Biomedical Engineering Reference
In-Depth Information
Surface energy is also related with surface hydrophobicity. Whereas surface energy
describes interactions with a range of materials, surface hydrophobicity describes these
interactions with water only. Since water has a high capacity for bonding, a material of
high surface energy (i.e., high bonding potential) can have more interactions with water
and consequently will be more hydrophilic. Therefore, hydrophobicity generally decreases
as surface energy increases. Hydrophilic surfaces such as glass have high surface energies,
whereas hydrophobic surfaces such as polytetrafluoroethylene (PTFE) or polystyrene have
low surface energies.
The determination of solid-vapor (
γ
sv
) and solid-liquid (
γ
sl
) interfacial tensions is impor-
tant in a wide range of problems in pure and applied sciences. Due to the immerse difficul-
ties involved in the direct measurement of surface tension when a solid phase is involved,
indirect approaches are usually required. Several approaches have been used to estimate
solid surface tensions, but among these methods, contact angle measurements are believed
to be the simplest.
Contact angle can be measured by establishing the tangent (angle) of a liquid drop with
a solid surface at the base. The possibility of estimating solid surface energies from contact
angles relies on a relation that has been recognized by Young [35]. The contact angle of a
liquid drop on a solid surface is defined by the equilibrium of the drop under the action
of three interfacial tensions (Figure 2.8): solid-vapor,
γ
sv
, solid-liquid,
γ
sl
, and liquid-vapor,
γ
lv
. This equilibrium relation is known as Young's equation:
γ
lv
cos
θ
Y
=
γ
sv
-
γ
sl
(2.12)
where
θ
Y
is the Young contact angle, or the measured contact angle.
The calculation of the solid surface tension
γ
sv
from the contact angle of a liquid of surface
tension
γ
lv
begins with Young's equation. Of the four parameters in Young's equation, only
θ
and
γ
lv
are readily measurable.
θ
, of course, can be determined during the test, and
γ
lv
can be determined and calculated using the pendant drop method by fitting the shape of
the suspended droplet (in a captured video image) to the Young-Laplace equation, which
relates the interfacial tension to drop shape:
1
1
∆
P
=
+
(2.13)
γ
R
R
1
2
Gas
γ
Iv
Liquid
γ
sv
θ
Y
γ
sl
Solid
FIGURE 2.8
Contact angle and respective interfacial tensions component of a liquid sample.
