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Rubber-metal composite material Gallery: The dazzling winners of the
stiffens up in response to stress 15th annual Smithsonian photo
contest
Michael Irving | February 19th, 2018

2 PICTURES

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of electricity

Examples of the new material that turns from rubber to metal under stress: (le t) a normal strip
of the material, (middle) a twisted strip of the stiffened material holding its new shape, and
(right) a demonstration of the strength of the material (Credit: Christopher Gannon)

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Rubber and metal are usually thought to sit at opposite ends of the stiffness Did scientists really just discover a
spectrum, but a new composite material developed at Iowa State University new organ in the human body?
could switch from the flexibility of the former to the stiffness of the latter
after being bent, twisted or squeezed. Chinese space station set for
uncontrolled Easter reentry
To develop the material, the team built on work in "undercooling," a
technique that lets metals remain in a liquid state at temperatures far SPONSORED CONTENT
cooler than when they'd usually solidify. Some of the same Iowa State RECOMMENDED FOR YOU
researchers previously experimented with the stuff as a heat-free
alternative to soldering. PDFCROWD

In the new research, the team first exposed droplets of melted metal to
oxygen, creating an oxidized shell around them that keeps them in a liquid
form. These tiny bubbles of undercooled metals are then gently mixed into
an elastomer material.

Those bubbles will break open when the rubber in which they're
embedded is flexed, bent, twisted, squeezed or put under any other kind of
mechanical pressure. The liquid metal flows out and fuses together,
hardening to form a kind of metal mesh inside the rubber. In their lab tests,
the researchers found the material would stiffen by up to 300 percent,
allowing it to support up to 50 times its own weight.

"A device with this material can flex up to a certain amount of load," says
Michael Bartlett, co-lead author of the study. "But if you continue stressing

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it, the elastomer will stiffen and stop or slow down these forces." PDFCROWD

Judging by the technique, it sounds like a one-way street. Once it's metallic
and hard, the material is not likely to return to its original rubbery state.
Other stress-induced stiffness-switching materials aren't so permanent, like
the one developed last year at the University of Michigan that used a unique
surface geometry of struts and hinges.
Its stiffness may not be reversible, but the new material would still have
plenty of applications. The researchers say it could be used to protect vital
sensors or components of soft robots or wearable electronics, or to support
delicate tissues in biological uses.

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Since you don't necessarily want a rubber that will turn to metal at a
sneeze, the researchers say the material could be tuned to withstand a
certain amount of stress, and only harden up after a set threshold is
crossed. This kind of tuning could be done by changing the metal inside the
particles (currently a bismuth, indium and tin alloy), using different-sized
particles or changing the soft material surrounding them.

The research was published in the journal Materials Horizons.

Source: Iowa State University

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TAGS #BUBBLE #FLEXIBLE #IOWA STATE UNIVERSITY #MATERIALS #METALS
#RUBBER

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