Borrowing an idea from nature could lead to technology
capable of producing full-colour prints in a fraction of a second,
according to South Korean engineers.
Many insects and birds owe their bright colours to the
interaction of light with finely-patterned surface textures, rather
than relying on pigments. The iridescent colours of a peacock's
tail are largely a result of the interaction of light with just one
biological material – melanin rods.
Engineers have long experimentedMovie Camera with replicating
these so-called structural colours in synthetic materials, and now
Sunghoon Kwon's team at Seoul National University in South Korea
has managed it.
Their M-Ink can be used to produce any colour in the visible
spectrum and could lead to a new method of cheap and fast
full-colour printing, Kwon says.
Just add nanoparticles
M-Ink contains three ingredients: magnetic nanoparticles 100 to
200 nanometres across, a solvation liquid, and a resin.
The nanoparticles disperse throughout the resin, giving the ink
a brown appearance. But when an external magnetic field is applied,
the nanoparticles immediately snap to the magnetic field lines,
forming chain-like structures.
The regularly-spaced nanoparticle chains interfere with incoming
light, so that the light reflected from the surface is of a
particular colour. Adjusting the magnetic field strength shifts the
spacing of the field lines and changes the colour, says Kwon.
"If you want to control the angle of the magnetic field [to
create curves in the image, for instance] you can combine multiple
electromagnets," he says.
Finishing off
Meanwhile the solvation liquid creates repulsive forces between
the magnetic nanoparticles, ensuring they do not simply clump
together in the ink.
Once the desired colour is produced, the nanoparticles can be
fixed in place by exposing the ink to UV light, which cures the
resin. The researchers' system uses maskless lithography to shine
UV light over just the areas of the image that are of the desired
colour. By then changing the magnetic fields and the UV light
pattern, it's possible to build up a full-colour image.
"We first set the magnet to tune colour to red and then shine UV
for 0.1 seconds [to fix the red areas of the image] and then change
to blue, for 0.1 seconds again, then green," says Kwon. "You can
pattern A4-size [letter-size] full-colour prints within a
second."
However, Kwon cautions that the prototype system takes a few
seconds to print because it can shift the magnetic field strength
only relatively slowly (see video, above).
Not like a DVD
He says that other potential applications include counterfeit
inks: "You can build papers displaying unique features on
application of an external magnetic field."
The team is also working on reversible colour fixing, which
could have applications in colour-changing gadgets.
Zhong Lin Wang at the Georgia Institute of Technology says the
approach is an interesting one, but thinks an existing method
involving laser beam patterning of a plastic surface – the method
responsible for the iridescent pattern on the surface of a DVD –
might be simpler.
Kwon says the two technologies are fundamentally different:
"Ours can change the colour of the whole plastic substrate, not
just the carving on the surface," he says. The UV fixation method
could turn out cheaper than using a laser beam, he adds.
Journal reference: Nature Photonics, DOI:
10.1038/nphoton.2009.141