A dispersion of a gas in a liquid or solid. The gas globules may be of any size, from colloidal to macroscopic, as in soap bubbles. Bakers' bread and sponge rubber are examples of solid foams. Typical liquid foams are those used in fire-fighting, shaving creams, etc. In such foams, the liquid must have sufficient cohesion to form an elastic film, e.g., soap, oil, protein, fatty acids, etc. Surfactant-induced foams have been developed to increase the efficiency of fuel cells. Foams made by mechanical incorporation of air are widely used in the food industry, e.g., whipped cream, egg white, ice cream, etc. Useful foams for automobile seats, mattresses, and similar uses are made from natural and synthetic latexes, e.g., polystyrene, polyurethane. A glass foam is based on sodium silicate and rock wool, and vitreous ceramic foams are also available. Metals can be caused to foam. Concrete foams are also in general use. Foams designed for fire extinguishment are agglomerations of small bubbles of gas produced by two methods: (1) by chemical reaction between aluminum sulfate and sodium bicarbonate to generate carbon dioxide (chemical foams), and (2) by mixing or agitation of air with water containing the foaming ingredients (mechanical foams). The two types are equally efficient in fire-extinguishing ability. Besides the foaming ingredients, the foams contain stabilizing agents to assure permanence; there are many of these, for example, soaps, proteins, extract of licorice root, fatty acids, and sulfite liquors. The ingredients of chemical foams are assembled in two separate units, which generate the foam on blending. Fire foams are used primarily on fires in hydrocarbon liquids (class B fires). There are many special types tailored for specific uses. Some fire-protection systems for aircraft include an instant-generating foam for cabin interiors, using a 2.5% aqueous solution of alkyl sulfonate and rigid polyurethane foams for use in fuel tanks.See Fire Extinguishment.An unusually stable foam that remains intact much longer than fire-fighting foams has been developed at Sandia National Laboratories. It results from a synergistic action caused when a water-soluble polymer and a fatty alcohol are blended with a solvent and a surfactant. A possible agricultural use would be in insecticide application.
