Emulsions are heterogeneous systems consisting of at least one immiscible liquid dispersed in another in the form of small droplets of diameter usually greater than 0,1 μm. Such systems are thermodynamically unstable but they can be persistent if stabilized by surface-active components. Emulsions can be water-in-oil, which are referred to as regular emulsions, as well as oil in-water, which are called reverse emulsions.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards