Explosive Spontaneous Emulsification*

We investigated the spontaneous emulsification characteristics of an aqueous containing carboxy-functionalized Fe3O4 paramagnetic nanoparticles emersed in a toluene solution of triamine-modified polystyrene (PS-triNH2), a cationic surfactant.  The NPs and surfactants interact at the interface between the fluid to form nanoparticle surfactants, NPSs, reducing the interfacial tension (IFT).  With increasing areal density of NPSs, the IFT decreases to a point where a spontaneous emulsification occurs, with microdroplets spontaneously forming at the interface. In the presence of an external magnetic field induced dipolar interparticle interactions trap the droplet in an oversaturated state. Upon removal of the magnetic field, an explosive relaxation towards equilibrium is observed via a plume of ferromagnetic liquid microdroplets that are jettisoned from the surface of the parent drop. The ability to externally trigger or suppress spontaneous emulsification, through this highly efficient energy conversion mechanism, introduces a unique energy storage system.