92168.pdf

272 Emulsions and foams

Foam drainage

Initially, the liquid films of a foam will be relatively thick and

drainage will take place mainly by gravitational flow throughout the

whole of these films. The foaming agent plays an important role even

at this stage in restricting this flow to a level where local disturbances

and consequent film rupture is minimised. When the films have

attained a thickness of the order of micrometres, gravitational flow

down the laminar regions will become extremely slow. The predom-

inant drainage mechanism will then involve liquid being discharged

locally at positions of inter-film contact (known as Plateau borders),

where the liquid capacity is relatively high. Subsequent drainage then

takes place through the network of Plateau borders that exists

throughout the foam.

Figure 10.2 Plateau border at a line of intersection of three bubbles. Owing to the

curvature of the liquid-gas interface at A, the pressure of liquid at A is lower than at

B, thus causing capillary flow of liquid towards A

The discharge of liquid from the laminar part of a thin film is
governed by the pressure of the liquid in this region compared with
that of the liquid in the Plateau borders. At least three factors are
likely to be involved, van der Waals attractive forces favour film
thinning, and the overlapping of similarly charged electric double
layers opposes film thinning (see Chapter 8). The other important
factor is a capillary pressure, which favours film thinning. This arises
because the pressure of the adjacent gas phase is uniform and,
therefore, the pressure of liquid in the Plateau borders, where the
interface is curved, is less than that of liquid in the laminar film.
Depending on the balance of these forces, a film may either thin
continuously and eventually rupture, or attain an equilibrium
thickness. Any propagated structure within the film may significantly