A foam is a dispersion of a gas in a liquid, semi-solid or a solid continuous phase. Examples of foams are cappuccino foam, ice cream, toppings, whipped creams, chocolate mousses, cast-iron, aerated concrete, polyurethane foam, beer foam, etc. These examples have the gas dispersed as bubbles in a continuous phase.
There are also examples that are called foams but that are in reality sponges. A sponge structure differs from a foam structure by having two continuous phases. In a sponge structure besides the continuous liquid, semi-solid or solid phase also the gas phase is continuous and examples of such systems are many bakery products such as bread, sponge cake and waffle.
Foams can be generated by, for instance, agitation at the gas-liquid interphase, introducing gas bubbles into the liquid phase through a glass sinter or a grit, by nucleation of gas bubbles in a supersaturated liquid, by excessive pressure gradients or by microbial activity as can be observed in fermented products. Foaming is mainly a physical phenomenon in which, of course, the chemical composition of the system plays a crucial role.
The lifetime of a foam is for different reasons of great importance. Undesired foam can create serious problems, ranging from occasional annoyance to major disruption on production processes. For example, foam can interfere with process instruments, sensors, pumps and filters, slow down drying of products as a result of the slower drainage of the liquid to be removed from the product, create bubbles in coatings or separation and segregation of process ingredients.
Controlling the instability of a foam can be a functional part in production processes as, for instance, textile printing, distillation processes, gas washing or in products as beer and champagne.
For the understanding of controlling foams it is a prerequisite to be aware of the factors that contribute to foam stability. The thin liquid films between the gas bubbles in a foam play a crucial role in the stability of a foam. In the stability of the thin liquid films the surface rheological properties play a major role and depend on the surface active components present in the continuous phase.