Emulsions: basic principles

Emulsions are metastable colloids made out of two immiscible fluids, one being dispersed in the other, in the presence of surface active agents. Emulsion droplets exhibit all the classical behaviours of metastable colloids: Brownian motion, reversible phase transitions as a result of droplet interactions that may be strongly modified, and irreversible transitions that generally involve their destruction. They are obtained by shearing two immiscible fluids leading to the fragmentation of one phase into the other. Because the lifetime of emulsions may become significant (more than a year), they become good candidates for various commercial applications. All these applications have already led to an important empirical control of these materials, from their formation to their destruction. Besides this empirical background, which is widespread among the various specific applications, the basics of emulsions are certainly progressing and we aim in this paper to give an overview of the most recent advances. We will particularly focus on interdroplet forces, reversible phase transitions and colloidal structures, monolayer adhesion and emulsion gels, coarsening and destruction and finally some stability criteria for double emulsions.