dilational modulus, hence for proteins. Consequently, a foam of small
bubble size and made with protein as the surfactant drains relatively slowly.
Drainage is further impeded by the liquid viscosity being high, but this
strategy for improving stability has its limits: a very high viscosity will
greatly hinder making a foam. What should help is giving the liquid a yield
stress. It is, however, not easy to predict what its magnitude should be. In a
foam layer of heightH, the maximum gravitational stress equalsrwatergH
[cf. Eq. (11.1)]. Its magnitude would then be of order 1 kPa, but the stress is
to a considerable extent counterbalanced by the surface tension gradients
mentioned. The author is not aware of systematic studies. In practice,
gelation can be achieved in various ways, leading to more or less ‘‘solid
foams’’ (see also Section 11.2.4); for instance, by heat treatment of egg white
foams. Another mechanism to counteract drainage is to provide the liquid
with small hydrophilic particles, e.g., protein aggregates; the film then
cannot become much thinner than the particle size.
- Thin films. It may take a long time before a film drains until the
gravitational stress is counterbalanced by the colloidal disjoining pressure,
i.e., a thickness of order 10 nm. However, at the top of a foam, film thinning
can be due to the evaporation of water, and this can happen very much
faster. Thin films may readily rupture, as discussed. Protein layers at the film
surfaces appear to provide reasonable stability, especially if the layer is
coherent, e.g., due to intermolecular cross-links, as discussed. Also mixtures
of proteins that have opposite electric charge, and that thereby give a
coherent adsorption layer, appear to be suitable. The presence of some
small-molecule surfactant may greatly impair film stability. - Films with hydrophobic particles.It is often observed, especially in
protein-stabilized foams, that the presence of small lipidlike particles is quite
detrimental to foam stability. Such extraneous particles are often present,
and they can cause rupture of relatively thick films. Possible mechanisms are
depicted in Figure 13.19, and it may be useful first to consult Section 10.6.1
on contact angles. In cases (a) and (b) a particle becomes trapped in a
thinning film and then makes contact with both air bubbles. Because of the
obtuse contact angley, the curvature of the film surface where it reaches the
particle becomes high, leading to a high Laplace pressure. Hence the water
will flow away from the particle, leading to film rupture. In cases (c) and (d),
contact with one A–W surface may suffice. An oil droplet reaching the
surface will suddenly alter its shape, to a flat lense, which induces flow of the
water in the film away from the droplet; if the film is fairly thin this may
cause its rupture, as depicted. If the contact angle equals 180 8 , the situation
as depicted in (d) can arise. It may also occur if the particle is partly solid
and contains a substance, generally a liquid, that can displace protein from
the film surface. For this to occur, the spreading pressure [Eq. (10.12)] has to