emulsion

Oil and water do not mix — shake them and they split back apart. An emulsion is the trick that keeps them mixed: one of the liquids is beaten into tiny droplets and scattered through the other, so the two stay blended as a single creamy product, like mayonnaise or vinaigrette held together by whisking.

Formally, an emulsion is a dispersion of one immiscible liquid (the dispersed phase) as droplets within a second liquid (the continuous phase). There are two basic types: oil-in-water, where oil droplets sit in water, and water-in-oil, where water droplets sit in oil. The droplets are usually in the coarse range, giving most emulsions their characteristic milky-white opacity.

Because two immiscible liquids naturally have a high interfacial tension and want to reduce their contact area, an emulsion is thermodynamically unstable: left alone, the droplets drift together and the phases separate. An emulsifying agent is therefore essential — it forms a film around each droplet and lowers interfacial tension, slowing creaming and coalescence enough for a usable shelf life. Emulsions let formulators mask oily tastes, deliver oil-soluble drugs in a watery form, and make smooth topical creams and lotions.