Kinetic factors of foam stability

The kinetic factors of foam stability as well as of the stability of any disperse system, are mainly determined by the stabilising ability of the surfactant adsorption layers. This action is spread over all structural elements of the foam. 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

The stabilising ability of the surfactant adsorption layers are due mainly to the fact that they ensure immobility of the surface layers of films and borders and do not allow the origination of a convective transport. They create also a definite dependence of the border profile on the surfactant kind and pressure gradient. Along with that, the stabilising role of the 

Two types of elasticity could be distinguished equilibrium (Gibbs elasticity) and dynamic (Marangoni elasticity). According to Gibbs the modulus of elasticity of the film is 

The equilibrium elasticity is observed in the process of extension (or contraction) of the film when there is an equilibrium between the film surface and bulk. It is a consequence of the decrease in equilibrium surfactant concentration when the film is extended. (Nonequilibrium elasticity corresponds to the extension, when there is no equilibrium in the film). 

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