foam

a dispersed system with a cellular internal structure. A foam consists of gas or vapor bubbles separated by thin layers of liquid. Owing to the size of the bubbles, which varies from fractions of a millimeter to several centimeters, foams are classified as coarse dispersion systems.

The total volume of gas that is included within foams may exceed the volume of the dispersion medium, that is, the volume of the liquid layers, by a factor of several hundreds (see DISPERSION MEDIUM). The ratio of the volume of a foam to the volume of the liquid phase is the foam’s multiplicity factor. In highly dispersed foams, the bubbles convert into polyhedral cells, and the liquid layers into films that are several hundreds or, in some cases, several tens of nanometers thick. Such films form a framework that is somewhat stable and elastic, and thus, foams have the properties of structured systems (see DISPERSE STRUCTURE and GELS).

One of the major characteristics of foams is time stability, which can be expressed by the time that is required for a 50-percent reduction of the original volume or height of a layer of foam; among other evidences of a foam’s time stability is the change in the degree of dispersion. Foaming takes place either by dispersion of a gas in a liquid medium or by release of a nascent gas phase within the bulk of a liquid. Stable, highly dispersed foams can be obtained using foaming agents—substances that stabilize foams. These substances facilitate foaming and hinder the drainage of liquid from the foam films, thus preventing coalescence of the bubbles. Like stabilizers of emulsions and of lyophobic colloid systems, they reduce surface tension and create an adsorptive surface with positive disjoining pressure. Soaps, soaplike surfactants, and some soluble polymers are especially efficient stabilizers in aqueous mediums, forming layers on the interface of the liquid and gas phases with highly pronounced structural and mechanical properties. An increase in the viscosity of the dispersion medium increases the stability of a foam. Pure liquids with low viscosity do not foam.

Many types of stable foams with carbon dioxide as the gas phase are widely used in fire extinguishers. These foams are produced either directly in the extinguisher or in another type of foam generator. Foam flotation is used to concentrate valuable minerals. Many liquid and semiliquid food products are foamed and subsequently hardened, for example, breads, biscuits, and various types of confectioneries and creams. Solid, structural cellular materials, for example foam glass, foamed slag, expanded plastics, and porous rubbers, are also obtained by foaming originally liquid suspensions, melts, solutions, or polymer mixtures.

Antifoams are used to destroy foams or to prevent foaming, since in several technological processes, especially in the chemical, textile, and food-processing industries, foaming is undesirable. Effective antifoams are surfactants that displace foaming agents from the surface of the liquid but do not themselves stabilize the foam. They include various alcohols, ethers, and alkylamines. Sometimes, foams are removed by high temperatures, by mechanical means, or simply by settling.