Figure 1. (a) Two immiscible liquids, such as oil and water, will phase separate into a layer of the less dense liquid on top of a layer of the more dense liquid below with a flat interface to minimize the interfacial and gravitational energies. (b) A surfactant, generally soluble in the continuous phase, preferentially adsorbs on the oil–water interface and exchanges with monomers and micelles in solution. In this example, the surfactant is soluble in the water phase, so a direct emulsion is anticipated. (c) Shear is applied to the system, causing the oil to break up into droplets that are coated with surfactant and are inhibited from coalescing due to the interfacial repulsion. As the emulsion is sheared, larger oil droplets are stretched, undergo a capillary instability, and rupture into smaller droplets. (d) After the shear has been stopped, the emulsion can persist for many years, and a fraction of the shear energy applied is stored in the greater surface area of the droplets.
