Foam drainage initial stage

Eq. (5.35) describes satisfactorily the kinetics of the change in capillary pressure of a foam at the initial and final drainage stages. In t vs. 1/r2 co-ordinates, Fig. 5.11 presents the experimental data 4-5 min after drainage initiation. They are well approximated by a linear dependence. Furthermore, the time for establishing an equal capillary pressure depends considerably on the surfactant kind and foam film type. Although the cylindrical model is. 

An important aspect of foam drainage theory represents the behaviour of foam liquid phase in the initial stage of foam formation. Moreover, this behaviour is of major industrial significance, since it determines the way in which properties of frozen low expansion ratio foams and solid polymer foams, used as thermal insulation of soil, are regulated [3],... 

The time of the onset of liquid outflow from the foam Tr is another significant characteristic of the initial stage of drainage. An approximate relation of and the foam column height has been found from the experimental data obtained for the drainage of a Volgonate foam... 

This is a very fast drainage rate and if it continued the foam would collapse within seconds. However, the assumption of a constant lamella thickness is, of course, not correct for more than the initial stage. As Uquid drains the lamella thins and the rate of drainage by gravity rapidly decreases (see the lamella thiekness dependency in Equation 13.5). Later on the main destabilization mechanism can be gas diffusion. 

Foam films can be produced by pulling a frame out of a reservoir containing a surfactant solution. Three stages can be identified (1) initial formation of the film that is determined by the withdraw velocity (2) drainage of the film within the lamella, which causes thinning with time and (3) aging of the film, which may result in the formation of a metastable film. 

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