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Physics

Cooling with a twist

Cooling systems based on twisting and untwisting fibers can reduce water temperature by 7 °C

Twisting and untwisting rubber fibers, fishing lines, or nickel-titanium wire can cause their temperature to change by as much as 20 ºC. When fibers are twisted, their temperature rises. They then come into thermal equilibrium with the surrounding environment and cool down a few degrees. When then allowed to relax and untwist, they cool by the same number of degrees as they warmed. The range of heating and cooling will depend on the type of material, the number of fibers, and the amount of twist. The more tautly they are twisted, the more the wires warm, and the more they will cool when twist is released. A paper describing this hitherto unknown effect, published on October 11 in Science, notes that some of the cooling produced by manipulating materials this way can be transferred to neighboring substances and used in refrigeration applications, such as refrigerators and air conditioners.

In an experiment reported in a study by researchers from the United States, China, and Brazil, a stream of running water was cooled by between 4.7 °C and 7.7 °C as it flowed over a device that had been cooled by the unraveling of a twisted bundle of three nickel-titanium wires measuring 0.7 millimeters (mm) in diameter. “We call this twistocaloric cooling,” said corresponding author Ray Baughman, director of the Alan G MacDiarmid NanoTech Institute at the University of Texas in Dallas, in a press release. Baughman’s group has previously experimented with twisting and coiling wires and even carbon nanotubes for different applications, such as producing energy or making artificial muscles.

The new cooling technique is a variant of a phenomenon that has been known for 200 years, called elastocaloric cooling. “Since 1805, we have known that rapid stretching of a rubber material will raise its temperature,” explains coauthor Douglas Galvão of the Gleb Wataghin Institute of Physics at the University of Campinas (IFGW-UNICAMP), Brazil. This property is relatively straightforward to understand: stretching a fiber will make it hotter; releasing it to its original shape will cool the material by as much as it warmed. “The discovery that twisting causes the temperature of a structure to vary significantly has several potential applications,” says Galvão.

Temperature change occurs because mechanical deformation (stretching or twisting) decreases entropy, a thermomechanical quantity that measures the disorder of particles in a physical system. This causes the material to give off heat to the surrounding environment. When the fiber is relaxed, the opposite effect occurs and temperature is reduced. In the study, the researchers used X-ray crystallography to observe this molecular rearrangement within the twisted and untwisted fibers as they warmed and cooled. They also found that twisting and untwisting wires produces temperature change twice as effectively as stretching and releasing those same materials. In some experiments, wires were coated with paints that change color in response to temperature variations, making the change of temperature visually observable.

The cooling effect was demonstrated in experiments with rubber fibers, fishing lines, and nickel-titanium wire

Twist-based cooling has another practical advantage over stretch-based cooling: it takes less space to twist elastic material than it does to stretch it. “Creating a refrigerator using the classic elastocaloric cooling effect would be impracticable,” explains physicist Alexandre Fontes da Fonseca of IFGW, a coauthor of the study. To produce a sufficient amount of cooling, the device would need to be six or seven times larger than the original length of the rubber fibers to accommodate the stretching and relaxing of the threads. Several studies have suggested that this problem could be solved by using materials at nanometric scale—which would not require large amounts of space to be stretched—in refrigeration equipment. Twistocaloric cooling of macroscopic fibers offers another approach. “The discovery that twisting fibers produces the same thermal effect as stretching them means that using solid materials rather than gases in refrigeration systems could be feasible,” says Fonseca.

Today’s refrigerators use compressed gases to cool the air inside. While this process is efficient, it has environmental drawbacks. Up until a few years ago the most common refrigeration gases were chlorofluorocarbons (CFCs), which degrade the ozone layer—an atmospheric layer that filters out ultraviolet radiation from the sun. After these compounds were banned in many countries, including Brazil, they were replaced by hydrofluorocarbons (HFCs) in most refrigerators and air conditioning systems. And while HFCs are not ozone-depleting substances, they have a powerful greenhouse effect, contributing to the abnormal warming of the earth’s atmosphere.

This makes alternatives to refrigerant gases an important field of research. Some of the alternative methods being considered rely on changes in electrical currents or magnetic fields or the mechanical movement of solid materials (such as stretching and twisting wires) to produce cooling in closed systems, like refrigerators.

To be commercially viable, an alternative method of cooling would need to be inexpensive and efficient at optimizing heat exchange between the cold-producing material (the rubber fiber or fishing line in the case of the newly reported experiment) and the environment being cooled, such as air in a refrigerator or in an air conditioner. This is a challenge for which today’s gas-based appliances would be ideally suited were it not for the climate impact of the gases they use. “While twist-based refrigeration is still less effective than conventional systems, it is more effective than other alternative cooling methods described in the literature,” says Fonseca.

Project
Center for Computational Science and Engineering (CECC) (nº 13/08293-7); Grant Mechanism Research, Innovation, and Dissemination Centers (RIDC); Principal Investigator Munir Salomão Skaf (UNICAMP); Investment R$24,519,718.86 (overall project).

Scientific article
WANG, R. et al. Torsional refrigeration by twisted, coiled, and supercoiled fibers. Science. Oct 11, 2019.

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