Emulsion droplets can be stabilized against coalescence by many types of interfacially active species, including proteins, small-molecule surfactants, polymers, nanoparticles and microparticles. When the fluid-fluid interface of an oil-inwater (o/w), or water-in-oil (w/o), emulsion is stabilized by adsorption of particles at the interface, a "Pickering emulsion" is obtained in which the particles occupy the fluid-fluid interface and prevent, or retard, droplet coalescence. Pioneering work of Ramsden [1] and Pickering [2] on paraffin/water emulsions containing solid particles, such as iron oxide, silicon dioxide, barium sulfate, and kaolin clays demonstrated the critically important role of particles in such interfacial stabilization. Theoretical treatment of nanoparticle-stabilized droplet structures, reported for example by Pieranski [3] and Binks, [4][5][6] describe the reduction of the overall surface energy of the system due to nanoparticle interfacial segregation as a function of particle size and the relative interfacial energies of the system (oil-water, oil-particle, and water-particle).The tendency of particles to localize to oil-water interfaces opens opportunities to fabricate new materials based on the individual and collective properties of the particles, including: 1) the optical properties derived from colloidal crystallization, [7,8] 2) self-assembled conducting structures, [9,10] and 3) encapsulation and release technologies. [11,12] For example, Weitz and co-workers reported the oil-water interfacial assembly of polystyrene microspheres into hexagonally packed capsule structures termed "colloidosomes." [13] For nanoparticles, the energy holding each particle at the fluid-fluid interface is smaller than that for microscale objects; nonetheless, there have been numerous reports of nanoparticle-stabilized emulsion droplets, in which the droplets are stable for hours, days, or longer. CdSe QDs, [14,15] Au NPs, [16][17][18] Fe 3 O 4 NPs, [19] "Janus" nanoparticles [20] and plantderived virus particles [21] all prove amenable to stabilizing fluid-fluid interfaces of different types. Moreover, embedding functional ligands on nanoparticle surfaces provides access to further droplet stabilization through chemical cross-linking of the ligands, thereby converting these dynamic self-assembled systems into robust structures. [22] Double-emulsion droplets, whether water-in-oil-in-water (w/o/w), or oil-in-water-in-oil (o/w/o), are attractive for providing a means to control release from the inner phase to the outer phase, while effectively shielding the interior phase from the continuous phase. Double emulsions can be prepared by a one-or two-step emulsification process, in the presence of a relatively hydrophilic surfactant that stabilizes o/w droplets, and a relatively hydrophobic surfactant that stabilizes the w/o interfaces.[23] Such emulsions have been generated during phase inversion processes, that is, when the continuous phase of the immiscible liquid-liquid dispersion becomes the dispersed phase. We n...
