Ballistic Ejection of Microdroplets from Overpacked Interfacial Assemblies

TL;DR

This study demonstrates how magnetic fields can trigger explosive microdroplet ejection from liquid interfaces with paramagnetic surfactants, enabling controlled droplet generation and potential applications in delivery systems and microrobotics.

Contribution

It reveals a novel magnetic field-induced explosive emulsification mechanism for controlled microdroplet ejection from liquid-liquid interfaces.

Findings

Magnetic fields modulate nanoparticle surfactant assembly at interfaces.

Rapid removal of magnetic field causes explosive microdroplet ejection.

Controlled droplet generation can be used for chemical delivery and microrobots.

Abstract

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.