protein surfactants can cause the surfaces to be fully stagnant. It is indeed
observed that such surfactants cause significantly slower drainage than
small-molecule surfactants do, for a foam of the samerand bubble size
distribution.
Question
In Figure 11.3, two cross sections through a Plateau border with adjacent films are
depicted. Assume now that these pictures represent two different situations but at the
same height in the foam. Which of the parameters listed below can be the cause of the
difference? Reason for each parameter whether it would be higher in (a) or in (b) or
has no effect:
The surface tension
The density of the liquid
The age of the foam
The viscosity of the liquid
The pressure above the foam (for this parameter, the question has two
different correct answers, depending on what is assumed to be constant).
11.2.3 Overrun
Ideally, all of the liquid in a vessel is incorporated in the foam made by
beating or air injection, but this is often not achieved. A layer of liquid
remains, with a layer of foam on top. The amount of foam obtained will
result from a balance between making the bubbles, drainage, and
coalescence. Several variables affect these processes and thereby the
resulting overrun. Coalescence of bubbles at the top of the foam with the
air above will especially decrease overrun.
Surfactant Concentration. The first requirement is that sufficient
surfactant is present to cover the total air surface produced (i.e., give it a
surface load comparable to the plateau valueG?). Assume an overrun of
300 %, i.e.,j¼ 0 :75, and an average bubble diameterd 32 ¼ 60 mm, this
implies a specific surface areaA¼ 6 j=d 32 ¼ 0 : 05 m^2 per ml foam, or 0.2 m^2
per ml liquid. Assume, moreover, thatG?¼3mg?m^2 ; this corresponds to
0.6 mg of adsorbed surfactant per ml liquid. Since some excess of surfactant
is needed to reach full surface coverage (cf. Figure 11.15b), we would need at