From compasses and data storage devices to MRI devices, the technologies that improve our lives all rely on solid-state magnets. What would magnetic devices look like if they could be made of liquid? With the help of liquid phase 3D printing, this idea has become a reality.
The results were published July 19 in the journal Science under the title Reconfigurable Ferromagnetic Droplets. This research will promote the manufacture of printable magnetic liquid devices and has a broad application prospect.
Professor Thomas Russell, who led the work, explained: "We have made a new material that acts as both a liquid and a solid magnet. No one had ever observed this phenomenon before. This opens up a new field for the study of magnetic soft materials." Russell is a visiting scholar at Berkeley Lab and a professor in the Department of Polymer Science and Engineering at the University of Massachusetts Amherst. For the past seven years, Russell's project has been working to develop a new material -- an all-liquid structure that can be 3D-printed.
We know that many substances have microscopic magnetic moments inside them that can point in the same direction because of an external magnetic field. In ferromagnetic materials, the coupling between magnetic moments will ensure that the material remains magnetic after the external magnetic field is removed. In paramagnetic materials, once the external magnetic field is removed, the thermal fluctuations rapidly break the coupling between the magnetic moments so that the material no longer has a magnetic macrostate.
It is a mixture of MHD nanoparticles dispersed in a liquid. At room temperature, the thousands of nanomagnetic poles are difficult to arrange uniformly due to the thermal motion of the nanoparticles, so the magnetic fluid is paramagnetic. In the external magnetic field, the interaction of gravity, surface tension and magnetic attraction between nanoparticles can produce a spike structure on the MHD surface.
Liu, PhD student at Beijing University of Chemical Technology, is the lead author of this paper. He joined Professor Russell's research group in the autumn of 2016 and selected Fe3O4 nanoparticles with magnetic response characteristics as the model material for research. Later, during my exchange study at the University of California, Berkeley in 2017, inspired by Peter Fischer, a professor who studies magnetic materials, I changed my research direction from the microscopic theory of magnetic nanoparticle interface self-assembly to the development of macroscopic all-liquid magnetic devices. His curious question was: "If a magnetic fluid can be temporarily magnetic, how can it be permanently magnetic, behave like a solid magnet, but still remain liquid?"
When squeezed, the liquid becomes a magnet.
Russell and Liu plan to do this by experimenting with a previously developed liquid phase 3D printing technology. With the help of nanoparticles and surfactants, the technique can print stable water phase structures in the oil phase. It's like a drop of oil falling into water. When shaken, the broken oil drops reassemble due to surface tension and shrink into a ball. When added to the cleanser, these small molecular surfactants can effectively prevent the oil droplets from aggregating, allowing many tiny droplets to remain stable. What is done here is similar, but the magnetic fluid material dissolved in water is sequentially injected into the organic phase.
In the experiment, the negatively charged magnetic nanoparticles (carboxylated ferrotetroxide magnetic nanoparticles, Fe3O4-COOH NPs) with a diameter of about 20 nm were dispersed in the aqueous phase. Then, millimeter-sized water droplets were injected into the oil phase. The billions of nanoparticles wrapped in the droplets attracted to the positively charged surfactant (POSS-NH2) dissolved in the adjacent oil phase at the water-oil interface. In situ self-assembly formed magnetic nanoparticles -- surfactants, which were adsorbed at the water-oil interface.
As the nanoparticles gather, the water-oil interface becomes crowded with them, forming a solid-like shell that blocks the phase transition. The shell formed by the "interface plugging" effect can stabilize the liquid with various non-equilibrium morphologies, and the liquid device with any morphologies can be prepared by using 3D printing technology. Finally, the magnetic fluid changes from paramagnetism to ferromagnetism, which means it becomes a liquid magnet, and this interface prevents the phase transition.
The researchers placed the printed droplets next to an electromagnetic coil to make them magnetic. As expected, the coil pulled the ferromagnetic droplets toward itself. But when they changed the direction of the coil's magnetic field, something unexpected happened.
Like coordinated swimmers, the droplets move in unison, creating graceful swirls that "seem to dance," Mr. Liu said. These droplets somehow become permanent magnets. "We couldn't believe it," Russell said. Before this study, people thought permanent magnets had to be solid."
