Theory of Defoaming

Foams are thermodynamically unstable but are prevented from collapsing by the following properties  

The stability of a foam can be explained by the Gibbs elasticity (E). The Gibbs elasticity results from reducing the surface concentration of the active molecules in equilibrium when the film is extended. This causes an increase in the equilibrium surface tension o, which acts as a restoring force. 

A is the area of the surface. In a foam, where the surfaces are interconnected, the time-dependent Marangoni effect is important. A restoring force corresponding to the Gibbs elasticity will appear, because only a finite rate of absorption of the surface-active agent, which decreases the surface tension, can take place on the expansion and contraction of a foam. Thus the Marangoni effect is a kinetic effect. 

The surface tension effects under nonequilibrium conditions are described in terms of dilatational moduli. The complex dilatational modulus e of a single surface is defined in the same way as the Gibbs elasticity. The factor 2 is not used in a single surface. 

In a periodic dilatational experiment, the complex elasticity module is a function of the angular frequency  

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