  Double emulsions, due to their compartmental structures, are essential in
food, agricultural, and pharmaceutical applications. Traditionally, double
emulsifications rely on the presence of both oil-soluble and water-soluble
surfactants or external stimuli responsive materials and require sequential
droplet formation settings or unique fluidic designs. We report on unusual
phenomenon where double emulsions are spontaneously formed as soon an aqueous
nanoparticle dispersion is placed in contact with an oleic micellar solution.
Nanoscale water droplets nucleate in oil in the form of swollen micelles.
Nanoparticles form a water-shell encapsulating the saturated oil phase with
swollen micelles over time. Remarkably, we find that the gradual
surface-activation of nanoparticles is key in self-double emulsification and
controlling the emulsion intensity. We build on this new discovery and design a
novel system for double emulsion formation. This approach is a scalable
self-sequential strategy for preparing core-shell double emulsions that
disperses nanoparticles in the opposite phase by employing micelles as
transport vehicles. Incorporating nanoparticles into spontaneous emulsification
systems opens novel routes for designing emulsion-based materials.
