  Spontaneous emulsification, resulting from the assembly and accumulation of
surfactants at liquid-liquid interfaces, is an interfacial instability where
microdroplets are generated and diffusively spread from the interface until
complete emulsification. Here, we show that an external magnetic field can
modulate the assembly of paramagnetic nanoparticle surfactants (NPSs) at
liquid-liquid interfaces and trigger an oversaturation in the areal density of
the NPSs at the interface, as evidenced by a marked reduction in the
interfacial tension, {\gamma}, and corroborated with a magnetostatic continuum
theory. Despite the significant reduction in {\gamma}, the presence of the
magnetic field does not cause stable interfaces to become unstable. Upon rapid
removal of the field, however, an explosive ejection of a plume of
microdroplets from the surface occurs, a dynamical interfacial instability
which is termed explosive emulsification. This explosive event rapidly reduces
the areal density of the NPSs to its pre-field level, stabilizing the
interface. The ability to externally suppress or trigger the explosive
emulsification and controlled generation of tens of thousands of microdroplets,
uncovers an efficient energy storage and release process, that has potential
applications for controlled and directed delivery of chemicals and remotely
controlled soft microrobots, taking advantage of the ferromagnetic nature of
the microdroplets.
