting) textured surface. This case refers to the
theory of impregnation [41]. A droplet on a rough wetting surface has a smaller
contact angle than the Young contact angle, according to Wenzel's law. However,
it has been observed that in some cases, imbibition occurs (i.e., a part of the liquid
forms a film on the substrate). With the same notations,
r
as the roughness of the
surface and
f
as the Cassie ratio, one can define a critical contact angle by
1
-
f
cos
crit
θ
=
(3.94)
r
-
f
If the Young contact angle
q
is such that
q
<
q
crit
, then the liquid wets the sur-
face (i.e., a liquid film spreads on the surface). In the opposite case, the drop is in
the Wenzel regime with a contact angle given by the Wenzel law. The two possible
morphologies are shown in Figure 3.75.
Relation (3.94) is very similar to (3.93) for hydrophobic substrates. We verify
that, for a flat surface
r
®
1, the surface is wetted only if the Young contact angle is
q
= 0. For a microporous substrate (
r
®
¥),
q
crit
=
π
/2. More generally, (3.94) defines
Figure 3.74
Superhydrophobicity requires a Cassie/Wenzel diagram with a very small coefficient
f
and a large coefficient
r
.
