What follows is adapted in part from this Nebraska Today article by Scott Schrage.
Drop by drop, magnetism is entering a new phase—one that may point the way to liquid, morphing robotics and other fluid technologies that can do what their solid, static counterparts cannot.
When you think of magnets, you likely think of solid materials. The permanent magnets we use in everyday life, from sticking photos and letters to our refrigerators to picking up loose nuts and bolts when working on our house or cars, are called ferromagnets. Ferromagnets originate from tiny magnets on the atomic scale whose south and north poles are spontaneously aligned. To be useful, ferromagnets need to be strong, retaining their magnetization over time. By contrast, soft magnets like the steel in paper clips don’t ordinarily stick to one another, but when they are magnetized by a strong ferromagnet, you can string a whole series of paper clips together through magnetic attraction (Fig. 1). Once they are removed from the ferromagnet, however, the attraction between them fades quickly.
Until very recently, ferromagnetism had only been found to exist in solid states, and only in a limited number of materials. So-called ferrofluids are formed by dissolving billions of iron oxide nanoparticles in water or alcohol. Because the nanoparticles are so small, they only exhibit magnetic behavior when an external magnetic field is applied (Fig. 2), similar to the behavior of the paper clips. (For more on ferrofluids, see “The Rise of Ferrofluids,” Chemistry World.)
But physicists wondered if it might be possible to create a liquid that would be permanently magnetic, like a refrigerator magnet with fluid properties. They designed an experiment similar to making vinegar and oil salad dressing at home. If you try to mix oil and vinegar, you’ll find that no matter how much you shake them up, the two liquids do not stay mixed. The oil floats on top of the vinegar. If you look closely, however, you will see that water droplets of varying sizes have spread from the vinegar through the oil. A kind of molecule called an emulsifier—one such molecule is found in eggs and is added to vinegar and oil to make mayonnaise—binds to both liquids and keeps the droplets suspended. Similarly, the researchers dissolved iron oxide nanoparticles in water and then dropped millimeter-sized droplets of this solution into oil with an emulsifier that kept some of the nanoparticles trapped at the surface of the water droplets (Fig. 3). While the iron oxide nanoparticles by themselves did not behave as ferromagnets due to their tiny size, the researchers found that when they applied and then removed a magnetic field to the mixture, the iron oxide nanoparticles on the surface of the water droplets became ferromagnetic, creating a ferromagnetic liquid!
These materials are a playground for scientists—every time we change the conditions, we see new behaviors emerge. Recently, for example, researchers added nonmagnetic nanoparticles to the mixture of iron oxide particles, which they expected to weaken the ferromagnetism of the droplets. Not only did the droplets’ magnetism remain much stronger than they expected, however, but the addition of the nonmagnetic particles produced distinct patches of ferromagnetic fluid with their magnetism aligned in the same direction. In Fig. 4, the colored dots represent magnetic nanoparticles at the water-oil interface, and the arrows show the direction of their magnetization. In the droplet on the left, only iron oxide nanoparticles are present, and the orientations of their magnets are more or less random. In the droplet on the right, the iron oxide has been mixed with nonmagnetic nanoparticles, creating a more orderly arrangement of magnets that can be controlled and tuned. For example, by rotating the external magnetic field, researchers found they could rotate the patterns on the droplets.
Not only does this create incredibly beautiful new shapes, but it allows scientists to study how rapidly the ferromagnetic surface of nanoparticles forms under different conditions. And while the vinegar droplets in your salad dressing are spherical, the researchers found that ferromagnetic liquids could “lock in” nonspherical shapes such as ellipsoids or rods, as shown in Fig. 5. The result is a permanently magnetic, rigid liquid that could enable 3D printing of structured liquids (see Fig. 5) and pave the way for all-liquid microrobotics with tailored structural and magnetic properties!