Ballistic Ejection of Microdroplets from Overpacked Interfacial Assemblies

• Published in: Advanced functional materials 2023-05, Vol.33 (20), p.n/a
• Main Authors: Wu, Xuefei, Bordia, Gautam, Streubel, Robert, Hasnain, Jaffar, Pedroso, Cássio C.S., Cohen, Bruce E., Rad, Behzad, Ashby, Paul, Omar, Ahmad K., Geissler, Phillip L., Wang, Dong, Xue, Han, Wang, Jianjun, Russell, Thomas P.
• Format: Article
• Language: English
• Subjects: Assembly; Density; Dynamic stability; Ejection; Emulsification; Energy storage; explosive emulsification; Ferromagnetism; Interface stability; Interfaces; magnetic field; Magnetic fields; magnetic nanoparticle surfactants; Materials science; Microrobots; Nanoparticles; overpacked interfacial assemblies; Reduction; Remote control; Surface stability; Surface tension; Surfactants
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202213844

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, it is shown that an external magnetic field can modulate the assembly of paramagnetic nanoparticle surfactants (NPSs) at liquid–liquid interfaces to trigger an oversaturation in the areal density of the NPSs at the interface, as evidenced by a marked reduction in the interfacial tension, γ, and corroborated with a magnetostatic continuum theory. Despite the significant reduction in γ, 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. An explosive emulsification behavior induced by oversaturating the surface of a ferrofluid droplet is discovered by exposing the droplet to a magnetic field, markedly reducing the interfacial tension well below the equilibrium value. Upon releasing the field, tens of thousands of microdroplets are ballistically ejected from the interface. Consequently, the interfacial tension rapidly returns to the equilibrium value.