Using a modified 3D printer, an international team of researchers led by Lawrence Berkeley National Laboratory has made a new material that is both liquid and magnetic, opening the door to a new area of science in magnetic soft matter. Their findings, published in the journal Science, could lead to a revolutionary class of printable liquid devices for a variety of applications from artificial cells that deliver targeted cancer therapies to flexible liquid robots that can change their shape to adapt to their surroundings.
Array of 1 mm magnetic droplets: fluorescent green droplets are paramagnetic without any jammed nanoparticles at the liquid interface; red are paramagnetic with nonmagnetic nanoparticles jammed at the interface; brown droplets are ferromagnetic with magnetic nanoparticles jammed at the interface. Image credit: Xubo Liu et al / Berkeley Lab.
“We’ve made a new material that is both liquid and magnetic. No one has ever observed this before. This opens the door to a new area of science in magnetic soft matter,” said Professor Tom Russell, from Berkeley Lab and the University of Massachusetts, Amherst.
Professor Russell and colleagues are developing a new class of materials — 3D-printable all-liquid structures.
Recently, they came up with the idea of forming liquid structures from ferrofluids, solutions of iron-oxide particles that become strongly magnetic, but only in the presence of another magnet.
“We wondered, if a ferrofluid can become temporarily magnetic, what could we do to make it permanently magnetic, and behave like a solid magnet but still look and feel like a liquid?” he said.
To find out, the team used a 3D-printing technique to print 1 mm droplets from a ferrofluid solution containing iron-oxide-nanoparticles just 20 nm in diameter.
Using surface chemistry and sophisticated atomic force microscopy techniques, co-authors Paul Ashby and Brett Helms of Berkeley Lab revealed that the nanoparticles formed a solid-like shell at the interface between the two liquids through a phenomenon called ‘interfacial jamming.’
“This phenomenon causes the nanoparticles to crowd at the droplet’s surface, like the walls coming together in a small room jampacked with people,” Professor Russell said.
To make them magnetic, the scientists placed the droplets by a magnetic coil in solution. As expected, the magnetic coil pulled the iron-oxide nanoparticles toward it. But when they removed the magnetic coil, something quite unexpected happened.
Like synchronized swimmers, the droplets gravitated toward each other in perfect unison, forming an elegant swirl. Somehow, these droplets had become permanently magnetic.
“We almost couldn’t believe it. Before our study, people always assumed that permanent magnets could only be made from solids,” Professor Russell said.
Through magnetometry measurements, the researchers 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.
They also found that the droplet’s magnetic properties were preserved, even if they divided a droplet into smaller, thinner droplets about the size of a human hair.
“Among the magnetic droplets’ many amazing qualities, what stands out even more is that they 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 even to the shape of an octopus — all without losing their magnetic properties,” Professor Russell said.
“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.”
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Xubo Liu et al. 2019. Reconfigurable ferromagnetic liquid droplets. Science 365 (6450): 264-267; doi: 10.1126/science.aaw8719
