emulsion

An emulsion is a heterogeneous mixture of two immiscible liquids in which one liquid is dispersed as droplets within the other. The dispersed phase forms droplets that range from submicrometer to several micrometers in diameter within the continuous external phase. Emulsions are typically stabilized by emulsifiers—surface-active agents that adsorb at the liquid–liquid interface and reduce interfacial tension to form a protective interfacial film.

Two common types are oil-in-water (O/W), where oil droplets are dispersed in water, and water-in-oil (W/O), where water droplets are dispersed in oil. Examples include milk and many salad dressings (O/W) as well as butter and certain creams (W/O). Emulsions can also involve solid particles at the interface, giving rise to Pickering emulsions.

Stability is not thermodynamically favored, so emulsions tend to separate over time by creaming or sedimentation, coalescence of droplets, flocculation, or Ostwald ripening. Stability is enhanced by appropriate emulsifiers, proteins, or solid stabilizers, by controlling droplet size and distribution, and by using electrostatic or steric stabilization through charged species or polymers. The interfacial film strength and droplet charge (zeta potential) influence longevity.

Emulsification is achieved through high-shear mixing, homogenization, microfluidization, ultrasonic dispersion, or phase-inversion techniques that can alter the phase type (O/W vs W/O). Characterization focuses on droplet size distribution, rheology, and interfacial tension.

Applications span food, cosmetics, pharmaceuticals, paints, and agrochemicals. Emulsions are also used in drug delivery and parenteral nutrition, where biocompatible oil and water phases enable controlled release and transport.