The rules about what makes a good magnet may not be as rigid as scientists thought. Using a mixture containing magnetic nanoparticles, researchers have now created liquid droplets that behave like tiny bar magnets.
Magnets that generate persistent magnetic fields typically are composed of solids like iron, where the magnetic poles of densely packed atoms are all locked in the same direction (SN: 2/17/18, p. 18). While some liquids containing magnetic particles can become magnetized when placed in a magnetic field, the magnetic orientations of those free-floating particles tend to get jumbled when the field goes away — causing the liquid to lose its magnetism.
Now, adding certain polymers to their recipe has allowed researchers to concoct permanently magnetized liquid droplets. These tiny, moldable magnets, described in the July 19 Science, could be used to build soft robots or capsules that can be magnetically steered through the body to deliver drugs to specific cells.
To make liquid magnets, the team submerged millimeter-sized droplets of a watery solution containing iron oxide nanoparticles in oil peppered with polymers. Those polymers drew many of the magnetic nanoparticles to the droplets’ surfaces and pinned them there, forming a densely packed shell of nanoparticles around each particle-rich droplet.
Exposing one of these droplets to a magnetic field forces the magnetic poles of its nanoparticles to point in the same direction. Nanoparticles on the droplet’s surface are crowded so closely that, when the magnetic field shuts off, their magnetic orientations can’t jostle out of alignment, the team found.
What’s more, the surface particles’ collective magnetism is strong enough to keep nanoparticles free-floating throughout the rest of the droplet in line. “So the whole droplet behaves like a solid magnet,” says study coauthor Xubo Liu, a materials scientist at Beijing University of Chemical Technology.
Like conventional bar magnets, the droplets’ opposite poles attract and their matching poles repel, and dividing up a single magnetized droplet produces smaller pieces with their own north and south poles. Liu and colleagues fashioned simple, spherical and cylindrical droplets, but 3-D printing or molding techniques could create malleable magnets with more complex forms, Liu says.
Liquid magnets could help soft robots get around, says Remi Dreyfus, a chemical physicist with the French national research agency CNRS, who is currently conducting research at a joint CNRS-Solvay-University of Pennsylvania lab in Bristol, Pa. Rather than rely on inflatable air pouches or electric current to move — which tether robots to wires or tubes — bots injected with liquid magnetic material could be remotely controlled with magnetic fields (SN Online: 9/19/18).
These droplets might also be combined to manufacture new kinds of materials, such as magnetic sponges or stretchy polymers, says Dreyfus, whose commentary on the study appears in the same issue of Science. “I’m sure people will have many ideas” for how to put ultrasoft magnets to work, he says.