Explosive Emulsification Induced by an Electric Field

We explore the behavior of explosive emulsification of a droplet of an aqueous dispersion of carboxy-functionalized nanoparticles (NPs) immersed in a toluene solution of triamine-modified polystyrene (PS-triNH₂), a cationic surfactant under a dc electric field. The E-field biases the movement of the charged NPs and the stability of the NPs surfactants (NPSs) at the interface. For a positively charged interface, the NPs are depleted from the interface, due to the electric potential gradient, while, regardless of the field direction, the adsorption rate of the PS-triNH₂ to the interface is increased due to electric field-induced flows. As a result, the NPSs formed at a negatively charged interface have a higher binding energy by anchoring the NPs to the ligands, spontaneous emulsification is suppressed, leading to an oversaturation of the interface and an interfacial tension much lower than the equilibrium value. Dipolar interactions between the NPSs are absent when the field is removed, triggering an explosive emulsification, where tens of thousands microdroplets (d ~4 μm) are ejected from the interface with velocities of up to 5 mm/s. The propulsion of the droplets occurring during this explosive event is sufficient to move a droplet (d=1.8 mm) at a velocity of 8 mm/s. The interface, oversaturated with NPSs, represents a unique energy storage system.