Magnetics Business & Technology - November/December 2019 - 18

RESEARCH & DEVELOPMENT

Scientists Print Magnetic Liquid Droplets to Create a Revolutionary New Material at
Berkeley National Lab
Scientists at Lawrence
Berkeley National
Laboratory have
made a new material
that is both liquid and
magnetic, opening the
door to a new area of
science and potential
product development
in magnetic soft matter. Basically, they
have managed to
print magnetic liquid
droplets. Their findings
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 in California 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
the lab's Materials Sciences Division, has focused on developing
a new class of materials - 3D-printable all-liquid structures. Their
findings were published July 19 in the journal Science.
The image above shows an array of 1 millimeter 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.
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.
To find out, they used a 3D-printing technique they had developed with former postdoctoral researcher Joe Forth at the lab
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 the 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

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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.

Permanently magnetized iron-oxide nanoparticles gravitate
toward each other in perfect unison.
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
at the lab and a doctoral student at Beijing University of Chemical Technology and is credited for the accompanying photos.
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, they confirmed they indeed had 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

November/December 2019

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Magnetics Business & Technology - November/December 2019

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