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the SFE values of the intrinsic Si and the C
F
coated Si substrate
4
8
2
2
are 52.96 mJ/m
and 13.51 mJ/m
, respectively. Thus, applying a
C
F
coating to a Si substrate effectively decreases the wettability.
4
8
Figure 4.5
Apparent CAs in relation to etching time for SiNWs and
hydrophobic C
layer (the blue dashed box represents a plain
Si substrate without SiNWs): (a) Effect of etching time on the
surface roughness of intrinsic Si substrates without C
F
4
8
; (b)
Effect of etching time on Si substrates with a deposited C
F
4
8
layer. Reprinted with permission from Ref. [20]. Copyright
2011 American Chemical Society.
F
4
8
Bhushan et al. [26,
27] and Jung et al. [28-30] have extensively
investigated the Cassie-Baxter to Wenzel regime transition that
seems to depend on of the droplet curvature. They start with a small
water droplet suspended on a super-hydrophobic surface consisting
of a regular array of circular pillars with diameter
D
, height
H
and
pitch
as shown in Fig. 4.6a. The curvature of a droplet is calculated
from the Laplace equation, which relates the pressure inside the
droplet to its curvature. The droop of the droplet (
P
) in the recessed
region is maximum at the midpoint between diagonally adjacent
pillars, as shown in Fig. 4.6b, which is given by
d
2
-
(
2
PD R
) /
8
),
1
where
is the droplet radius. When the droop is much greater than
the depth of the cavity, in this case the height
R
1
of the pillars (Eq.
4.6), then a transition from the Cassie-Baxter to the Wenzel regime
occurs.
H
2
-
≥
(
2
PD RH
) /
(4.6)
1
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