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self-cleaning ability (Fig. 4.9b). Nosonovsky et al. [32] derived a
relationship for CAH as a function of roughness, given as
q
-
q
cos
cos
sin
aO
rO
qq
-=-
(
1
f R
)
adv
rec
LA
-
q
q
-
q
cos
cos
rO
aO
=-
(
1
f R
)
(4.10)
LA
(cos
R
q
+
)
2
1
0
is the fractional flat geometric area of the liquid-air
interface under the droplet;
where
f
LA
q
q
R
is the roughness; cos
and cos
a0
r0
are advancing and receding CA, respectively, of the smooth surface.
a
Figure 4.9
(a) Tilted surface profile (tilt angle,
) with a liquid droplet;
q
q
, respectively
(Adapted from Ref. [34]). (b) Self-cleaning effect with high CA
and low CA hysterics.
advancing and receding CAs are
and
adv
rec
Having obtained a theoretical description of a surface in the
perspective of wetting properties, we are in a position to understand
and appreciate the observations in natural surfaces around us.
4.3 Natural Surfaces
4.3.1 Lotus Leaf
The lotus (
) leaf is a well-known example of a
hydrophobic surface [35-41] and it has been considered as a model
super-hydrophobic surface (Fig. 4.10) [41]. The leaf surface is full
of papillose epidermal cells with asperities, or papillae, making
the surface very rough. In addition to the superficial microscale
roughness, the surface of the papillae has hierarchical sub-micron-
sized asperities (Fig. 4.10c,d) which are made of three-dimensional
epicuticular waxes having tubular or platelet-like structures (Fig.
4.10d) . These hierarchical structures trap an air film between
Nelumbo nucifera
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