A coiled fiber at room temperature (orange, top) cools when untwisted (brown, middle) before returning to room temperature over time (bottom).
(Photo : University of Texas)

The commercial and industrial demand for cooling techniques and technologies are currently based on the thermodynamic principles of refrigeration cycles by compression and expansion. However, previous studies have noted that these processes contribute to global warming, which is why scientists have been trying to devise new ways to cater to this demand.

In a recent issue of Science, scientists from China, the United States, and Brazil report that they have developed a technique called twistocaloric cooling, which produces a cooling effect after unwinding twisted fibers. The scientists explained the underlying principle—change in entropy due to physical deformation—in their research by using a rubber band. When a rubber band is stretched and held in that position for a while, the material extracts heat from its surroundings, then increasing their temperature. As the rubber band is released, it not only goes back to its original shape but also experiences a change in temperature, so it cools down. The same principle applies for twisting and unwinding of different materials. These materials are called mechanocaloric, which is a property of a solid that allows experiencing a change in temperature upon physical deformation.

To prove the principle, the researchers used natural rubber, polyethylene from fishing lines, and nickel–titanium wires. They first twisted the various materials to form coils or coils of coils called supercoils. The materials were then coated with temperature-sensitive paint. In a video from the University of Texas, a twisted and coated fiber of rubber at room temperature is shown. After unwinding the coiled fiber, the material cooled down, which is shown by its change in color from orange to brown. After some time, the material was brought back to its original temperature, hence to its original color, as a result of the simultaneous heat exchange with the environment.

In one of the many tests performed, the researchers report that the technique was able to decrease the surface temperature of supercoiled rubber by 27.9 degrees Fahrenheit, or 15.5 degrees Celsius. However, this was only illustrative of how the technique could cool down the surface temperature of the material. And since the goal of the researchers was to look for a new refrigeration technique, they built what they called a "twist fridge." The twist fridge was equipped with a three-ply nickel–titanium alloy wire. While the coiled wire was unwound, water flowing over the material dropped its temperature by 13.9 degrees Fahrenheit, or 7.7 degrees Celsius.

Future researches will be made to work on the actual and scaled application of the new technique. One of the authors, Dr. Ray Baughman, says that this will be a challenging task. "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," he said. "The opportunities include using performance-optimized twistocaloric materials, rather than the few presently studied commercially available candidates."