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droop of the droplet (
),
the droplet touches the bottom of the cavity between the pillars.
The critical velocity is determined at which the droplet touches the
bottom by equating the Laplace pressure to the dynamic pressure.
For developing a composite interface, the velocity should be lower
than the critical velocity given as
d
) is larger than the height of the pillar (
H
g
32
2
H
PD
V
<
(4.9)
2
r
(
-
)
4.2.4 Droplet Evaporation
Jung et al. [28, 29] also reported about the successive photographs
of a droplet evaporating on the patterned surface as shown in
Fig. 4.7a. At first they started with a droplet of radius approximately
700
m within 180 s. However, in another
60 s the droplet size first decreased to 360
µ
m, which became ~480
µ
µ
m but suddenly increased
to 470
m. The first three droplets (Fig. 4.7a) were in the Cassie-
Baxter state, and its size gradually decreases with time. However, the
transition from the Cassie-Baxter to Wenzel regimes occurred when
the radius of the droplet reached 360
µ
m on the patterned surface.
Light passed through below the first droplet, indicating that air
pockets exist indicating the Cassie-Baxter state. But for the last image,
when the radius of the droplet increased to ~470
µ
m, no air pocket
was visible below it. Clearly, the last droplet was in the Wenzel state.
The transition criterion was displayed in terms of droplet
size. The critical radius, below which the Wenzel state appeared,
of a droplet deposited on the patterned Si with different pitch
values coated with 1,1,2,2-tetrahydroperfluorodecyltrichlorosilane
(PF
µ
) was measured during the evaporation experiment [28, 29].
Fig. 4.7b shows the relation between the critical radii of the
droplets on patterned Si, coated with PF
3
, with known pitch values.
Figure 4.7b compares experimental results (circles) with theoretical
data (solid line) calculated using Eq. 4.6. The results show that the
critical radius of impalement is in good quantitative agreement with
the theoretical value and that a higher critical radii droplet could
3
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