The classic red-and-white horseshoe magnet may have worked for Wile E. Coyote, but it can be a little inflexible. Now researchers at Lawrence Berkeley National Laboratory (LBNL) have created a more malleable tool: a miniscule liquid-based magnet made from nanoparticles.
Such flexible magnets could be useful in places where rigid ones cannot go, including soft robots or flexible electronics. And although they are not yet ready for practical applications, the liquid-based magnets reveal a new facet of nanoparticle behavior, which could pave the way for a novel range of magnetic materials, the researchers say.
To create the liquid-based magnets, researchers led by Thomas P. Russell, a polymer scientist at the University of Massachusetts Amherst and a visiting researcher at LBNL, started with a modified 3-D printer. First, they printed millimeter-sized droplets of liquid filled with magnetic nanoparticles. This liquid-particle mix is superparamagnetic—it is strongly attracted to a magnetic field, but as soon as the field disappears, so does the magnetism. In plain language, it is magnetic but not a magnet—a characteristic typical of fluids that can be magnetized, called ferrofluids.
Next, Xubo Liu, lead author of the new work and a researcher in Russell’s team at LBNL, added a surfactant to these droplets. A surfactant is a chemical that acts as an interface between oil and water. In this case, it drew the nanoparticles to the surface of each droplet so they formed a tightly packed shell around the liquid. Just like other ferrofluids, the encapsulated droplets were attracted to an external magnetic field. But the jam-packed shell brought out an unexpected new property: when Liu removed the magnetic field, the droplets held onto their magnetism. The process had made flexible, millimeter-wide permanent magnets, much like a classic bar or horseshoe magnet.
“We almost couldn’t believe it,” Russell says. “People always assumed that permanent magnets could only be made from solids.” The discovery was published last week in Science.
Unlike traditional solid magnets, these droplets can change shape. For example, in the video below, a spherical droplet gets sucked into a narrow tube, which deforms it into a cylinder. Despite being squashed and stretched, the droplet remains magnetic.
The droplets can also be reset: when the researchers crank up the pH of the liquid in which they are floating, the nanoparticle shells loosen, and the droplets become round and lose their magnetism. But they will regain it if the pH is lowered, because that change will jam the nanoparticles back into a shell around each droplet.
In the video clip below, the researchers demonstrate that the jam-packed shell of nanoparticles—and not some other phenomenon—makes these droplets permanently magnetic. The beige droplets have a dense magnetic nanoparticle shell. The green ones have a loosely packed shell. The red blobs have a packed shell, but it is composed of nonmagnetic particles. The beige droplets react strongly to a spinning magnetic field—as a permanent magnet should—whereas the red and green droplets do not.
“It’s not something I’ve seen before,” says Carlos Rinaldi, a professor of chemical engineering at the University of Florida, who also researches magnetic nanoparticles and was not involved in this study. “There is a whole field on soft magnetic matter, but I’ve never seen it done with droplets. I think they did it elegantly.”