Two Liquids: Continuous and Divided

The two liquids in an emulsion can be thought of as the container and the contained: one liquid is broken up into separate droplets, and these droplets are contained in and surrounded by the intact mass of the other liquid. In the usual shorthand, an “oil-in-water” emulsion is one in which oil is dispersed in a continuous water phase; “water-in-oil” names the reverse situation. The dispersed liquid takes the form of tiny droplets, between a ten-thousandth and a tenth of a millimeter across. The droplets are large enough to deflect light rays from their normal path through the surrounding liquid, and give emulsions their characteristically milky appearance.