Biomedical Engineering Reference
In-Depth Information
1.3
ı
(left) and
a flat water film with a CA of 0
ı
(right) before and after the films were exposed to UV
illumination, respectively [
143
]. (
b
) Cycles of reversible superhydrophobicity/superhydrophilicity
transformation of the films by alternating between UV irradiation and storage in the dark [
143
]
Fig. 9.7
(
a
) Photographs of a spherical water droplet with a CA of 154
˙
materials such as WO
3
[
151
], V
2
O
5
[
152
], SnO
2
[
153
], and some metal alloys
[
154
] have been explored. Reversible wetting states (hydrophilicity/hydrophobicity)
transition has been observed under the amplifying effect of roughness.
In addition to semiconductor inorganic oxides, some organic materials containing
photochromic functional groups are able to reversibly change conformations of two
chemical species triggered by UV/visible irradiation [
155
-
160
]. Therefore, they
are expected to be used in superhydrophobic coatings to reversibly control wetting
transition by UV/visible irradiation. For instance, azobenzene layers transform from
trans
isomer to
cis
isomers upon UV irradiation and this process is reversed by
visible irradiation. Meanwhile, the wetting behavior of azobenzene layers change
depending on different irradiations, because the
trans
isomer has a smaller dipole
moment exhibiting a higher water CA, while the
cis
form has a higher dipole mo-
ment showing lower water CA. Usually, the change of water CA on flat azobenzene
surface is less than 10
ı
upon alternating UV and visible irradiation [
161
]. To amplify
the wetting transition, Jiang et al. [
162
] have prepared a self-assembly azobenzene
