Structure of Dry Foams

The Plateau borders in a foam form a network into which liquid from the films collects, and from which liquid is drained away by gravity. This drainage process, which is important in foam beds used for mass transfer or separation, has been modeled by several groups, starting with Haas and Johnson (1967 see Gururaj et al. 1995 for a recent review). 

Once excess liquid has drained and the foam is relatively dry, if film rupture is rare, then the primary coarsening mechanism will be gas diffusion through the liquid films, which allows some bubbles to expand at the expense of others, which shrink and eventually disappear. The chemical potential of the gas in a bubble is proportional to F/a, where a is the bubble radius (see Fig. 9-30). Thus, the flux of gas per unit area of bubble surface, which is proportional to the chemical potential, goes as a . Since the surface area per bubble, across which mass flux occurs, is proportional to a. the rate of change of bubble volume, dV/dt. is 

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Figure 12.3b depicts three thin films. The juncture of multiple thin films is called a Plateau border, after the Belgian physicist who first described the equilibrium structure of dry foams [24]. In three dimensions thin films define a surface, and Plateau borders are quasi-ID objects. The point of intersection of several Plateau borders is, in turn, known as a vertex or node. Increasing the liquid content causes the bubbles to become more rounded, ultimately attaining a circular/spherical shape. In the dry foam limit, the thin film thickness goes to zero and the radius of curvature R at each vertex vanishes. The liquid content in Figure 12.3 is therefore seen to be low—the foam is dry rather than wet. 

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