foam

The froth on a beer or the lather of shampoo is a foam: a tangle of gas bubbles trapped inside a thin web of liquid. Although it looks frivolous, foam is a genuine dispersed system in which gas is the dispersed phase and liquid is the continuous phase.

Pure liquids cannot hold a foam for long because the thin liquid films between bubbles drain and rupture in moments. A foam persists only when a surfactant adsorbs onto the bubble surfaces, forming films that resist drainage and rupture and lend the liquid lamellae a self-healing elasticity. The result is a coarse, often unstable dispersion of bubbles separated by surfactant-stabilised liquid films.

In pharmaceutics foam appears as both nuisance and product. Unwanted foaming during the mixing, filling, or shaking of surfactant-containing liquids slows manufacturing and causes dosing errors, and is countered with an antifoaming agent. Deliberately, medicated foams deliver corticosteroids to the scalp or rectum and spread drugs easily over skin, while sclerosant foams are injected to treat varicose veins. The caveat is that foam stability and bubble size are sensitive to formulation and handling, so reproducibility requires tight control.