Systems such as magnetic data storage devices and MRI body scan machines rely on magnets made from solid materials. Now, using a modified 3D printer, scientists have made magnetic devices from liquids.
The 3D-printable all-liquid structures are formed from ferrofluids, which are solutions of iron-oxide particles that become strongly magnetic in the presence of another magnet. A 3D printing technique was used to print 1-millimeter droplets from a ferrofluid solution containing iron-oxide nanoparticles just 20 nanometers in diameter (the average size of an antibody protein). Using surface chemistry and atomic force microscopy, it was shown that the nano-particles formed a solid-like shell at the interface between the two liquids through a phenomenon called “interfacial jamming.” This causes the nanoparticles to crowd at the droplet’s surface.
To make them magnetic, the scientists placed the droplets by a magnetic coil in solution; the magnetic coil pulled the iron-oxide nanoparticles toward it. When the magnetic coil was removed, the droplets gravitated toward each other in unison, having become permanently magnetic.
All magnets, no matter how big or small, have a north pole and a south pole. Opposite poles are attracted to each other, while the same poles repel each other. Through magnetometry measurements, the scientists found that when they placed a magnetic field by a droplet, all of the nanoparticles’ north-south poles — from the 70 billion iron-oxide nanoparticles floating around in the droplet to the 1 billion nanoparticles on the droplet’s surface — responded in unison, just like a solid magnet. Key to this finding were the iron-oxide nanoparticles jamming tightly together at the droplet’s surface. With just 8 nanometers between each of the billion nanoparticles, together they created a solid surface around each liquid droplet.
When the jammed nanoparticles on the surface are magnetized, they transfer this magnetic orientation to the particles swimming around in the core, and the entire droplet becomes permanently magnetic —just like a solid. The droplet’s magnetic properties were preserved even if they divided a droplet into smaller, thinner droplets about the size of a human hair.
The magnetic droplets also change shape to adapt to their surroundings, morphing from a sphere to a cylinder to a pancake, or a tube as thin as a strand of hair, or to the shape of an octopus — all without losing their magnetic properties. The droplets can also be tuned to switch between a magnetic mode and a nonmagnetic mode. And when their magnetic mode is switched on, their movements can be remotely controlled as directed by an external magnet.