A colloidal suspension of one liquid dispersed within another. The dispersed phase has droplet sizes usually less than 1 mm. Butter is an example of a water-in-oil emulsion, while mayonnaise is an example of an oil-in-water emulsion. Surfactants or emulsifiers are surface-active agents and used to stabilize emulsions. Detergents possess both hydrophilic and hydrophobic parts to their molecules to behave as emulsifiers. In the offshore oil industry, emulsions form at the interface of water and oil in crude oil gravity separators. Sufficient hold-up time is used to separate the emulsion, or alternatively, surface-active agents are used to encourage separation.