Biomedical Engineering Reference
In-Depth Information
9.5.1
Transparent and Antireflective Superhydrophobic
Coatings
Transparent superhydrophobic coatings behave intensively applied in various appli-
cation including architecture or automobile windows, eyeglasses, optical windows
for electronic devices, etc.
The comment microstructured surfaces are, however, not transparent due to Mie
scattering [
185
] from their rough surfaces. The Mie scattering occurs whenever the
surface structures larger than the visible light wavelength. In this case, the wide
range of irradiated light is scattered by the structured surface depending upon the
size of surface structures, incident angles of light, and differences of refractive
indices between the air and materials.
To make superhydrophobic surfaces transparency, the dimensions of roughness
should be lower than the wavelength of visible light (ca. 380-760 nm) [
186
].
According to this criterion, several superhydrophobic coatings techniques have
been developed to achieve the integration of superhydrophobicity and transparency.
For the applicable concern, Nakajima et al. have prepared hard superhydrophobic
thin films by a phase separation method to avoid abrasion and contamination. The
hardness of the fabricated film is almost at the same level as normal silica-based hard
coatings, meanwhile the transmittance of the film is high in the visible wavelength
range [
95
]. Moreover, Nakajima et al. endows the superhydrophobic transparent film
with self-cleaning properties by introducing TiO
2
photocatalyst. Only 2% of TiO
2
was added into the superhydrophobic coating, and the self-cleaning is activated by
photo-decomposition of organic contaminants under sunlight irradiation [
187
]. It
was found that the photo-decomposition rate of the fluorinated silane hydrophobic
outermost layer was very low due to the high stability of C-F bonds, while stains
were photo-decomposed relatively much faster.
While the technique of inorganic coatings has been gaining rapid development,
the study of organic transparent superhydrophobic coatings is still in its early
stages. Compared to inorganic coating, organic coating structures are more flexible,
easier to fabricate, and comparatively not fragile. Till now, a few endeavors have
been made to fabricate organic transparent superhydrophobic coatings. Fresnais
et al. used O
2
-CF
4
double plasma modification of LDPE for a transparent su-
perhydrophobic surface coating [
188
]. In Teshima et al. work, low-temperature
CVD or plasma-enhanced CVD was adopted for hydrophobic coating [
70
]. Yabu
et al. used block copolymers containing equimolar fluorinated acrylate and methyl
methacrylate monomers to the fabrication of superhydrophobic surfaces. Through
careful controlling the condensation of water droplets and final evaporation of the
solvent and water, a honeycomb film with around 300 nm was obtained [
133
].
Meanwhile, the antireflective properties of superhydrophobic coatings have been
developed to enhance their transparency. Xu et al. have reported the fabrication
of simultaneous superhydrophobic and antireflective coatings on the basis of silica
colloidal suspension sol-gel methods and the lowest reflectivity on one-sided film
reached 0.03% [
14
]. Bravo et al. utilized a LBL process to make a transparent
