An emulsion is a mixture of two or more immiscible liquids, one of which is dispersed in the other in the form of very fine droplets. Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). The dispersed-phase particles do not settle, and there is sufficient interfacial tension between the phases to prevent their separation. The size of the dispersed-phase droplets in an emulsion is usually 1 to 1000 micrometers (µm).
The word emulsion comes from the Latin word for “milk,” which is the most familiar type of emulsion. Milk is an emulsion of fat droplets in water. Butter is an emulsion of water in fat, and margarine is an emulsion of water in oil. Emulsions are used in many foods, such as mayonnaise, salad dressings, sandwich spreads, and ice cream. They are also used in cosmetics and pharmaceuticals.
A number of factors can destabilize an emulsion, causing the dispersed phase to coalesce (come together) and separate from the continuous phase. These factors include changes in temperature, changes in the pH, the addition of certain compounds, and mechanical agitation.
Emulsions are used in printing to produce half-tones (shades of gray) and color photographs. In lithography, an oil-based ink is dispersed in water, and the mixture is applied to a printing plate. The hydrophobic (water-repelling) ink adheres to the printing surface, while the hydrophilic (water-attracting) water is repelled. When the printing surface is brought into contact with a sheet of paper, the ink is transferred to the paper.
In intaglio printing, such as engraving and etching, an emulsion of wax or resin is used to protect the areas of the printing plate that are not to be printed. The printing ink is then applied to the exposed areas of the plate, and the excess is removed. The plate is then placed on a press, and the paper is run through the press, where it picks up the ink from the engraved or etched areas of the plate.
In offset lithography, the printing plate is a metal plate with a coating of light-sensitive material. The plate is exposed to light, which hardens the light-sensitive coating in the areas that are to remain white. The plate is then developed, which removes the unhardened areas of the coating.