Page 84 - Handbook of Adhesives and Sealants

52 Chapter Two

surface), the liquid tends to go to its lowest energy state—that of a
sphere. The surface energy of a pure liquid is easily obtained because
this is simply its surface tension, .
LV
Surface tension and surface energy are numerically identical for liq-
uids. Surface energy is generally given in units of millijoules per meter
2
squared (mJ/m ), while surface tension is given in units of dynes/
centimeter (dynes/cm) or Newtons per meter (N/m). The surface ten-
sions of organic liquids and of most inorganic liquids rarely exceed the
value for water (32 dynes/cm).
The surface energies of liquids are readily determined by measuring
2
1
the surface tension with a duNouy ring or Wilhelmy plate as shown
in Fig. 2.1. With the duNouy ring, a clean platinum ring is placed
under the surface of the test liquid and the liquid is slowly moved
downward until the ring breaks through the liquid surface. The force
is recorded, and by means of appropriate conversion factors, the sur-
face tension of the liquid is calculated. The Wilhelmy plate is a similar
method which measures the force of a liquid on a plate passing
through its surface.
Another method of measuring surface tension is the ‘‘drop weight/
3
drop volume’’ method. Here, the average volume of test liquid to cause
a drop to fall from a carefully calibrated syringe is used to calculate
the surface tension of the liquid.
Whereas, the surface tension of a liquid is a real surface stress, the
same cannot be said of a solid surface. With a solid, work is done in
stretching a surface and not in forming the surface. For a solid surface,

Wilhelmy Plate

Figure 2.1 Wilhelmy plate and
du Nouy ring methods of mea-
suring surface tension of a liq-
uid. (Courtesy: Kruss USA).
¨
duNouy Ring