Ferromagnetic liquid droplets with adjustable magnetic properties

Abstract

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.

Ferrofluids and bio-ferrofluids: looking back and stepping forward

“…460,461 The two-dimensional layer of magnetic nanoparticles generated on the curved interface between two immiscible liquids is the result of the restriction of translational and rotational degrees of freedom and increased dipolar interactions between close packed nanoparticles. 462…”

Reversible morphology-resolved chemotactic actuation and motion of Janus emulsion droplets

“…Utilizing these dynamics, a series of artificial soft-matter robots on the micro- or nanoscale powered by chemical fuels have been reported. Motion can be mediated by (photo-)catalytic reactions at their interface 11 , 12 , surface-encoded translation pathways 13 , by bacteria 14 , or via different physical stimuli, including light, sound, magnetic, or electric fields 15 – 19 .…”

Aquabots

“…Ferrofluid droplets can overcome this limitation, where extreme deformability can be achieved by controlling interfacial tension or droplet size, enabling navigation through highly confining channels, but upon removal of the magnetic field, shape is lost. However, liquid droplets can be structured into nonequilibrium shapes by the interfacial jamming of nanoparticles (NPs), and, if the NPs are paramagnetic, ferromagnetic liquid droplets (FMLDs) , can be produced that can be actuated and structured. , However, FMLDs have only been generated at the water–oil interface, which is not compatible with biological systems. The solid components in soft robots are also not amenable to surface reactions, unlike their counterparts in nature.…”