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Refrigerators of the future may be inspired by the weird physics of rubber

A new refrigeration technique harnesses the ability of rubber and other materials to cool down when released from a tight twist.

ByKatherine J. WuNOVA NextNOVA Next
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The future of refrigeration might just come with a twist. Image Credit: AndreyPopov, iStock

Around the turn of the 18th century, English philosopher John Gough stumbled upon the weirdness of rubber.

When stretched, the elastic material felt hotter, then cooled below baseline upon release—observations that Gough, who’d lost his eyesight as a child, made with his lips.

The temperature difference wasn’t huge (though Gough did go on to publish his results in 1805). And though prototypes of elastically powered refrigerators have popped up in recent years, they’re less energetically efficient than those that run on vapor compression, a fossil-fuel-guzzling method found in kitchens and air conditioners worldwide.

Materials like rubber, it might seem, aren’t destined to cool the foods and office spaces of the future.

Zunfeng Liu, a materials scientists at Nankai University in China, thinks differently. In a paper published today in the journal Science, he and his colleagues make the case that the natural properties of rubber can still be leveraged to achieve some serious chill—just not in a way that researchers have considered before.

A stretch, Liu says, is great for cooling. But what’s even better is a twist: a motion that can contort material without hogging the extra space that stretched fibers tend to consume. Liu and his team, a group co-led by the University of Texas at Dallas’ Ray Baughman, have found that twisting and untwisting strands of rubber, as well as other everyday items like fishing line and sewing thread, can also make temperatures plunge—with double the cooling efficiency of simple stretch and release.

“This is a beautiful piece of experimental work,” says Sharon Gerbode, a soft-matter physicist at Harvey Mudd College who was not involved in the study. “It gives us a new way of approaching the extremely energy-intensive process of refrigeration...just by saying, ‘Let’s twist this and see what happens.’”

The team has already applied their twisted technique to something that’s, well, pretty cool: a simple “twist fridge” that can lower the temperature of a teaspoon or so of water by around 10 degrees Fahrenheit in a single, 30-second cycle, Liu says.

That’s not much water, and the technology is still in its infancy. But with more tinkering, Baughman says, the team’s discovery could someday provide an alternative to traditional cooling systems, which, in addition to consuming fossil fuels, produce gobs of greenhouse gases and account for about 20 percent of global electricity consumption.

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When materials like rubber are coiled up and released, their temperatures drop. Image Credit: MovieAboutYou, iStock

The scientific oomph behind the breakthrough boils down to a couple concepts in thermodynamics. Because rubber is a polymer, it’s made up of subunits that manifest as long chains, which, when left limp, tangle as haphazardly as loose headphone wires. A stretch or twist, however, tugs these strands into alignment, making them more orderly.

Rubber’s initial state of disarray teems with energy—energy that has to be released when its molecules get organized. That energy is released as heat, temporarily warming the elastic and its surroundings. When it’s released from contortion, rubber slips back into a more energetic jumble. This time, the molecules absorb heat from their surroundings and funnel that energy into resuming a state of chaos, allowing the rubber and whatever’s nearby to cool.

Rubber’s cool factor has been known for centuries. But so far, the research on it has pretty much been limited to stretch, Liu says. “We just wondered if there would be additional benefit if we exerted twist.”

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Natural rubber fibers in various stages of twist: twisted, partially coiled, fully coiled, and fully supercoiled. Image Credit: Wang et al. Science, 2019

And so, using fibers suspended between motorized attachment points, Liu, Baughman, and their colleagues twisted rubber until it coiled, then supercoiled, or entered an extra-wound-up state. After the warmed up material returned to room temperature, the researchers released it, and found its temperature dropped by up to 28 degrees Fahrenheit, besting another elastic that had been stretched to six times its length. The two strategies could also be combined: Rubber fibers that were stretched and twisted cooled off by up to 30 degrees Fahrenheit.

Though there are some variations at the molecular level, the same broad “energy in, energy out” principles applied for other substances, including fishing line and sewing thread, Liu says, though neither material could top rubber’s cooling ability.

These tests were mostly proof of principle, Baughman explains. “People don’t usually think of fishing line as elastic,” he says. “But if you exert so much twist that it coils...it can do some heating and cooling. We were amazed.”

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“Even with relatively widespread and cheap materials...they can induce this big effect, and with such a simple motion,” says Svetlana Boriskina, a mechanical engineer at the Massachusetts Institute of Technology who was not involved in the study. “Just knowing that is practical and useful.”

The best-performing material the team tested, Liu says, was nickel titanium (also known as nitinol) wire, an elastic alloy that’s been used in medical devices and implants. A single nitinol wire twisted up could produce a cooling effect of 31 degrees Fahrenheit; adding three more wires to the mix eked out another 7 or so degrees of chill. In a final experiment, the team bundled up three of these wires in a small chamber and flowed water over them. As the wires untwisted inside the tiny refrigerator, they cooled, bringing down the temperature of the surrounding fluid as well.

Nitinol isn’t as cheap as rubber, Liu says, but it’s hardier: While elastics lose their resilience fairly quickly, the metal alloy retained its cooling powers through 1,000 cycles of twisting and untwisting.

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Study author Zunfeng Liu holds a super-coiled rubber fiber in his laboratory. Image courtesy of Zunfeng Liu, Nankai University

To power an actual refrigerator or air conditioner long-term, though, a material would probably need to withstand several million cycles of twisting, says Ichiro Takeuchi, a materials scientist at the University of Maryland who wasn’t involved in the study. It’s also unclear how other prototypes would fare when scaled up to a larger size.

Still, Borskina, Gerbode, and Takeuchi all praise the study for its elegant and creative approach. In their first published pass, Gerbode points out, the researchers achieved greater cooling efficiencies than what’s typical for vapor compression systems. “That’s pretty impressive,” she says.

In the world of refrigeration, the twist may still be the new kid on the block, Baughman says. But it’s already established itself as a pretty cool contender, he says. “This could create enormous savings for the world.”

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