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Figure 4.6
(a) Surface height maps of the patterned Si surface (5
µ
m
diameter, 10
m pitch pillars) using an optical
profiler. Reprinted with permission from Ref. [30]. Copyright
2008 American Chemical Society. (b) A liquid droplet
suspended on a hydrophobic surface consisting of a regular
array of circular pillars. The maximum droop of the droplet (
µ
m height, 10
µ
)
can be found in the middle of two pillars that are diagonally
adjacent. Reprinted with permission from Ref. [30]. Copyright
2008 American Chemical Society.
d
Jung et al. [30] explained the dynamic effect of a bouncing water
droplet for the Cassie-Baxter to Wenzel regime transition. They
observed that when a water droplet hits a super-hydrophobic surface
with a velocity
, a liquid-air interface is formed below the droplet
when the dynamic pressure is less than the Laplace pressure. The
Laplace pressure,
V
r
, can be written as
L
2
rg
=
=
gd
-
2
/
R
16
/(
2
P D
)
(4.7)
L
1
g
where
is the surface tension of the liquid-air interface, and the
dynamic pressure,
p
, of the droplet is given by
d
=
1
2
2
r
p
V
(4.8)
d
r
where
is the mass density of the liquid droplet, other symbols
have the same meaning as described in Eq. 4.7. When the maximum
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