Biomimetic Superhydrophobic Surface Fabrication by Plasma Processing - Biomimetic Architectures by Plasma Processing - page 154

a consequence, water cannot wet the surface, and spherical water
droplets roll off the surface of the lotus leaves. This suggests that two
important factors are responsible for the superhydrophobicity of
surfaces: (i) rough surface morphology and (ii) low surface energy.
Unlike the surface textures of the lotus leaf which are asymmetric,
surface of the carpet grass leaf exhibits parallel lines with consid-
erable roughness (Fig. 5.1b). Due to this unique structural feature,
a water droplet on them usually has a preferential sliding mode or
direction, which therefore makes it possible for the guided transpor-
tation of water droplets. The lotus leaf and the carpet grass leaf are
examples of superhydrophobic surfaces with low water adhesion. In
contrast, there are some other superhydrophobic surfaces on which
water droplets are firmly adhere on the surface. A popular natu-
ral superhydrophobic surface which exhibits such characteristics
is a rose petal [4]. The microstructures of hierarchical micropapil-
lae and nanofolds are the most typical in petal surface morphology.
Similar superhydrophobicity can also be observed in the petals of
other flowers, such as camellia. Figure 5.1c shows that the camel-
lia petal surfaces are composed of a periodic array of micropapillae
and nanofolds on each papilla. The hierarchical micro- and nano-
structures endow the camellia petals with sufficient roughness, and
therefore, contribute to its superhydrophobicity. Their difference in
size as compared with those on the lotus leaves is another reason
for the unique adhesive property of the flowers' petals [5]. The wa-
ter droplet is expected to enter into the larger grooves of the petal,
and steadily stay on the petal surface even when it is turned upside
down.
These examples show that unique micro- and nanostructures of
biological surfaces strongly affect their special surface properties.
These findings therefore have inspired the creation of artificial
superhydrophobic surfaces with potential applications such as
self-cleaning, micro-droplet manipulation, water or oil separation,
friction reduction, and anti-icing or fogging [6].

5.2.2 Surface Wettability

The physical interaction between liquid droplets and solid surfaces
is a point of interest for classifying surface wettability. The most
common method for the wettability measurement involves looking
at the profile of the drop and measuring the angle formed between
the solid surface and the tangent line at the contact point, called the
“contact angle” (

q

). At the beginning, the contact angle of a liquid