Plateau border pressure on foam column

Influence of Plateau border pressure on foam column destruction 

An intrinsic property of a polyhedral foam is the reduced pressure in its Plateau borders. At the moment of foam formation the pressure in the borders depends mainly on the foam expansion ratio, dispersity and surface tension (see Eqs. (4.9) and (4.10)). At hydrostatic equilibrium the border pressure is expressed by Eqs. (1.37) and (1.38). 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

The lifetime of a foam being subjected to a pressure drop is affected by the mode of foam formation, the foam column height and foam dispersity. The foam column height 

In order to collect information about the influence of the pressure drop on the lifetime of foams with different types of foam films, foam columns of small heights (2-3 cm) were studied. It was found that the time needed to reach hydrostatic equilibrium pressure (the outflow of the excess liquid ceases) was considerably smaller (5-6 times) than the foam lifetime. This is realised at small pressure drops (up to 5-10 kPa). For that reason it is believed that in these experiments the foam column destruction runs mainly under equilibrium conditions (referring to hydrostatic pressure and drainage). 

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