Physical Chemistry of Foods

(generally water) between air bubbles will be present. As illustrated

in Figure 10.29a, the water will flow downward, and if no surfactant

is present, the A–W interfaces cannot carry a stress and the water

will flow as if there were no interfaces. In other words, the water

falls down like a drop. This then means that the foam immediately,

i.e., within seconds, disappears, as is commonly observed. If a

surfactant is present, a situation as in Figure 10.29b will generally

exist. The downflow of water immediately causes formation of a

FIGURE10.29 Foam lamellae. (a) Downflow of water between two air bubbles in
the absence of surfactant. (b) Same, in the presence of surfactant. (c) Gibbs
mechanism of film stability; surfactant molecules depicted by short lines. The arrows
indicate motion of the surface and of the bordering liquid. See text for explanation.
Highly schematic.