Biomedical Engineering Reference
In-Depth Information
Figure 3.17
Wetting is said to be total when the liquid spreads like a film on the solid surface.
droplet on a plastic substrate. In the case of partial wetting, the line where all three
phases come together is the triple line.
A liquid spreads on a substrate like a film if the total energy of the system is
lowered by the presence of the liquid film (Figure 3.18). The surface energy per unit
surface of the dry solid surface is
g
SG
; the surface energy of the wetted solid is
g
SL
+
g
LG
. The spreading parameter
S
determines the type of spreading (total or partial)
S
γ
=
-
γ γ
+
(3.22)
(
)
SG
SL
LG
If
S
> 0, the liquid spreads on the solid surface; if
S
< 0, the liquid forms a droplet.
When a liquid does not totally wet the solid, it forms a droplet on the surface.
Two situations can occur. If the contact angle with the solid is less than 90°, the
contact is said to be
hydrophilic
if the liquid is aqueous, or more generally wetting
or lyophilic. In the opposite case of a contact angle larger than 90°, the contact is
said to be
hydrophobic
with reference to water or more generally not wetting or
lyophobic (Figures 3.19 and 3.20).
3.5 Contact Angle: Young's Law
3.5.1 Young's Law
Surface tension is not exactly a force; its unit is N/m. However, it represents a force
exerted tangentially to the interface. Surface tension can be looked at as a force per
unit length. This can be directly seen from its unit, but it may be interesting to give
a more physical feeling by making a very simple experiment (Figure 3.21) [10]. Take
a solid frame and a solid tube that can roll on this frame. If we form a liquid film of
Figure 3.18
Comparison of the energies between the dry solid and the wetted solid.
