The beetles’ nanorods look white because
they reflect nearly all light. But other structures
can scatter light in a way that makes them
appear intensely black. In 2018, Dakota McCoy
at Stanford University in California and her
colleagues investigated the wings of birds of
paradise kept as specimens in museums and
found that some have nanostructures that
scatter nearly all light in this way.
The team found that a typical black feather
reflects between 3 and 5 per cent of light, but
when viewed from certain angles, the birds of
paradise feathers reflected less than 0.3 per
cent. Part of the work involved coating the
feathers in gold dust, to make them amenable
to analysis by electron microscope – and they
still looked black even then. McCoy says this
super-blackness helps set off the vibrant
colours elsewhere in the birds’ plumage,
which they use to attract mates.Beat the heat
Strange as it sounds, structural colour isn’t
just about colour. Because the size of the
nanostructures involved determines the
wavelength of radiation they affect, slightly
larger structures scatter infrared radiation –
otherwise known as heat – not visible light.
Nature hasn’t missed this. Some butterflies
are thought to use such effects to absorb heat
and warm their bodies ready for flight. Other
insects use it to beat the heat. Saharan silver
ants scurry across the desert, where air
temperatures can hit 50°C. A few years ago,
we discovered that their bodies are covered
in microscopic prism-like hairs that reflectalmost all heat. This reduces their body
temperature by a couple of degrees,
helping them survive conditions in which
other creatures perish.
In 2020, materials scientist Han Zhou at
the Shanghai Jiao Tong University in China
looked at a species of longhorn beetle that
lives on active volcanoes in Indonesia, where
ground temperatures can top 70°C. She found
the beetle is covered in highly reflective
prisms that keep it cool.
Inspired, Zhou wondered if she could
design a structural colour coating that could
be applied to buildings to lower temperatures.
We already paint houses white for this reason,
of course, as part of our quest to hit net-zero
carbon emissions. But Zhou suspected that
structural colour could perform better than
regular white paint.
She developed a polymer film coated
in microscopic pyramids and a random
arrangement of ceramic particles. She says this
mimics the effect of the beetles’ nano-prismsand the coating reflects around 95 per cent of
light. Zhou tested it by applying the material
and a sheet of white paper to a car bonnet. She
found that the snazzy new polymer kept the
car about 5°C cooler.
Others, including Shen and the team that
discovered the secrets of the Cyphochilus
beetles’ scales, say they are also working on
highly heat-reflecting structural materials. But
would these really be better than white paint?
David Sailor at Arizona State University
and his colleagues recently conducted a
modelling study in which they compared
the cooling effects of white paint, which
is about 70 per cent reflective, with a
supercooling coating that is about 95 per cent
reflective. They modelled this across eight US
cities and found that the reduction in demand
for energy for cooling could be more than
doubled when using the structural coating
compared with white paint.
Structural coatings could also be more
environmentally benign to produce. White
paint often uses titanium dioxide as a pigment.
“You usually extract it by melting rocks,”
says Benjamin Droguet, who also works in
Vignolini’s lab. “It is definitely not something
which is good for the environment.” Because
their colour comes from structure, not specific
chemicals, iridescent materials can be made
from almost anything, including renewable
materials like recycled plastic or cellulose
derived from wood pulp.
An unexpected bonus of supercooling
structural colour materials is that they
don’t have to be white. Not everyone wants
a white building, and they can be so bright
that they dazzle passers-by. The trouble is
that conventional coloured paints absorb a
lot of infrared radiation, so white is usually
the best option by far for keeping cool.
To get around this, Yuan Yang at Columbia
University in New York and his colleagues have
developed a coating with two layers: one has an
iridescent structure that reflects 90 per cent of
infrared light, then regular coloured paint is
added on top. Tests have shown that a black
version can keep surfaces almost 16°C cooler
than standard black paint.
Yang says the coatings work, though, in any
shade or hue. The cities of our net-zero future
will need to be cooler, but that doesn’t mean
they can’t be as colourful as a rainbow. ❚A slime mould
stores spores
in iridescent
structures
called
sporangiaMichael Allen comes in
any colour you like as
long as it is readAN
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SA
ND
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76 | New Scientist | 18/25 December 2021“ Iridescent
materials
can be made
from almost
anything”