A new twist on staying cool: X-rays unravel the mechanism

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  • Published: Oct 15, 2019
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: A new twist on staying cool: X-rays unravel the mechanism

A cool yarn

An X-ray crystallographic study reveals how twisting fibres gives rise to a novel cooling effect that might be used in low-energy refrigeration systems. This natural rubber fiber has been coated with a paint that changes color when the fiber changes temperature. University of Texas at Dallas researchers and their colleagues recently showed that when the rubber fiber is twisted, it heats up. After reaching room temperature, the fiber cools as it is subsequently untwisted.

An X-ray crystallographic study reveals how twisting fibres gives rise to a novel cooling effect that might be used in low-energy refrigeration systems.

Researchers at The University of Texas at Dallas, USA, and Nankai University in China have found that twisting and untwisting fibres can have a cooling effect. Writing in the journal Science, the team explains how natural rubber, polyethylene fishing line, and nickel titanium wire when twisted undergo what the team calls "twistocaloric cooling". Team leader Ray Baughman has dubbed devices that would exploit this phenomenon "twist fridges" and working alongside the team of Zunfeng Liu at Nankai University in Tianjin, the researchers hope that twist fridges will be utilized in a wide range of cool applications.

Refrigeration and air-cooling and conditioning are energy-intensive processes and finding sustainable low-energy alternatives is now high on the agenda in terms of solutions to combat pollution and carbon emissions as well as reduce our dependence on fossil fuel energy sources. Indeed, refrigeration and air conditioning consume about a fifth of global electrical power generation. As the developing world continues to advance, there is increasing pressure around the globe for novel cooling technologies.

The researchers point out that when you stretch a rubber band it warms up slightly. Releasing that stretch, releases the heat cooling it. This well-known phenomenon is called elastocaloric cooling. There are other materials that can lead to cooling through other phenomena, including electrocaloric and magnetocaloric cooling.

Calorie count

"This elastocaloric behaviour of natural rubber has been known since the early 1800s. But to get high cooling from a rubber band, you have to release a very large stretch,” Baughman explains. “With twistocaloric cooling, we found that all you have to do is release twist.” The team, had been working on carbon nanotube yarns and ordinary nylon thread as well as polyethylene fishing line with a view to developing artificial muscle materials for robotics and microelectromechanical systems (MEMS) and other technologies. They were seeing how they could twist stretched rubber fibres and saw coiling and supercoiling of the material. When the twist was released suddenly, there was a cooling effect of 15.5 degrees Celsius. Releasing the twist and the stretch simultaneously led to even greater cooling, 16.4 degrees Celsius.

“By employing opposite directions of twist and coiling, we engineered fibres that cool when stretched,” explains Baughman. “This is quite unusual behaviour since ordinary materials heat up when stretched.” They saw an almost 21 degrees Celsius drop in temperature in parallel experiments with bundles of nickel titanium wires.

It was X-ray crystallography that revealed the underlying mechanism leading to this phenomenon at the molecular level. "We found that releasing stretch from a coiled fiber results in partial conversion of a low entropy phase into a high entropy phase,” Liu explains. “This phase change leads to the twistocaloric cooling.”

Refrigeration with a twist

Of course, the stretching and twisting processes will require energy input. Moreover, those very processes will ultimately cause physical damage to the fibres or wires. “Many challenges and opportunities exist on the path from these initial discoveries to the commercialization of twist fridges for diverse large- and small-scale applications,” Baughman concedes. “Among the challenges are the need to demonstrate refined devices and materials that provide application-targeted cycle lifetimes and efficiencies by recovering part of the inputted mechanical energy. The opportunities include using performance-optimized twistocaloric materials, rather than the few presently studied commercially available candidates.”

Related Links

Science 2019, online: "Torsional refrigeration by twisted, coiled, and supercoiled fibers"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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