New displays possible from colour-changing magnetised liquid
25 July 2007
Researchers at the University of California, Riverside (UCR) have
discovered that it is possible to change the colour of a liquid consisting
of very small particles of iron oxide suspended in water simply by varying
the strength of a magnetic field.
The discovery has potential to greatly improve the quality and size of
electronic display screens and to enable the manufacture of products such as
erasable and rewritable electronic paper and ink that can change colour
A suspension of iron oxide in water changing colour under a
magnetic field, with increasing strength of the field from left to right.
Credit: Yin laboratory, UCR.
The results were
published in Angewandte Chemie International Edition’s online edition
and in print in issue 34 of the journal.
“The key is to design the structure of iron oxide nanoparticles through
chemical synthesis so that these nanoparticles self-assemble into
three-dimensionally ordered colloidal crystals in a magnetic field,” said
Yadong Yin, an assistant professor of chemistry who led the research.
nanoparticle is a microscopic particle whose size is measured in nanometers,
a nanometer being a billionth of a meter. A colloid is a substance comprised
of small particles uniformly distributed in a liiquid — milk, paint and
blood are examples of colloids.
“By reflecting light, these crystals, also
called photonic crystals, show brilliant colours,” Yin said. “Ours is the
first report of a photonic crystal that is fully tunable in the visible
range of the electromagnetic spectrum, from violet light to red light.”
Iron oxide (formula: Fe3O4) nanoparticles are
“superparamagnetic,” meaning that they turn magnetic only in the presence of
an external magnetic field. In contrast, “ferromagnetic” materials become
magnetized in a magnetic field and retain their magnetism when the field is
The researchers used the superparamagnetic property of iron oxide
particles to tune the spacing between nanoparticles, and therefore the
wavelength and colour of the light reflected, by changing the strength of
the external magnetic field.
“Other reported photonic crystals can only
reflect light with a fixed wavelength,” Yin said. “Our crystals, on the
other hand, show a rapid, wide and fully reversible optical response to the
external magnetic field.”
Photonic materials such as those used by Yin and
his team could help in the fabrication of new optical microelectromechanical
systems and reflective colour display units. They also have applications in
telecommunication (fibre optics), sensors and lasers.
“What should make
the technology commercially attractive is that iron oxide is cheap,
non-toxic and available in plenty,” Yin said. "A different colour for each
pixel can be assigned using a magnetic field,” he said. “The advantage is
that you need just one material for all the pixels. Moreover, you don’t need
to generate light in each pixel. You would be using reflected light to
create the images.”
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