| Abstract: Emulsions can be formed from virtually any pair of immiscible fluids in the presence of a suitable surfactant. Moreover, inverse emulsions (water drops in oil), as compared to direct emulsions (oil drops in water) can sometimes be formulated without any surfactant. In all emulsions, the primary mechanism for instability is through coalescence. This occurs when two droplets approach one another to form a thin film of the continuous fluid between the neighboring interfaces, and a hole spontaneously forms through the intervening fluid layer causing the two drops to merge into a single drop. The metastability of the thin film separating drops will govern the volume fraction of dispersed phase, f , that can be incorporated. Indeed, an emulsion can remain stable at f much larger than close packing, but ultimately fails at a characteristic volume fraction f*. However, failure can occur through two distinct scenarios: As additional dispersed phase is mixed into the emulsion at f*, the “solid” emulsion generally convert into a dilute inverted emulsion in which the previous continuous phase becomes the dispersed phase; in a second scenario, as additional dispersed phase is mixed into the emulsion at f*, the “solid” emulsion fractures and breaks into globules containing the initial emulsion, each globule being dispersed within the previous dispersed phase. By exploring the behavior of a variety of surfactant-stabilized emulsions and comparing it to the behavior of surfactant-free inverse emulsions, we will attempt to rationalize the origins of these two distinct universal scenarios. |