Refrigerator works by twisting and untwisting fibres
The cooling system relies on the fact that some materials undergo significant changes in entropy when deformed. As far back as 1805 – when the concepts of thermodynamics were first being developed – it was known that ordinary rubber heats up when stretched and cools down when relaxed. In principle, such mechanocaloric materials could be used in place of the gases that change entropy when compressed and expanded in commercial refrigeration systems. Replacing gas-based systems is an important environmental goal because gaseous refrigerants tend to degrade the ozone layer and are powerful greenhouse gases.
In their experiments, Liu and Baughman’s team studied the cooling effects of twist and stretch changes in twisted, coiled and supercoiled fibres of natural rubber, nickel-titanium and polyethylene fishing line. In each material, they observed a surface cooling as high as 16.4 °C, 20.8 °C, and 5.1 °C respectively, which they achieved through techniques including simultaneous releases of twisting and stretching, and unravelling bundles of multiple wires.
Supercoiled fibres
The team also made supercoiled fibres of natural rubber in which the twisting and coiling were done in opposite senses (clockwise and anticlockwise). Much to their surprise, they found that these structures cooled when stretched, rather than heated.The team also looked at microscopic changes in the materials. An X-ray diffraction crystallography study of the polyethylene fishing line revealed changes in molecular structures associated with the transition from low to high entropy phases. The team identified this process as the cause of the effect, which they have dubbed “twistocaloric” cooling.
The team faces many challenges in creating commercially-viable twist fridges, including the need find a material that is not degraded by being repeatedly twisted and untwisted. So far, they have only explored few commercially-available materials, but now plan to expand their research to seek-out materials that have optimized mechanical and twistocaloric properties. If realized on commercial scales, twist fridge technologies could provide climate-friendly solutions to meeting our rapidly expanding demand for cooling.
The research is described in Science.
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