Foam Definition and Examples

What Is a Foam in Chemistry?

Soap Bubble Foam
Soap bubble foam is an example of a foam. Air is trapped in the soap mixture to form the bubbles. Andre Karwath/Creative Commons

Foam Definition

A foam is a substance made by trapping air or gas bubbles inside a solid or liquid. Typically, the volume of gas is much larger than that of the liquid or solid, with thin films separating gas pockets.

Another definition of a foam is a bubbly liquid, particularly if the bubbles or froth are undesirable. Foam can impede the flow of a liquid and block gas exchange with air. Anti-foaming agents may be added to a liquid to help prevent bubbles from forming.

The term foam may also refer to other phenomena that resemble foams, such as foam rubber and quantum foam.

How Foam Forms

Three requirements must be met in order for a foam to form. Mechanical work is needed to increase surface area. This can occur by agitation, dispersing a large volume of gas into a liquid, or injecting a gas into a liquid. The second requirement is that surfactants or surface active components must be present to decrease surface tension. Finally, the foam must form more quickly than it breaks down.

Foams may be open-cell or closed-cell in nature. Pores connect the gas regions in open-cell foams, while closed-cell foams have enclosed cells. The cells are usually disordered in arrangement, with varying bubble sizes. The cells present minimal surface area, forming honeycomb shapes or tessellations.

Foams are stabilized by the Marangoni effect and by van der Waals forces. The Marangoni effect is a mass transfer along the interface between fluids because of surface tension gradient. In foams, the effect acts to restore lamellae -- a network of interconnected films. Van der Waals forces form electric double layers when dipolar surfactants are present.

Foams are destabilized as gas bubbles rise through them. Also, gravity pulls liquid downward in a liquid-gas foam. Osmotic pressure drains lamellae because of concentration differences throughout the structure. Laplace pressure and disjoining pressure also act to destabilize foams.

Examples of Foams

Examples of foams formed by gases in liquids include whipped cream, fire retardant foam, and soap bubbles. Rising bread dough may be considered a semisolid foam. Solid foams include dry wood, polystyrene foam, memory foam, and mat foam (as for camping and yoga mats). It's also possible to make a foam using metal.

Uses of Foams

Bubbles and bath foam are fun uses of foam, but the materials have many practical uses, too.

  • Fire retardant foam is used to extinguish fires.
  • Solid foams may be used to engineer strong yet light materials.
  • Solid foams are excellent thermal insulators.
  • Solid foams are used to make flotation devices.
  • Because solid foams are light and compressible, they make an excellent stuffing and packing material.
  • A closed-cell foam called a syntactic foam consists of hollow particles in a matrix. This type of foam is used to make shape memory resins. Syntactic foams are also used in space and deep-sea exploration.
  • Self-skin or integral skin foam consists of a dense skin with a lower density core. This type of foam is used to make shoe soles, mattresses, and baby seats.