Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 164

Without Salt

With Added Salt

FIGure 8.3 Thickness of foam films and added ions.

called kugelschaum seeross). The foam fills the empty space in the bottle, and under
normal conditions, it barely spills out. However, under some abnormal conditions,
the foam is highly stable and starts to pour out of the bottle, which is considered
undesirable. In fact, if a batch of beer shows this property, it is mixed with other
batches. There are some reports that addition of heavy metal ions could change the
foaming characteristics (Birdi, 1989).
Regarding foam stability, it has been recognized that the surface tension under
film deformation must always change in such a way as to resist the deforming forces.
Thus, tension in the film where expansion takes place will increase, while it will
decrease in the part where contraction takes place. A force exists that tends to restore
the original condition, which is film elasticity, defined as

E ilm = 2 A (dγ/d A ) (8.1)

where E ilm and A are the elasticity and area of the film, and γ is the surface tension
of the surface deformed.
Bubble formation in champagne is another important area of application. The size
of the bubbles is found to be important for taste to make an impact. Stability has also
much impact on the looks and taste. The taste of impact is related to the size and
number of bubbles, and also how long the bubbles are stable.
The stability of any foam film is related to the kinetics of thinning of the TLF. As
the thickness reaches a critical value, stability becomes critical. It was recognized at
a very early stage by Gibbs that an unstable state of thickness will conform when the
film diverges from bulk system properties. The measure was mentioned to be in the
range of 50-150 Å. This state is called the black film, and the random motion of the
molecules may easily cause a rupture of TLF. The factors determining drainage of
the fluid in these films are the flow of liquid under gravity and the mean velocity not