Biomedical Engineering Reference
In-Depth Information
of the thin air layer covering superhydrophobic surfaces under water has been
studied by Poetes et al. experimentally. Their results reveal the stability of the air
layer depending to the immersion depth: the deeper depth, the faster decay of the air
layer [
192
].
One of the benefits from air-retaining property under water is the fluidic drag
reduction. Truesdell et al. provided an experimental result to conclude that the struc-
tured superhydrophobic surface reduces the fluidic drag reduction by decreasing the
water/solid contact area and forming of a thin layer of air. The thin layer of air
establishes an new boundary condition from no slip to limited slip [
193
]. Watanabe
et al. were among the first to report the laminar flow drag reduction phenomenon
of superhydrophobic coatings for Newtonian fluids [
194
]. Recently, the theoretical
and experimental details of the fluidic drag reduction of superhydrophobic surfaces
in laminar flow were studied thoroughly. According to Ou et al. [
195
,
196
]
experimental results, superhydrophobic surface reduces pressure drop of over 40%
and results in apparent slip lengths exceeding 20 mm. More importantly, no drag
reduction is observed for smooth hydrophobic surfaces. Through detailed velocity
measurements, the author demonstrated that slip along the shear-free air-water
interface trapped within microstructure is the primary mechanism responsible for
the drag reduction phenomenon [
195
,
196
]. Alternatively, Shi et al. have compared
superhydrophobic and hydrophobic coating on gold threads confirming that for the
first time that a superhydrophobic coating can effectively reduce the fluidic drag
for objects moving in water Experimental results show that with the same initial
propulsion the velocity of a superhydrophobic-coated gold thread is as much as 1.7
times as that of a normal hydrophobic-modified gold thread [
197
].
9.5.4
Enhancing Water Supporting Force
As an another important aspect of air-retaining property, researchers found that
superhydrophobic coatings can provide extra supporting force for floating objects on
the water surface and for objects immersed underwater, like impressive supporting
force of water striders' legs [
197
]. Shi et al. mimic the strider's leg by gold
threads and have presented experimental data of a maximum upward forces per
centimeter of gold threads with superhydrophobic coating (contact angle 156
ı
)and
hydrophobic coating (contact angle 110
ı
). The results show that the thread with
superhydrophobic coating overall provides 0.4 mN supporting force per centimeter
more than the one with hydrophobic coating. The authors concluded that the air
trapped in the cavities of superhydrophobic surfaces providing extra buoyancy
forces, which can reduce the apparent density of a water strider to 0.71 g cm
3
in water [
10
,
78
,
197
].
