emulsion
/ ih-MUL-shun /
Shake a bottle of oil and vinegar hard and it turns cloudy and creamy — for a while. The oil has been smashed into countless tiny droplets scattered through the vinegar, so it looks like one milky liquid. Leave it on the table and the droplets find each other, merge, and the oil floats back to the top. That fleeting milky mixture of two liquids that do not normally mix is an emulsion.
More precisely, an emulsion is a colloid in which one liquid is dispersed as fine droplets throughout another liquid it does not dissolve in — almost always oil and water in some form. There are two flavours: oil droplets spread through water (oil-in-water, like milk) or water droplets spread through oil (water-in-oil, like butter). Which one forms depends on the proportions and on what else is present to stabilise the droplets.
Emulsions matter across the kitchen, the medicine cabinet, and the factory: mayonnaise, salad dressing, cream, paint, and many lotions and drugs are emulsions. The honest difficulty is that two unwilling liquids resist staying mixed — the droplets always tend to merge back and separate. Making an emulsion last needs an emulsifier, a substance that coats the droplets and stops them from coalescing.
Mayonnaise is a stable emulsion of oil droplets in water (well, in egg-and-vinegar liquid). The egg yolk's lecithin coats each oil droplet and keeps them apart, which is why mayo stays thick and creamy instead of splitting into oil and water.
An emulsion is droplets of one liquid suspended in another, kept apart by an emulsifier.
An emulsion is a liquid-in-liquid colloid; do not confuse it with a foam (gas in liquid) or an aerosol (liquid or solid in gas). The emulsifier that keeps the droplets apart is usually a surfactant, with one end that likes oil and one that likes water.