Revolutionary material could lead to 3D-printable magnetic
liquid devices for the fabrication of flexible electronics, or artificial cells
that deliver targeted drug therapies to diseased cells
July 18, 2019 -- 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 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 at the Department of Energy’s
Lawrence Berkeley National Laboratory (Berkeley Lab) have done just that. Their
findings, to be published July 19 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 Tom Russell,
a visiting faculty scientist at Berkeley Lab and professor of polymer science
and engineering at the University of Massachusetts, Amherst, who led the study.
“This opens the door to a new area of science in magnetic soft matter.”
For the past seven years,
Russell, who leads a program called Adaptive Interfacial Assemblies Towards
Structuring Liquids in Berkeley Lab’s Materials Sciences Division and also led
the current study, has focused on developing a new class of materials –
3D-printable all-liquid structures.
Russell and Xubo Liu,
the study’s lead author, came up with the idea of forming liquid structures
from ferrofluids, which are solutions of iron-oxide particles that become
strongly magnetic 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 Russell.
Jam sessions: making magnets out
of liquids
To find out, Russell
and Liu used a 3D-printing technique
they had developed with former postdoctoral researcher Joe Forth in Berkeley
Lab’s Materials Sciences Division 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, staff
scientists Paul Ashby and Brett Helms of Berkeley Lab’s Molecular Foundry revealed that the nanoparticles 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, “like the walls coming together in a small room jampacked
with people,” said Russell.
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 Liu, who is a graduate
student researcher in Berkeley Lab’s Materials Sciences Division and a doctoral
student at the Beijing University of Chemical Technology.
Somehow, these
droplets had become permanently magnetic. “We almost couldn’t believe it,” said
Russell. “Before our study, people always assumed that permanent magnets could
only be made from solids.”
Measure by measure, it’s still a
magnet
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.
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, Russell and Liu
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 Russell.
Among the magnetic
droplets’ many amazing qualities, what stands out even more, Russell noted, is
that they change shape to adapt to their surroundings. They morph 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, Russell added.
Liu and Russell plan
to continue research at Berkeley Lab and other national labs 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.
“What began as a
curious observation ended up opening a new area of science,” said Liu. “It’s
something all young researchers dream of, and I was lucky to have the chance to
work with a great group of scientists supported by Berkeley Lab’s world-class
user facilities to make it a reality,” said Liu.
Also contributing to
the study were researchers from UC Santa Cruz, UC Berkeley, the WPI–Advanced
Institute for Materials Research (WPI-AIMR) at Tohoku University, and Beijing
University of Chemical Technology.
The magnetometry measurements
were taken with assistance from Berkeley Lab Materials Sciences Division
co-authors Peter Fischer, senior staff scientist; Frances Hellman, senior
faculty scientist and professor of physics at UC Berkeley; Robert Streubel,
postdoctoral fellow; Noah Kent, graduate student researcher and doctoral
student at UC Santa Cruz; and Alejandro Ceballos, Berkeley Lab graduate student
researcher and doctoral student at UC Berkeley.
Other co-authors are staff
scientists Paul Ashby and Brett Helms, and postdoctoral researchers Yu Chai and
Paul Kim, with Berkeley Lab’s Molecular Foundry; Yufeng Jiang, graduate student
researcher in Berkeley Lab’s Materials Sciences Division; and Shaowei Shi and
Dong Wang of Beijing University of Chemical Technology.
This work was supported by the
DOE Office of Science and included research at the Molecular Foundry, a DOE
Office of Science User Facility that specializes in nanoscale science.
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