Foam is the union of two phase systems consisting of gaseous bubbles which are separated by liquid lamella (liquid foam) or solid walls (solid foam). Liquid foams as interfacial phenomena, where surfactants are involved in their production and stabilization, are the subject of this glossary.
When gas bubbles are present in a surfactant solution, surfactant molecules adsorb at the surface of the bubble and reduce the surface tension between the bubble and the volume phase. If a bubble rises, this produces bubbles which are enclosed by a thin film (lamella). These lamellae are able to bond with others, thus forming a foam. A lamella consists of a double layer of surfactant, one side of which consists of the upwardly transported surfactant molecules and the other side of which consists of the molecules which were previously present at the surface.
Lamellae are stabilized by the adsorption enthalpy of the surfactant molecules. They are destabilized by the internal pressure (Laplace pressure) and by gravity, which causes the liquid to flow out of the lamellae (drainage). The destabilizing effects lead to the liquid foam gradually collapsing. Surfactant solutions can differ considerably with regard to foamability and foam stability.
Foam is very important in industry. Foamable surfactant solutions are used in large quantities for products in the cosmetics, body-care and foodstuffs industries, in cleaning, for substance separation by means of foam (floatation) and in firefighting.
Just as important is the prevention of foam in processes such as printing, pumping, cooling and lubrication. Defoamers or antifoamers, which destabilize foams or prevent their formation, are used here.
Measuring methods (selection)
- Foamability measurement: A sample is foamed, e.g. with a stream of air, with the help of a stirrer or by letting liquid fall from a reservoir into a receiver solution (Ross-Miles method). After foaming, the height of the foam achieved is determined.
- Foam stability measurement: The foam height is observed for a sample which has been foamed in a defined manner. The rate of decay of the foam can be evaluated on the basis of different parameters.
- Foam structure analysis: Foam is video-recorded using a special prism for achieving a two-dimensional cut through the foam structure. The video is analyzed by software in order to calculate the number of bubbles per area as well as the bubble sizes and their statistical distribution.
- Conductivity measurement (determination of liquid content): The liquid content of the foam can be determined based on a comparison of the conductivity of the starting solution and the foam formed. Height and time-dependent measurements enable the foam to be characterized with regard to drainage and stability.