Biomedical Engineering Reference
In-Depth Information
Fig. 9.3
Schematic illustration of TCL on MnO
2
films with different patterns. The solid lines
demonstrate the possible solid-liquid-air interface contact line, and the dash lines demonstrate the
liquid-air boundary for a droplet. (
a
) Formation of a continuous contact line on the L-MLS, which
exhibits large adhesion. (
b
) TCL on S-MLS. (
c
) Formation of a dash-line-like TCL forms on BCS.
(
d
) Formation of a highly discontinuous dotlike TCL forms on TNS, which exhibits extremely
small adhesion. Reprinted with permission from ref. [
33
]. Copyright (2011) American Chemical
Society
the contact angle hysteresis correlates to the continuity of TCL formed differently
on varied patterns. When drops are in contact with a mesh-like porous structure,
the TCL is continuous contact with the roughness features and large contact angle
hysteresis is found. Comparatively, for a surface consisting of isolated roughness
features, like posts shown in the literature, they assumed that the energy barriers de-
creased between metastable states, giving rise to a lowered contact angle hysteresis.
Oner and McCarthy followed the same principle, highlighting the significance of
“destabilizing the contact line” in a way to obtain superhydrophobic materials [
33
].
Recently, Zhao et al. [
34
] have proposed two strategies for controlling the TCL
continuity adopted to tune the adhesion on MnO
2
films. The surface topography
and patterns of MnO
2
films were simplified according to the high-contrast black
and white SEM images shown in Fig.
9.3
. The solid lines describe the possible TCL
for a droplet in contact with these patterns, showing different continuity [
32
,
35
].
When a droplet is placed on most surfaces, it will come to rest at a local energy
minimum. The contact line will be pinned at a metastable state, and there will be
an energy barrier for any advancing or receding of the water droplet on the surface
[
32
,
36
]. This kinetic barrier difference in the advancing and the receding modes
gives rise to the CA hysteresis, leading to the adhesive force. As for a mesh-like
structured (MLS) film, continuous linelike TCL is formed on the top of nanowall
