An international team led by researchers at The University of Texas at Dallas and Nankai University in China has discovered a new technology for refrigeration that is based on twisting and untwisting fibres.
In research published in the Science journal, they demonstrated twist-based refrigeration using materials as diverse as natural rubber, ordinary fishing line and nickel titanium wire.
Dr Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas said: “Our group has demonstrated what we call ‘twistocaloric cooling’ by changing the twist in fibres. We call coolers that use twist changes for refrigeration ‘twist fridges’.” Baughman is a corresponding author of the study, along with Dr. Zunfeng Liu, a professor in the State Key Laboratory of Medicinal Chemical Biology in the College of Pharmacy at Nankai University in Tianjin.
Stretching a rubber band heats the rubber, and releasing the stretch cools it; this is called elastocaloric cooling. Other solid substances for cooling include electrocaloric and magnetocaloric materials, which cool via changes in electric and magnetic fields, respectively.
Baughman continued: “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. With twistocaloric cooling, we found that all you have to do is release twist.”
In the experiments, the scientists stretched rubber fibres, then twisted them until they not only coiled, but also supercoiled. Fast release of the twist resulted in surface temperature cooling of 15.5°C. Releasing both the twist and the stretch from the rubber produced even higher cooling of 16.4°C.
The twistocaloric cooling also worked for fishing line. The researchers inserted twist into a non-elastic polymer fishing line until coils formed. Stretching the coiled fibre caused heating, while stretch release produced a maximum surface cooling of 5.1°C.
Baughman added: “By employing opposite directions of twist and coiling, we engineered fibres that cool when stretched. This is quite unusual behaviour since ordinary materials heat up when stretched.”
To investigate the origin of the cooling effect in the fishing line, the researchers turned to X-ray crystallography, which allowed them to determine what was happening on the molecular level when twist was changed by stretching a coiled fibre.
Liu detailed: “We found that releasing stretch from a coiled fibre results in partial conversion of a low entropy phase into a high entropy phase. This phase change causes twistocaloric cooling.”
Large reversible cooling was also achieved by removing twist from nickel titanium wires and by unplying bundles of these wires. A maximum surface cooling of 17°C was observed when the researchers untwisted a single wire. Unplying a four-wire bundle produced even higher cooling of 20.8°Celsius.
The researchers placed a three-ply nickel titanium wire cable in a device they built that cooled a stream of water by up to 7.7°C when the cable was unplied. “By using further cycles of twist and twist release, much higher cooling can be achieved,” Liu said.
In another set of experiments, they coated the different types of fibres with thermochromic paint, which changes colour in response to temperature variations produced by twisting fibres or stretching coiled fibres. Such fibres could be used for remotely readable sensors of strain and twist, as well as for colour-changing textiles for clothing.
Baughman concluded: “Many challenges and opportunities exist on the path from these initial discoveries to the commercialisation of twist fridges for diverse large- and small-scale applications.
“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-optimised twistocaloric materials, rather than the few presently studied commercially available candidates.”