Explosive spontaneous emulsification

Xuefei Wu, Beijing University of Chemical Technology, Lawrence Berkeley National Laboratory
Gautam Bordia, Lawrence Berkeley National Laboratory, University of California
Robert Streubel, University of Nebraska-Lincoln
Jaffar Hasnain, Lawrence Berkeley National Laboratory, University of California
Ahmad K. Omar, Lawrence Berkeley National Laboratory, University of California
Phillip L. Geissler, University of California - Berkeley
Dong Wang, Beijing University of Chemical Technology
Han Xue, Chinese Academy of Sciences
Jianjun Wang, Chinese Academy of Sciences
Thomas P. Russell, Beijing University of Chemical Technology, Lawrence Berkeley National Laboratory, University of Massachusetts, Tohoku University

Document Type Article

Used by permission.

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 the 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, the stored free energy is released through an explosive ejection of a plume of microdroplets, a dynamical interfacial instability which we term explosive spontaneous emulsification. This explosive event rapidly reduces the areal density of the NPSs to its pre-field level, stabilizing the interface. The ability to externally trigger or suppress spontaneous emulsification, through this efficient untapped energy storage and release process, has potential applications for micro-propulsion systems and remotely controlled soft microrobots.