Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 4.
Different wetting regimes: (a) ideal Young regime, barrierless TPC line and (b) contact
angle hysteresis, pinned TPC line.
Figure 5.
Advancing
a
and receding
r
contact angles.
recent movement of the TPC line. In Fig. 4, two different wetting regimes—ideal
and real—are shown.
The difference between the so-called advancing
a
and receding
r
contact an-
gles is called contact angle hysteresis:
Y
=
a
−
r
.InFig.5,theadvancingand
receding contact angles are illustrated.
The difference can depend on the time interval between the movement and mea-
surement, on contamination, and on many aspects of the solid surface state. The
work of adhesion while receding is larger. It is essential to measure both contact
angles and report the contact angle hysteresis to fully characterize a surface. The
hysteresis can be classified in thermodynamic and kinetic terms. Roughness and
heterogeneity of the surface are sources for hysteresis in thermodynamic sense. Ki-
netic hysteresis is characterized by time-dependent changes in contact angle which
depends on deformation, reorientation and mobility of the surface as well as liq-
uid penetration. Advancing and receding contact angle measurements are possible
force-driven if the drop volume will be increased or decreased.
B. Wetting on Rough Surfaces
Surface roughness can be viewed as surface energy fluctuations [1], which act as
barriers for the TPC line propagation. As shown in Fig. 6, the TPC line is anything
but straight or smooth and its motion is scarcely studied. From the viewpoint of fluid
mechanics, there is
no-slip
boundary condition [3]. In wetting experiments, suitable
precautions have to be used to separate dynamic effects from static hysteresis. For
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