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Hehlen and Mueller put their heat switches to the ultimate test with a rough, first-ever demonstration of thin-film refrigeration and were able to achieve—wait for it—cooling by 0.1°C. That may not sound earth-shattering, and it doesn’t prove thin-film refrigeration will Heat Flow Stop and Go the electrodes, carrying heat from one to the be commercially viable any time soon. other with every revolution. They achieved a “It just proves it works,” says Hehlen. “And factor of greater than 50 in conductivity change, This is cool: Los Alamos scientists Markus it proves the versatility of the heat switches.” converting a fluid that’s normally nonconduc- Hehlen and Alex Mueller have devised a thin- tive, similar to fiberglass insulation or fleece, film heat switch—a layer of material that into one that approaches the thermal conduc- can go from thermally insulating to thermally tivity of a metal. conducting with the flip of a switch. It takes —Craig Tyler Initially, these heat-switch films could be advantage of fluid motion to achieve its used in specialty applications, such as tempera- dramatic change in thermal conductivity. Before a pot of water on a lit stovetop comes ture control for satellite electronics—which can Microbiome References Required face the Sun one minute and the extreme cold to a boil, the upper surface of the water can of empty space the next. Then they might prove remain much cooler than the water closer to Imagine a jigsaw puzzle with five billion useful in thermal management of computers, the burner. But once the water starts boiling, pieces that shows a picture of a prairie. There other electronics, or even buildings—replacing are no buildings, no trees, and no people; the fluid motion quickly distributes the heat conventional insulation with something thin- throughout the water. Hehlen and Mueller’s there’s just grassland and sky. Now imagine ner and more adaptable. Hehlen and Mueller heat switch relies on a similar effect, using that you have a dozen such puzzles—one of also believe they can make their thin-film electrohydrodynamic technology to generate a prairie in Kansas, another in Iowa, maybe a heat switches flexible enough to be used in motion in a dielectric fluid, thereby improving Canadian prairie for good measure—and all temperature-controlled clothing, effectively heat conduction. But unlike boiling water, the 60 billion puzzle pieces are together in one allowing the wearer to switch between a light bag. Without the pictures from the box lids dielectric fluid can be set in a thin layer that doesn’t feel wet any more than a liquid-crystal cotton tee and a wool sweater without having and without knowing how many puzzles there to change clothes. display (LCD) screen does. are, you must assemble them all correctly and In fact, the heat switches may someday To make the heat switch, a dielectric fluid simultaneously. How are you supposed to tell enable compact, thin-film refrigeration is sandwiched between two electrode plates. the difference between this piece showing to compete with today’s bulky and noisy When a voltage is applied across the plates, blue sky and that piece showing blue sky? Or vapor-compression-based refrigerators and air determine whether a particular patch of grass charged particles within the fluid move in response, effectively stirring the fluid. Because conditioners. The trick will be to intersperse is from Kansas or Iowa? This, says Los Alamos suitable electrocaloric material layers—which bioinformatics expert Patrick Chain, is the chal- of this motion, heat is transported across the thin fluid layer from one electrode to the other. can be heated and cooled by the application lenge of metagenomics. of an electric field—between heat switch When the voltage is switched off again, the Metagenomics is the field of genetics fluid stops moving and greatly inhibits the heat layers. By opening and closing the switches in concerned with the genomic sequencing of sequence while alternately raising and lowering microbial communities, the members of which flow. So if a heat switch is inserted between electrocaloric-layer temperatures to draw heat cannot be easily isolated from one another. For a hot region and a cold one, it can be made to in and then push it away, it is possible to send preserve that temperature difference or not, example, the human microbiome consists of heat out of a cold region, against its natural with easy on-off electronic control. the hodgepodge of microorganisms (mostly flow direction—analogous to sending a ship Hehlen and Mueller designed and con- bacteria) that live on and within human bodies. structed a specially patterned electrode to make through a series of locks, uphill and overland Presently in vogue, this microbiome is increas- the dielectric fluid circulate smoothly between across the Panama Canal. ingly being shown to influence our health, 26 1663 August 2014