Emulsions and foams 271liquid drainage, or directly from a liquid of relatively low viscosity)*.
The nature of thin liquid films (as found in these concentrated foams)
has been the subject of a great deal of fundamental research.Foam stabilityOnly transitory foams can be formed with pure liquids and, as with
emulsions, a third (surface-active) component - a foaming agent - is
necessary to achieve any reasonable degree of stability. Good
emulsifying agents are, in general, also good foaming agents, since
the factors which influence emulsion stability (against droplet
coalescence) and foam stability (against bubble collapse) are some-
what similar.
The stability of a foam depends upon two principal factors - the
tendency for the liquid films to drain and become thinner, and their
tendency to rupture as a result of random disturbances. Other factors
which may significantly influence foam stability include evaporation
and gas diffusion through the liquid films. Owing to their high
interfacial area (and surface free energy), all foams are unstable in
the thermodynamic sense. Some distinction can be made, however,
between unstable and metastable foam structures. Unstable foams are
typified by those formed from aqueous solutions of short-chain fatty
acids or alcohols. The presence of these mildly surface-active agents
retards drainage and film rupture to some extent, but does not stop
these processes from continuously taking place to the point of
complete foam collapse. Metastable foams are typified by those
formed from solutions of soaps, synthetic detergents, proteins,
saponins, etc. The balance of forces is such that the drainage of liquid
stops when a certain film thickness is reached and, in the absence of
disturbing influences (such as vibration, draughts, evaporation,
diffusion of gas from small bubbles to large bubbles, heat, temperature
gradients, dust and other impurities), these foams would persist
almost indefinitely.
*SimilarIy, some solid foams (e.g. foam rubber) consist of spherical gas bubbles
trapped within a solid network, whereas others (e.g. expanded polystyrene) consist of
as little as 1 per cent solid volume and are composed of polyhedral gas cells separated
by very thin solid walls.