The surface free energy that exists between two immiscible liquid phases, such as oil and water. (Surface tension is the term for the energy barrier between a liquid and air. ) The energy barrier produced by interfacial tension prevents one liquid from becoming emulsified into the other. To form an emulsion, surface free energy must be lowered by adding a third component (an emulsifier) that seeks the interface. Spontaneous emulsification does not typically occur just by adding an emulsifier. Mechanical and heat energy are usually needed to break bulk liquid into droplets. Very low interfacial tension requires less external energy to form a stable emulsion.