Research Papers in Physics and Astronomy


Date of this Version



PNAS 2021 Vol. 118 No. 8 e2017355118


The assembly and jamming of magnetic nanoparticles (NPs) at liquid–liquid interfaces is a versatile platform to endow structured liquid droplets with a magnetization, i.e., producing ferromagnetic liquid droplets (FMLDs). Here, we use hydrodynamics experiments to probe how the magnetization of FMLDs and their response to external stimuli can be tuned by chemical, structural, and magnetic means. The remanent magnetization stems from magnetic NPs jammed at the liquid–liquid interface and dispersed NPs magneto-statically coupled to the interface. FMLDs form even at low concentrations of magnetic NPs when mixing nonmagnetic and magnetic NPs, since the underlying magnetic dipole-driven clustering of magnetic NP-surfactants at the interface produces local magnetic properties, similar to those found with pure magnetic NP solutions. While the net magnetization is smaller, such a clustering of NPs may enable structured liquids with heterogeneous surfaces.

Steubel PNAS 2021 Ferromagnetic liquid droplets SUPPL.pdf (277 kB)
Streubel Movie 1.mp4 (4485 kB)
The FMLD rotates in the oil phase dissolving POSS-NH2 ligands driven by the external rotating magnetic field and it is captured by the optical microscopy.

Streubel Movie 2.mp4 (3485 kB)
The merge of the unjammed droplets initially formed in the oil phase.

Streubel Movie 3.mp4 (4331 kB)
The rotating of dispersed assembled aggregates at the interface of the unjammed droplets.