Stabilising effects in foams

In order to generate foam, surfaces of thin liquid films always have to be stabilised by layers of surfactants, polymers or particles. This is why pure liquids never foam. Foaming is always accompanied by an increase in the interfacial area and, hence, its free energy. Thus, in a thermodynamic sense foams are basically unstable and are, therefore, sooner or later destroyed. The lifetime of a foam can span a remarkable range from milliseconds to very long duration. 

A surfactant film is adsorbed on the surfaces of the foam lamella and, depending on concentration, micelles may be present. If the foam lamella is stretched in the direction of the arrows, for instance by an external force, the surface film is depleted and the coverage is incomplete in this zone. This causes an increase of the surface energy in this zone and 

The foam properties of products are mainly governed by the surfactant system and the use of anti-foams discussed below. Besides this the chemical composition of the product or the washing liquor, for example electrolyte content and soil, strongly influences the foam properties. Physical parameters such as temperature and pH value or mechanical input in the system additionally have to be taken into account. 

It can be considered from the scheme that one has to distinguish between the foam kinetics, i.e. the rate of generation of foam under well defined conditions (air input and mechanical treatment) and the stability and lifetime of a foam once generated. The foam kinetics is also sometimes termed foamability in the literature. These quantities can be related to interfacial parameters such as dynamic surface tension, i.e. the non-equilibrium surface tension of a newly generated surface, interfacial rheology, dynamic surface elasticity and interfacial potential. In the case of the presence of oily droplets (e.g. an antifoam, a 

Correlation of experimental data with fundamental parameters 

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This is an important stabilising effect in foams which are formed from solutions of soaps, detergents, etc. If a film is subjected to local stretching as a result of some external disturbance, the consequent increase in surface area will be accompanied by a decrease in the surface excess concentration of foaming agent and, therefore, a local increase in surface tension (Gibbs effect). Since a certain time is... 

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