Biomedical Engineering Reference
In-Depth Information
2
lg
R
P D
(9.8)
Theoretically, Bartolo et al. modeled the critical Cassie and Wenzel transition in
“touching” and “sliding” scenario based on the vertical aligned pillars.
A Cassie and Wenzel transition takes place once the meniscus is so intensively
curved that either direct contact with the bottom of the surface or the local advancing
angle, which is defined as the advancing angle on the sidewalls of the roughness
structure, is reached, so that the meniscus can slide down to bottom. In drop impact
experiments, the energy necessary for inducing a Cassie-to-Wenzel transition as a
function of the post height was measured on the structured post array [
54
]. For
a transition to occur, Bartolo et al. found an approximately linear increase in the
energy barrier with increasing post height and reach maximum above a critical post
height. This trend suggests at the beginning the “touching” mechanism determines
the Cassie-to-Wenzel transition, and “sliding” scenario occurs after the critical post
height. This model has been further proved in his following research by investigating
the shape of the drop footprint during the Cassie-to-Wenzel transition [
52
]. For a
similar post-type structure, Zheng et al. simulated the hydraulic pressure at which a
transition begins,
P
Crit
,as:[
55
]
lg
cos
Y
.1 /
P
Crit
D
(9.9)
where
is the cross-sectional area of a post divided by its perimeter. In contrast,
Extrand's results suggested that
P
Crit
increases with an increase in the
P
-
parameter [
56
]. Furthermore, Liu and Lange simulated the critical pressure on a
surface composed of regularly arranged, microscale spheres [
57
]. Bormashenko
et al. revealed that a critical threshold parameter responsible for the Cassie-to-
Wenzel transition depends on the force per unit length of the triple line instead of
pressure [
49
].
Moreover, some recent experimental studies and simulations suggest that the
transition from the Cassie to the Wenzel state start from one or more nucleation
sites instead of over the entire drop footprint at once [
58
-
61
]. How the transition
proceeds is then determined by how easily the meniscus can move within the surface
structure.
Compared to the Cassie-to-Wenzel transition, the Wenzel-to-Cassie transition
has received much less attention, and it is mainly due to rare observation. The
Cassie-to-Wenzel transition is generally considered as an irreversible process for
the case where the Wenzel state is at the absolute energy minimum [
46
,
47
,
62
].
However, recent studies on condensation of water on microstructure indicate that
Wenzel-to-Cassie transitions are possible when the Wenzel state is metastable, and
the Cassie state stay within lower energy state [
42
]. The solid experimental results
show that Wenzel, Cassie, and mixed Wenzel-Cassie drops co-exist when water was
condensed onto microscale post-type surfaces [
41
,
42
]. In one series of experiments,
