Emulsion
Emulsion, a mixture of two liquids that ordinarily do not combine, such as oil and water. When stirred or shaken vigorously, the two liquids will form a temporary emulsion. Within a relatively short time, however, the two will separate into distinct layers. A permanent emulsion can be made by adding a third substance, called an emulsifier or emulsifying agent, to the mixture. Oil and water will form a permanent emulsion if soap or detergent is added as an emulsifier. Milk is a permanent emulsion of butterfat in water, with casein acting as the emulsifier. An emulsion is a colloid.
Emulsions are used extensively in medicines and cosmetics, in the form of salves, ointments, and creams. They are also used in many manufacturing processes, especially in the making of textiles, paper, and leather products. An emulsion of light-sensitive silver salts in gelatin, coated on a paper or acetate backing, is the basis of virtually all photography.