Biomedical Engineering Reference
In-Depth Information
sheets schemed in Fig.
9.3
(a). Because of the large energy barrier between the two
metastable pinning states, the film with large-sized MSL (L-MLS) generates a high
CA hysteresis and thus a very high adhesive force. With decreasing the mesh size,
the energy barrier decreases as well. Consequently, the adhesive force is reduced as
the mesh size shrinks (Fig.
9.3
(b)). Unlike MLS structure, a highly discontinuous
dotlike TCL forms on the tilted nanorod structured (TNS) film (Fig.
9.3
(d)). Herein,
the energy barrier between the metastable pinning states is negligible. As a result,
the TNS film is nonadhesive, and water droplets roll off effortlessly even when
the surface tilts slightly. Furthermore, an intermediate state between these two
distinct TCL modes can be obtained by mixing the patterns (linelike and dotlike
TCL, Fig.
9.3
(c)). On the hierarchical ball cactus-like structured (BCS) film, the
contact line is a combination of continuous linelike and discontinuous dotlike
TCL, illustrated as a separated dash-linelike TCL. Correspondingly, medium CA
hysteresis and adhesive force are observed on this pattern.
9.2.5
Wetting Transitions
On some rough surfaces, drops can coexist in both Wenzel and Cassie states,
depending on the way how the respective drop was deposited [
37
-
42
]. From
theoretical prediction, the Wenzel state is energetically favored for a given
r
and
, if the contact angle on a smooth material
Y
is below a critical value
Crit
.
Crit
can be obtained by equating the Wenzel and the Cassie equation [(
9.2
)and(
9.4
)]:
[
43
,
44
]
r
cos
Crit
D
cos
Crit
C 1
(9.7)
In reality, kinetic barriers may stabilize drops in a metastable state instead of
the minimum-energy state [
42
,
45
-
48
]. Consequently, drops in both Cassie and
Wenzel wetting states may appear on the same material. Theoretically, Patankar
derive a model on a post array to study the Cassie to Wenzel transition [
45
]. For
a hydrophobic surface (i.e.,
Y
>
90
ı
), the initial impalement of a drop on the post
array is found to be correlated with an increase in interfacial energy. Energy was
then recovered when the drop was in contact with the bottom of the post surface and
liquid-air interface was replaced by liquid-solid interface. Following similar line
of thought, Nosonovsky and Bhushan provide an energy diagram of the Cassie-to-
Wenzel transition, illustrating both states separated by an energy barrier [
48
].
Varied external stimuli have been applied to introduce a transition of drops
from the Cassie to the Wenzel state including pressure [
38
-
40
,
44
], vibrating [
49
],
electrical voltage [
50
,
51
], and evaporate [
48
,
52
]. In principle, the driving force for
the transition can be ascribed to the increased Laplace pressure
P
across the drops'
liquid-air interfaces [
53
]. The pressure difference
P
can be calculated by Laplace
equation, which correlates surface tension
lg
, and the radius of the meniscus:
