whereby most of the water is converted into ice; this makes a ‘‘solid
foam.’’
Particlesthat adhere to the A–W interface (see Section 10.6) can make
bridges between air bubbles, thereby forming a bubble network. The
prime example is whipped cream, where fat globules (partially solid
emulsion droplets) cover the air bubbles and also make a continuous
network in the aqueous phase; generally,j& 0 :5. Something similar
occurs in (high-fat) ice cream: see Figure 9.1.
When beatingegg white, which contains about 10%protein, part of the
protein becomes denatured at the A–W interface, thereby aggregat-
ing, and the particles so formed also cover the air bubbles and make
a continuous network. The air content can be much higher than in
ice cream.
Aheat treatmentcan convert some more or less liquid foam systems
into a solid foam. A prime example is, again, foam based on egg
white—such as foam omelettes and meringues—since at high
temperature protein denaturation occurs, which causes gelation.
Another well-known case is provided by bread, where baking causes
(a) the gas cells in the dough to grow and (b) a fairly stiff gel to be
formed of the continuous phase (the dough). Moreover, the foam is
converted into a sponge, because most of the thin films between the
gas cells break as they become brittle at high temperature.
(Question: Why is it necessary that the films break to obtain a
good loaf of bread?)
All of these systems can be made so as to have a high yield stress and
hence good stand-up properties. A high yield stress also prevents leakage of
liquid from the foam.
The most important foam property may be stability against various
physical changes. Something on this has been mentioned already, and more
will be given in Chapter 13.
11.3 BREAKUP OF DROPS AND BUBBLES
Making bubbles has been discussed in Section 11.2. Making drops is easy:
just stir a suitable surfactant solution with some oil and coarse drops are
obtained. The difficulty is to make small drops/bubbles. This occurs by the
breakup (disruption) of bigger ones. A drop must be strongly deformed to
be disrupted, and deformation is counteracted by the Laplace pressure,
which increases with increasing deformation (see Figure 10.21) and with
decreasing drop size. The ratio between external and internal stress acting