Foam
A colloidal system in which gas bubbles are dispersed throughout a liquid or solid medium.
Unit
nm
In depth
Foams need a surfactant to stabilise the thin liquid films between bubbles, otherwise drainage and coalescence collapse them quickly. In solid foams the continuous phase has set around the bubbles, giving a light, thermally insulating material.
Examples in the real world
Shaving cream and beer head as liquid foams; expanded polystyrene, bread and pumice as solid foams.