Scientific American - 09.2019

(Darren Dugan) #1
September 2019, ScientificAmerican.com 15

MARCUS DRYMON


Field Museum


MATERIALS SCIENCE

Cracked


Canvases


Microstructures in plastic
produce colorful portraits

For millennia humans have created art with
pigment-based paints, inks and dyes. Now
researchers have produced tiny plastic paint-
ings whose colors come from variations in
microscopic surface features instead.
Pigments are chemicals that absorb cer-
îDž³§žšîÿDþx§x³îšäD³lßx‹x`î ̧îšxßäî ̧
Çß ̧lø`xäÇx`ž‰`` ̧§ ̧ßäÍ
øîä ̧­x­Dîxߞ-
D§ääø`šDäîš ̧äx ̧³­ ̧ßǚ ̧UøîîxߋžxäÜ
iridescent blue wings and the striking feath-
ers of some hummingbirds—produce colors
based on the size and spacing of micro-
structures on their surfaces, which interact
ÿžîš§žšîÿDþx§x³îšä ̧…lž†xßx³î䞱xäÍ
Many plastics form minuscule cracks,
called crazes, when put under stress.
Typically these fractures occur randomly
îšß ̧øš ̧øîîšx­DîxߞD§Í
øî‰ßäîxĀÇ ̧䞳
some plastics to light beams can selective-
ly weaken them in places, where crazes
will appear when the plastic is stressed.
“You can actually control where the cracks
form,” says materials scientist and study
co-author Andrew Gibbons of Kyoto Uni-

versity in Japan. Depending on size and
` ̧³‰øßDîž ̧³jîšxäx`ßD`¦äD`îDä­ž`ß ̧-
äîßø`îøßxäîšDîÇß ̧lø`xäÇx`ž‰`` ̧§ ̧ßäÍ
Gibbons and his colleagues shone pow-
erful LEDs on thin pieces of plastic and then
dunked them in acetic acid, generating
crazes in the places preweakened by light.
5šxäx`ßD`¦äž³žîžD§§āßx‹x`îîšxäD­xÿDþx-
length of light to which the section of plastic
ÿDäxĀÇ ̧äxljD`` ̧ßlž³î ̧îšxäîølājÿšž`š
was published in June in Nature.
…îšxǧDäîž`žää ̧D¦xl§ ̧³xß ̧ßxĀÇ ̧äxl
î ̧šžšîx­ÇxßDîøßxäjîšx`ßD`¦ä`D³xĀÇD³l
î ̧ßx‹x`î§ ̧³xßÿDþx§x³îšäÍ5šx䞱x ̧…
each region hit by light and the thickness of
îšxǧDäîž`D§ä ̧ž³‹øx³`xš ̧ÿ…Dßîšx`ßD`¦ä
xĀÇD³lÍ5 ̧îxäîîšxžß­xîš ̧ljîšxßxäxDß`š-
ers produced miniature renderings of classic
paintings and even a Queen album cover.
(The smallest was 0. 25 millimeter across.)

ÙîÜäD³ž³³ ̧þDîžþxîÿžäîjÚäDāäÇ ̧§ā­xß
scientist Christopher Soles of the National
Institute of Standards and Technology,
who was not involved in the study. “Usual-
ly crazing in materials is a very bad thing,”
Soles notes, “but here the crazes are use-
ful.” He was surprised the process worked
ÿžîšä ̧­D³ālž†xßx³îîāÇxä ̧…ǧDäîž`äj
including polystyrene, polycarbonate and
acrylic glass—used in food containers, CD
cases and bulletproof glass, respectively.
Gibbons says crazing could potentially
create a durable plastic coating for cur-
rency or high-end merchandise to dis-
courage counterfeiters. And the micro-
structures can produce more than pretty
pictures. Eventually he hopes the tech-
nique could be used to create devices that
store microscopic amounts of liquid for
medical analysis. — Jennifer Leman

FIELD BIOLOGY

A Migrating


Snack


For young sharks, land-based
birds can be easy targets

=šx³‰äšxߞxäbiologist James Drymon
noticed feathers in the vomit of a tiger shark,
šx‰ßäîDääø­xlîšxāUx§ ̧³xlî ̧ä ̧­x
unfortunate seabird: a gull, perhaps, or a
Çx§ž`D³Í
øîÿšx³šxD³lšžäîxD­x³xîž-
cally sequenced the feathers, the results sur-
prised them: the quills came from a land-
based songbird called a brown thrasher. So
ÿšDîÿDäžîl ̧ž³ž³DxßäšDߦÜääî ̧­D`š
ž³îšxø§… ̧…$xĀž` ̧Õ
Drymon, a researcher at Mississippi
3îDîx7³žþxßäžîāÜä ̧DäîD§2xäxDß`šD³l
Āîx³äž ̧³ x³îxßjD³lšžä` ̧§§xDøxää ̧ßîxl

through the stomach contents of 105 juve-
nile tiger sharks between 2010 and 2018.
Nearly 40 percent had recently feasted on
birds that hail from dry land. In all, the scien-
tists counted 11 terrestrial bird species show-
ž³øÇ ̧³îšxäšDߦäÜ­x³øÍ5šxßxäø§îä
were published online in May in Ecology.
2xäxDß`šxßäšDþx¦³ ̧ÿ³䞳`xîšx¿ ́éćä
that sharks sometimes eat songbirds, “but
what was interesting to us was the preva-
lence” of the behavior, Drymon says. “This is

something that happens every year in a high
number” of sharks.
Every fall and spring, songbirds under-
take dramatic migrations across the Gulf of
$xĀž` ̧Í…UDlÿxDîšxß` ̧­xäD§ ̧³jîšxā
can be forced to land on the water—which is
x†x`îžþx§āDlxDîšäx³îx³`xÍÙ 5šxxäDîx
for the number of migrants that die because
of storm-related events is in the billions,”
Dr ymon says. He suspects that sharks have
long taken advantage of this twice-yearly
nutritional bounty raining down from the
skies, but scientists have only recently had
îšxx³xîž`î ̧ ̧§äî ̧` ̧³‰ß­äUāžlx³îž…ā-
ing partially digested feathers.
The results underscore how intercon-
nected marine and terrestrial ecosystems
can be, says University of Miami marine
ecologist Neil Hammerschlag, who was
not involved in the study: “It shows how
opportunistic and amazingly generalist
these sharks are.” — Jason G. Goldman

Girl with a Pearl Earring, by Johannes
Vermeer, re-created in polystyrene

ANDREW GIBBONS


Kyoto University


Songbird from
a tiger shark’s
stomach
Free download pdf