Biomedical Engineering Reference
In-Depth Information
Table 3.1
Values of Surface Tension of Different Liquids at the
Contact with Air at a Temperature of 20°C (Middle Column) and
Thermal Coefficient
a
(Right Column)
Liquid
g
0
a
Acetone
25.2
-
0.112
Benzene
28.9
-
0.129
Ethanol
22.1
-
0.0832
Ethylene-glycol
47.7
-
0.089
Glycerol
64.0
-
0.060
Methanol
22.7
-
0.077
Mercury
425.4
-
0.205
Perfluoro-octane
14.0
-
0.090
Polydimethylsiloxane
19.0
-
0.036
Pyrrol
36.0
-
0.110
Toluene
28.4
-
0.119
Water
72.8
-
01514
“stick” to the wall because of the Brownian motion. However, impurities contained
in the fluid, such as particles of dust or biological polymers like proteins, may well
adhere permanently to the solid surface because, at the contact with the solid in-
terface, they experience more attractive interactions. The reason is that the size of
polymers is much larger than water molecules and van der Waals forces are propor-
tional to the number of contacts.
Usually surface tension is denoted by the Greek letter
g
with subscripts referring
to the two components on each side of the interface, for example,
g
LG
at a liquid/gas
interface. Sometimes, if the contact is with air, or if no confusion can be made, the
subscripts can be omitted. It is frequent to speak of “surface tension” for a liquid
at the contact with a gas, and “interfacial tension” for a liquid at the contact with
another liquid.
According to the definition of surface tension, for a homogeneous interface
(same molecules at the interface all along the interface), the total energy of a surface
E
is
E
=
γ
S
(3.2)
where
S
is the interfacial surface area.
3.2.2 Surface Tension
In the literature or in the Internet there exist tables for surface tension values
[2, 3]. Typical values of surface tensions are given in Table 2.1. Note that surface
tension increases as the intermolecular attraction increases and the molecular size
decreases. For most oils, the value of the surface tension is in the range
g
~ 20-30
mN/m, while for water,
g
~ 70 mN/m. The highest surface tensions are for liquid
metals; for example, liquid mercury has a surface tension
g
~ 500 mN/m.
3.2.2.1 The Effect of Temperature on Surface Tension
The value of the surface tension depends on the temperature. Observing that the
surface tension goes to zero when the temperature tends to the critical temperature
