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(a)
(b)
q
Figure 4.4
) as a function of the roughness
factor (R) for various CAs of the smooth surface (
(a) CA for a rough surface (
C
q
) (Adapted
0
from Ref. 19); (b)
requirement for a hydrophilic surface to
become hydrophobic as a function of the roughness factor (
f
LA
R
)
q
0
for different values of
(Adapted from Ref. 18).
[20] proved the theoretical calculation of CA variation
with R by a nice experiment. To control the wetting characteristics,
they manipulated the surface roughness of the silicon substrate
by controlling the heights of the SiNWs. They created super-
hydrophilic surfaces on bare silicon wafers with high surface free
energy (SFE), and super-hydrophobic surfaces by using plasma-
enhanced chemical vapour deposition (PECVD) coated (20 nm
thickness) fluorine carbon (C
Kim et al.
) on the SiNWs. Figure 4.5a shows
the apparent CA measurements of various surfaces, including
intrinsic Si substrates. The bare Si substrate shows hydrophilic
behaviour with a static CA of 43.6° [21], and the apparent CA
decreases with the increasing etching time for longer SiNWs
(Fig. 4.5a). On the other hand, the intrinsic Si substrates coated
with C
F
4
8
exhibit hydrophobic properties with a CA of 105.8°, which
progresses toward the super-hydrophobic regime with increasing
SiNW height, as shown in Fig. 4.5b. The authors reported that the CA
transition from the hydrophilic to hydrophobic regime is due to SFE
manipulation of the Si substrate. The researchers have shown that
whether a finished surface is hydrophilic or hydrophobic depends
on the SFE of the initial substrate. General polymeric materials,
including fluorocarbon species, such as CF
F
4
8
are known
to have very low SFE values compared with metals or silicon [22,
, CF
, and C
F
2
3
4
8
23]. Using the van Oss method [24, 25], the authors verified that
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