Foam

Foam is a substance in which a gas phase forms fine dispersed bubbles within a continuous phase of liquid or solid. The gas is typically air, but other gases can be involved. The dispersed gas constitutes the dispersed phase and the liquid or solid surrounding the bubbles is the continuous phase. Foam structures are characterized by bubble size distribution, overall density, and the stability of the film surrounding each bubble.

Foams are commonly described as liquid foams or solid foams. Liquid foams consist of gas bubbles dispersed in a liquid, as in beer, whipped cream, or meringue. Solid foams consist of gas bubbles dispersed in a solid matrix, such as polyurethane foam, polystyrene foam, or metal foams. Open-cell foams have interconnected pores; closed-cell foams trap gas within sealed cells and are usually more rigid and insulative.

Stability arises from interfacial films around bubbles, usually aided by surfactants or polymers that reduce surface tension and provide elasticity. Drainage of liquid from the foam, coalescence of bubbles, and diffusion of gas between bubbles (Ostwald ripening) can lead to collapse. Additives, processing conditions, and temperature influence foam longevity and mechanical properties.

Foams are produced through agitation, gas injection, whipping, or chemical reactions that generate gas in situ. They have wide application: culinary foams in foods and beverages; packaging foams and cushioning; insulation and acoustic materials; and lightweight, energy-absorbing components in automotive, construction, and aerospace industries. Some firefighting foams and certain polymer foams may raise environmental or health concerns due to additives such as PFAS.