Printing magnetic liquid droplets

Inventors of centuries past and scientists of today have found ingenious ways to make our lives better with magnets - from the magnetic needle on a compass to magnetic data storage devices and even MRI (magnetic resonance imaging) body scan machines.

All of these technologies rely on magnets made from solid materials. But what if you could make a magnetic device out of liquids? Using a modified 3D printer, a team of scientists have done just that. Their findings, to be 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.

"We've made a new material that is both liquid and magnetic. No one has ever observed this before," said lead author of the study. "This opens the door to a new area of science in magnetic soft matter."

The team 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?" said the author.

To find out, the researchers used a 3D-printing technique they had developed 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 sophisticated atomic force microscopy techniques the Lab revealed that the nanoparticles formed a solid-like shell at the interface between the two liquids through a phenomenon called "interfacial jamming," which causes the nanoparticles to crowd at the droplet's surface, "like the walls coming together in a small room jampacked with people," said the author.

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. "Like little dancing droplets," said another author.

Somehow, these droplets had become permanently magnetic. "We almost couldn't believe it," said the lead. "Before our study, people always assumed that permanent magnets could only be made from solids."

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 nm between each of the billion nanoparticles, together they created a solid surface around each liquid droplet. Somehow, 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 authors explained.

The researchers 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, added the lead.

Among the magnetic droplets' many amazing qualities, what stands out even more, the authors noted, 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.

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, the lead added.

The authors plan to continue research to develop even more complex 3D-printed magnetic liquid structures, such as a liquid-printed artificial cell, or miniature robotics that move like a tiny propeller for noninvasive yet targeted delivery of drug therapies to diseased cells.

https://newscenter.lbl.gov/2019/07/18/new-laws-of-attraction-scientists-print-magnetic-liquid-droplets/