Stability of a real foam

As already mentioned the real foam is not a simple sum of films. Its behaviour and response to different disturbances is much more complex than the behaviour of individual foam films. This is so because the films in the foam have different size and shape, and the kinetics of establishing film equilibrium is more complicated since the process of foam drainage exerts significant influence. 

In the real polydisperse foam along with coalescence there always acts another process of internal collapse. This is the diffusion decrease in the specific surface which is accompanied by structural rearrangement, i.e. shift of knots and borders, and change in their orientation. This leads to the origination of various local disturbances (Act, Apa, AC, etc.). These local disturbances along with the rupture of individual films cause destruction either of other films and borders or of local volumes or of the whole foam (see Sections 6.5 and 6.6). Finally, various external factors can affect the foam (pressure drop, applied to the liquid phase reduced pressure of the liquid vapour above the foam, leading to evaporation the effect of antifoam droplets a-particle irradiation vibration, etc.). 

Let us consider the simplest model of a real foam built up of different by size bubbles but having equilibrium films. If the diffusion expansion of bubbles runs slowly, then the real (aggregative) stability with respect to coalescence will be preserved but the column height and dispersity will decrease as a result of gas diffusion transfer between bubbles in the foam and from the upper bubbles to the surrounding medium (if the foam is open). 

The stability of such a foam with respect to coalescence is owed not only to the condition n 0 but also to a certain film and border elasticity. A foam from NaDoS with CBF can serve as an example [51]. During the whole lifetime of this foam the decrease in the foam column height occurs by the mechanism of layer-by-layer destruction and the lowering of dispersity by diffusion (see Section 6.6). 

A qualitative evidence of the above are the data reported in [52]. It has been established that there is a correlation between the calculated rate of internal diffusion foam collapse and the experimentally determined rate. To obtain a stable foam from poor surfactants (alcohols, acids, etc.) under these conditions is hardly possible because of either insufficient dynamic elasticity of foam films or the lack of equilibrium elasticity (for films from insoluble surfactants). Furthermore, the n barrier for films from acid or alcohol solutions is low and the typical capillary pressures for a real foam are sufficient to induce disturbance of the film equilibrium and, respectively, foam collapse. 

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