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spacing factor, the CA drops to 120° indicating a transition from a
high roughness Cassie-Baxter regime to a low roughness, more
homogeneous, Wenzel regime. The results confirmed that the
transition, from Cassie-Baxter to Wenzel, takes place at a critical
value of 0.083 <
< 0.111 for both the series. Figure 4.17b showed
CAH and TA change as functions of the spacing factor. The PF
S
f
coated
flat Si surface showed the advancing CA of 116°, the receding CA of
82°, CAH of 34° and TA of 37°. The result show a good correlation
between the CAH data and the TA in the case of small values of the
CA, while for the large values of the CA, the correlation become
worse. CAH and TA show the same trends as CA with varying
3
S
. They
f
and show an abrupt minimum
in the value, which corresponds to the highest CA. The lowest
hysteresis and TAs are 5° and 3°, respectively, which were observed
at
gradually decrease with decreasing
S
f
S
= 0.111.
f
(a) Static CA (
for Series
1 (squares) and Series 2 (diamonds) of the experiments
compared with the Wenzel (dashed line) and Cassie-Baxter
(solid lines) models. (b) CAH (squares for the first series and
diamonds for the second) and tilt angle (TA) (crosses for the
first series and triangles for the second) as a function of
q
as a function of the spacing factor
S
Figure 4.17
S
f
.
Reprinted from Ref. [64] with permission from Royal Society
Publishing.
S
f
Shieh et al.
manufactured some very interesting two-tier
air-like structures which consisted of nanopillars and nanograss.
The nanopillars were fabricated using e-beam lithography and dry
plasma etching. On the other hand, nanograsses were produced
by using hydrogen plasma etching [66]. Their designed pillar
diameter was 100 nm, the pillar height was 1 µm, and the space
between the pillars ranged from 200 nm to 50 µm. The nanograss
[65]
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