Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 168

(a) (b)

FIGure 8.4 Foam structure of three-bubble (a) and four-bubble (b) aggregates.

(i.e., the sum of the area of two bubbles is larger than that of one bubble). This means
that system 3 is at a lower energy state than the initial stage 1 (γ d AII−III).
When three bubbles come into contact, the equilibrium angle will be 120°. The
angle of contact relates to system's equilibrium state. If four bubbles are attached to
each other, then the angle will at equilibrium be 109° 28° (Figure 8.4).

8.3.2.1 Foam Formation of beer and Surface viscosity
The surface and bulk viscosities not only reduce the draining rate of the lamella but
also help in restoration against mechanical, thermal, or chemical shocks. The highest
foam stability is associated with appreciable surface viscosity (η s ) and yield value.
The overfoaming characteristics of beer (gushing) has been the subject of many
investigations. The extreme case of gushing is when a beer, on opening, starts to
foam out of the bottle and, in some cases, empties the whole bottle. The relationship
between surface viscosity, η s , and gushing was reported by various investigators.
Various factors were described for the gushing process: pH, temperature, and metal
ions, which could lead to protein denaturation.
The stability of a gas (i.e., N 2 , CO 2 , air) bubble in a solution depends on its dimen-
sions. A bubble with a radius greater than a critical magnitude will continue to
expand indefinitely and degassing of the solution would take place. Bubbles with
a radius equal to the critical value would be in equilibrium, while bubbles with a
radius less than the critical value would be able to redissolve in the bulk liquid. The
magnitude of the critical radius , R cr , varies with the degree of saturation of the liq-
uid (i.e., the higher the level of supersaturation, the smaller the R cr ). The work, W,
required for the formation of the bubble of radius R cr is given by La Mer:

W = [16 π γ 3 ]/[3{p′ − p′′}] (8.7)

where γ is the surface tension of beer, p′ is the pressure inside the bubble, p′′ is the
pressure in the bulk liquid. It has been suggested that there is nothing unusual in the
stability of beer, and, although carbon dioxide is far from an ideal gas, empirical
work supports this conclusion. A possible connection between a high value of η s
and gushing has been reported. Nickel ion, a potent inducer of gushing, has been
reported to give rise to a large increase in the η s of beer. Other additives besides Ni,
such as Fe or humulinic acid, which cause gushing, have also been reported to give a