Biomedical Engineering Reference
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combined with filling such as silica nanoparticles. The surface property depends on
the preparing process of the sol and surface functional groups of the outcome gel
[ 83 , 85 , 109 - 115 , 117 , 124 ].
Sol-gel processing and self-assembly have been utilized by Shang et al. [ 125 ]
to prepare transparent superhydrophobic films. And the surface roughness was
tuned through the microstructures of the sol-gels by controlling hydrolysis and
condensation reactions of varied silica precursors over the sol-gel processing, while
introducing a monolayer through surface condensation reaction to modify surface
chemistry. The resulting silica-based coatings by direct dip-coating demonstrated
optical transparency higher than 90%. The advancing and receding WCAs were
about 165 and 115 ı , correspondingly, thus a relatively large contact angle hysteresis
of 50 ı .
Hikita et al. [ 83 ] have used the sol-gel method to fabricate super liquid repellent
surfaces, where colloidal silica particles and fluoroalkylsilane are used to control the
surface energy and roughness. The surfaces can exhibit repellency to both water and
oil, when films have optimal ratio between colloidal silica and fluoroalkylsilane.
Doshi et al. [ 126 ] used silica sol-gel to prepare 50-nm-thick surfaces with
switchable properties from superhydrophobic to superhydrophilic. A rough
and highly porous organosilica aerogel-like film was achieved through a
low-temperature, low-pressure technique, where coatings of mixed alkoxides 3,3,3-
trifluoropropyltrimethoxysilane and tetramethylorthosilicate make the substrate
superhydrophobic. CA reached 155-160 ı , and angles up to 170 ı was observed.
UV/ozone treatments were used to modulate the surface coverage of hydrophobic
organic ligands on the silica framework, adjusting the CA continuously varying
from superhydrophobic of 160 ı with the presence of organic ligands, to hydrophilic
less than 10 ı after removal of the organic ligands.
The merits of the bottom-up approach are the molecular precision in controlling
the chemistry composition and the thickness of the products. The weakness of this
method lies in the difficulty to anticipate the hydrophobic properties unless it is fully
completed.
9.3.3
Combination of Bottom-Up and Top-Down Approaches
The combination of bottom-up and top-down approaches is especially useful for
creating substrates with a two-scale roughness, similar to the structure of the
lotus leaf. The combination methods typically consist of two stages: the top-down
approach creating a rough surface and the subsequent bottom-up process producing
fine roughness. However, combination methods do not necessarily require a distinct
two-stage process. Take phase separation, for example; it involves casting of thin
film and succeeding phase separation by adjusting the environmental conditions.
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