blob, it tends to assume a spherical shape, which is the smallest surface area
possible for a given volume. An obvious example is a rain drop in air. If two
of such drops collide, they generally coalesce into one bigger drop, thereby
lowering total surface area. This is also commonly observed for fairly large
oil drops in (pure) water. Since any system tries to minimize its free energy,
it follows that at an interface between two phases free energy is
accumulated. This is called surface or interfacial free energy. For a
homogeneous interface, it is logical to assume that the amount of surface
free energy is proportional to surface or interfacial area. Consequently, the
surface (or interface) is characterized by itsspecific surface free energy. It can
be expressed in units of energy per unit area, i.e., J?m^2 in the SI system.
In Figure 10.1a a metal frame is depicted in which a piece of string is
fastened. By dipping the frame in a soap solution, a film can be formed
between frame and string. (Such a film cannot be made of pure water, as will
be explained in Section 10.7.) Figure 10.1b illustrates that this film too tries
to minimize its area. By pulling on the film, as depicted in c, its area can be
enlarged. The film thus exerts a tension on its boundaries, andthis tension
acts in the direction of the film surfaces. It is called thesurface tension,andit
is expressed in units of force per unit length, i.e., in N?m^1 in SI units.
Notice that it concerns a two-dimensional tension; in three dimensions,
tension (or pressure) is expressed in newtons per square meter. Since 1 J¼
1N?m, the surface tension has the same dimension as the specific surface
free energy. In fact, these two parameters have identical values (provided
FIGURE10.1 Illustration of surface tension. See text. The presence of a soap film is
depicted by gray. The weight in c is of order 1 gram.