Evanescent foam

The first method is quite difficult to reproduce due to the strong influence on the results that small contaminations or vibrations can have. The latter two are also difficult to reproduce since the foam generation and collapse is not always uniform, yet these methods are very commonly used. The dynamic foam tests are most suitable for evanescent foams since their lifetimes are transient. For more stable foams the static foam tests are more commonly used. 

Here, foam is generated by flowing gas through a porous orifice into a test solution as shown in Figure 2.17. The steady-state foam volume maintained under constant gas flow into the column is then measured. There are many variations of this kind of test [46,115,116], This technique is frequently used to assess the stability of evanescent foams. 

A dispersion of gas bubbles in a liquid, in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae ). Sometimes distinctions are drawn as follows. Concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral, are termed polyederschaum . Low-concentration foams, in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical, are termed gas emulsions , gas dispersions , or kugelschaum . See also Evanescent Foam, Froth, Aerated Emulsion. 

The first requirements for foam formation are thus surface tension lowering and surface elasticity. A greater elasticity tends to produce more-stable bubbles. But if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed because of the overwhelming effects of the gravitational and capillary forces. These foams are termed evanescent foams. Important properties that determine the stability will include bubble size, liquid viscosity, and density difference between gas and liquid. More-stable foams may require additional stabilizing mechanisms. 

Evanescent Foam A transient foam that has no thin-film persistence and therefore is very unstable. Such foams exist only where new bubbles can be created faster than existing bubbles can rupture. Examples air bubbles... 

The foaming capability and foam stability obtained from sparkling wines is usually tested by a dynamic foam stability method, as discussed in Section 2.6.2. Because these foams are evanescent and not really very stable, at least compared with the foams found in other industries, dynamic rather than static foam tests are the most suitable. In one version of the dynamic foam test, the Mosalux method, the foam heights are automatically measured using infrared beams and sensors [848],... 

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