Foam column decay in gravitational field

Three typical types of foam column decay due to gravity can be distinguished linear, logarithmic and stepwise kinetics [32], Besides, the foam often decays as a whole after a certain induction period without any visible signs of volume reduction [41]. 

Logarithmic decrease in foam column height is observed also for foams from concentrated solution of NaDoS and pentanol, containing a large quantity of solubilised xylol 

In several cases the lowest foam layers decay very slowly, which seams to be a characteristic feature of the kinetics of foam column destruction. The decrease in border capillary pressure can be regarded as the main reason for such a decrease in the rate of decay in gravitational field (see Section 6.5.2). At low surfactant concentrations the lower foam layers are stabilised, because the surfactant concentrates in them as a result both of internal foam collapse and decay of the upper layers. When the foam is destroyed by addition of antifoams, the delay of this process occurs because the antifoam solubilises in the surfactant solution during the breaking of the foam (see Sections 9.1 and 9.3). 

The most important factor regulating the rate of foam column destruction are the surfactant kind, electrolyte concentration and additives that determined the structural characteristics of the foam (dispersity, film type and thickness, etc.) and foam film stability. 

The total degradation time is often employed as a characteristic of the kinetics of foam column decay. Sometimes the time needed for breaking a certain part of the foam column (for example, 1/2 or 1/5 of its height) is also used. Obviously, either of these characteristics depend strongly on foam column height and foam dispersity. 

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Fig. 6.10. Kinetic curves of foam column decay in gravitational field curve 1 - NaDoS (103 mol dm 3) +...

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