Emulsions are heterogeneous systems composed of at least two immiscible liquids, water and oil, one of which is usually uniformly dispersed as fine droplets throughout the other liquid phase by a mechanical agitation process. Emulsions are considered as a type of liquid–liquid colloid. The phase existing as small droplets is called the dispersed phase and the surrounding liquid is known as the continuous phase. Emulsions are commonly classified as oil-in-water (O/W) or water-in-oil (W/O) depending on whether the continuous phase is water or oil.

Emulsions are thermodynamically unstable as the dispersed and continuous phases can revert back as separate phases, oil and water, by fusion or coalescence of droplets. However, emulsions are commonly stabilized by an emulsifying agent, often referred to as a surfactant. In general, after vigorous agitation of the two immiscible phases, the more rapidly coalescing droplets form the continuous phase. This is usually the liquid that is present in the larger amount – the greater the number of droplets, the higher the probability of collision and coalescence. Therefore, emulsification can be considered as the result of two competing processes that occur simultaneously, namely the disruption of bulk liquids to produce fine droplets and the recombination of the dispersed droplets back to the bulk liquids. Theoretically, the dispersed phase of an emulsion can occupy up to 74% of the phase volume, and such high internal phase O/W emulsions have been produced with suitable surfactants