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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.