Biomedical Engineering Reference
In-Depth Information
surfaces in a systematic way, (c) to illustrate different superhydrophobic states
and how to control the state transition, and (d) to address the multifunction of
superhydrophobic surfaces and their potential applications.
9.2
Theoretical Background and Mechanism of Wettability
9.2.1
Ideal Surfaces
The wetting angle of a water droplet on an ideal surface is determined by the
Young's law [
25
] and is based on the assumption of the balance among the cohesive
forces acting in the TCL shown in Fig.
9.2
(a) (see
9.1
).
F
SV
D F
LV
cos
Y
C F
SL
(9.1)
For many years, this equation has been reinterpreted in a way of thermodynamic
equilibrium by substituting forces by surface tension:
SV
SL
LV
cos
Y
D
(9.2)
where
SL
,
SV
,and
LV
refer to the interfacial energy in the solid-liquid, solid-
vapor, and liquid-vapor interface, respectively. Young's theory provides a very
simple description of the different wetting scenarios on flat surfaces with varied
surface energy. When the solid-vapor interface exhibits a high-surface energy,
theoretically the addition of
SV
and
SL
equals
LV
, and then the drop wets
completely the surface, reaching a superhydrophilic state. In the case of surfaces
with low surface tension, the water contact angle (WCA) increases. WCAs on
surfaces higher than 90
ı
are commonly defined as hydrophobic state, while WCAs
higher than 150
ı
are referred to as superhydrophobic state.
Fig. 9.2
Schematic illustration of wetting state: (
a
) drop on an ideal surface with contact angle
and
ij
indicated. (
b
) Wenzel state. (
c
) Cassie-Baxter state
