Magnetic materials: A magnet that can twist and turn

09/30/2019

Liquid droplets containing nanoparticles can have permanent magnetism

Three liquid droplets that have a permanent magnetization can be made to rotate in unison by applying a rotating magnetic field. The orange dye shows the swirling fluid flow created by the rotating droplets.

From Ref. 1. Reprinted with permission from AAAS.

A liquid that is a permanent magnet has been developed for the first time by an AIMR-led team1. In the future, it could be used for various applications, such as actuators in liquid robotic systems.

Liquids that become magnetic when placed inside a magnetic field have been known since the 1960s, when NASA scientists investigated their potential use as rocket fuel. However, as soon as the magnetic field is switched off, these liquids rapidly lose their magnetism due to the thermal motion of magnetic particles. Until now, the only known permanent magnets were solids.

Now, Tom Russell of the AIMR at Tohoku University and his collaborators in the USA and China have succeeded in making small liquid droplets that behave similarly to solid permanent magnets, but retain liquid characteristics.

The team used a modified three-dimensional printer to inject millimeter-sized water droplets containing iron oxide nanoparticles into an oil containing a surfactant. The nanoparticles congregated near the surfaces of the droplets, forming shell-like structures. When a magnetic field was applied, the nanoparticles became magnetized. But to the team’s surprise, the droplets retained some of this magnetization even when the magnetic field was turned off.

The researchers demonstrated their liquid magnets in various ways. For example, they made several of them rotate in unison by applying a rotating magnetic field (see image). Furthermore, by drawing a spherical droplet into a narrow tube so that it became cylindrical, the team showed that the droplets remained magnetic even on changing their shape. The magnetization remained even when a droplet was broken up into hundreds of smaller droplets, effectively forming multiple permanent magnets.

The magnetic droplets are much more than a scientific curiosity; they have various exciting applications. “Conventional solid magnets have made great contributions to the development of modern industry,” says Russell. “But what if magnets were soft, flowable and reconfigurable like a liquid? Magnets that could conform to spatial constraints? The reconfigurable ferromagnetic liquid droplets we have uncovered represent a milestone for the further development of magnetic materials. These amazing liquid magnetic materials will attract attention in biology, physics and chemistry.”

Since the researchers are uncertain about how their liquid retains its magnetism, discovering the mechanism is a major priority. “We do not understand the exact coupling since the length scales between the particles is too large for dipolar coupling,” says Russell. “And so we’re currently investigating the exact origin of the coupling.”

References

  1. Liu, X., Kent, N., Ceballos, A., Streubel, R., Jiang, Y., Chai, Y., Kim, P. Y., Forth, J., Hellman, F., Shi, S. et al. Reconfigurable ferromagnetic liquid droplets. Science 365, 264−267 (2019). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.