Biomedical Engineering Reference
In-Depth Information
solid-electrolyte capacitor. The superhydrophobic materials can also improve the
mass transport in the fuel cell system by optimize water repellency of the electrode,
because this will promote the removal of water produced during electrode reaction.
Li et al. have prepared partially oriented superhydrophobic CNTs film catalyzed
with Pt nanoparticles (30 wt%) and they found the superhydrophobic film can
moderate the mass transport difficulty [
204
].
9.5.7
Oil-Water Separation and Oil Absorption
Porous materials that are superhydrophobic but superoleophilic can be applied for
oil-water separation. In the study by Feng et al., a copper mesh coated with PTFE
is used to demonstrate oil-water separation [
205
]. It has been recorded that a falling
water droplet was stopped by the mesh due to the surface superhydrophobicity;
while an oil drop can easily pass through the mesh within 240 ms. Alternatively,
a membrane combining both the superhydrophobic and superoleophilic properties
can be used for selective absorption. Yuan et al. [
2
] have fabricated nanoporous
membranes which can selectively absorb oils up to 20 times the material's weight
in preference to water. This superwetting membrane can also be used to separate
one solvent from a mixture of solvents of very similar polarity. The technique
mechanism is similar to thin layer chromatography.
9.5.8
Microcondensation
Microcondensation of water on patterned superhydrophobic surfaces can be utilized
in water collection system. An intriguing example is provided by Namib desert
Stenocara
beetle's back [
3
], on which micrometer-sized patterns of hydrophobic
and hydrophilic region are able to capture water from humid air. The hydrophilic
region promotes water condensation from the fogs in the desert atmosphere and the
hydrophobic region results in water falling down to the beetle's mouth. To mimic
the micro-structures of the beetle's back, Rubner et al. [
206
] have fabricated su-
perhydrophobic/hydrophilic patterned surfaces via PAH/PAA/silica nanoparticles.
Water sprayed on the patterned surface only adhered to the hydrophilic region
and grew to spherical water droplet which is easier to fall off. Garrod et al.
[
207
] have applied a simple two-step plasma chemical methodology to fabricate
a microcondensor surface on which hydrophilic polymer array was deposited on
the superhydrophobic background. In a series of experiments, the author concluded
that patterned hydrophilic-hydrophobic surfaces were the most efficient for water
microcondensation and these results are compared to the hydrophilic-hydrophobic
pattern on the Namib Desert
Stenocara
beetle's back. Another water-collecting
example in nature is the capture silk of the cribellate spider
Uloborus walckenaerius
.
Zheng et al. [
208
] have found that the wet-rebuilt periodic spindle-knots structure
