An emulsion is a mixture of two liquids that do not normally combine, with one dispersed as fine droplets throughout the other. Oil in water and water in oil are the common arrangements, and which is which determines much of how the product behaves.
Emulsions are not naturally stable - the droplets tend to merge and the phases separate again. Keeping them together requires a surfactant or emulsifier positioned at the droplet surfaces, and mechanical energy to break the dispersed phase into droplets small enough to separate only slowly. See homogenisation.
Whether an emulsion is oil-in-water or water-in-oil affects how it feels, what it can be diluted with, and how it dries. An oil-in-water emulsion has water as the continuous phase, so it dilutes with water and feels lighter; water-in-oil behaves the opposite way. The same ingredients can often produce either, depending on how they are combined.
Emulsions appear across paints, adhesives, lubricants, agrochemicals and personal care products. Because stability is the main failure mode, emulsion products almost always require stability testing before a shelf life can be claimed. See emulsion manufacture.
Do not commit to a shelf-life claim on an emulsion before stability data exists in the final packaging. Emulsions that hold for weeks in the laboratory can separate over months in a warm warehouse, and the container material affects the outcome. Agree in advance what degree of separation is acceptable on delivery, as some emulsions cream slightly and redisperse on shaking without any defect existing - a point worth settling before the first consignment arrives.
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Last reviewed: August 2026. Regulation and guidance change — verify anything you rely on against the primary source. Tell us if something here is out of date.
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