foams and aerosols

Think of the head on a glass of beer and the mist from a spray bottle. The beer foam is countless gas bubbles trapped in thin films of liquid; the spray is countless tiny liquid droplets floating in air. Both are colloids where a gas is one of the partners, and both show how dramatically a little gas and liquid can transform each other's behaviour.

More precisely, a foam is a colloid of gas bubbles dispersed throughout a liquid (or sometimes a solid), and an aerosol is a colloid of tiny liquid or solid particles dispersed throughout a gas. A foam is gas-in-liquid; an aerosol is the mirror image, matter-in-gas. In a foam the liquid forms thin walls around the bubbles; in an aerosol the air is the continuous medium and the droplets or specks float freely within it.

These matter across daily life and the planet: shaving cream, whipped cream, and bubble wrap are foams; fog, clouds, smoke, hairspray, and the haze of air pollution are aerosols. Both rely on stabilisers — surfactants for foams, and various forces for aerosols — to keep from collapsing. The caveat is that both are usually fragile: foams drain and pop as their films thin, and aerosol droplets settle, evaporate, or merge over time.