Biomedical Engineering Reference
In-Depth Information
Fig. 9.1
SEM image of (
a
)
Nelumbo nucifera
surface which is characterized by microsized
papillae, (
b
) a water strider leg showing numerous oriented spindly microsetae, (
c
) hollow and
bridges structures of
Papilio ulysses
wings. Scale bars: (
a
)and(
b
)20
m, (
c
)1
m. Reprinted
with permission from ref. [
10
]. Copyright (2004) Nature Publishing Group
been reported. These surfaces are of practical interest due to their water-repellent,
antisticking, and self-cleaning properties. These properties are desirable for many
application, including raindrop self-cleaning [
1
], oil spill cleanup [
2
], water-capture
devices [
3
], laboratory-on-a-chip devices [
4
], bioinspired geckos/mussels feet [
5
],
and functional interface for cell and tissue engineering [
6
].
In nature, various species exhibit impressive water-repellent property and one
prominent example is the lotus leaf. Lotus leaf is well known for its self-clean
feature removing dust and mud by water droplets rolling off surfaces and is regarded
as a traditional symbol of purity in Buddhist societies. In 1997, Barthlott et al. [
7
]
have revealed for the first time the interdependence between the surface roughness
and the water-repellent property as well as dust particles adhesion. By comparing
and summarizing the surface structures and their wetting properties of different plant
leaves, the author found those with microsized papillae all exhibit contact angles
(CAs) larger than 150
ı
. Figure
9.1
(a) shows the SEM image of lotus leaf (
Nelumbo
nucifera
), from which fine-branched nanostructures on top of microsized papillae
can be observed. Following researchers have found that such dual-scale structure
surfaces combined with low-surface energy are crucial to design superhydrophobic
surfaces with large CA and small sliding angle (SA) [
8
,
9
]. Barthlott et al. also
coined the term “lotus effect” for the demonstrable superhydrophobic property and
since then researches on superhydrophobic surfaces have been activated to mimic
the nature.
Since functionally optimized surface structures are one of the key innovations
in the more than 400 million years of evolution of species, much more super-
hydrophobic surfaces with particular functions in wildlife have been explored.
The water strider's legs are structured with numerous superhydrophobic nanohairs,
which provide the impressive supporting force on water surface. The micrographs
(Fig.
9.1
(b)) revealed numerous oriented setae on the legs. These needle-shaped
setae endow a single leg with maximal supporting force about 15 times the total
body weight of the insect [
10
]. The secret of the
Stenocara
beetle surviving in
extremely arid habitat is due to its structured superhydrophobic back which is able
to collect water from the fog-laden wind in the morning [
3
]. The wings of many
