2020.03.04;WMar4th:Mpemba effect: The fastest way to heat certain materials may be to cool them first
Published by DB,
March 4, 2020
Mpemba effect: The fastest way to heat certain materials may be to cool them first
To heat a slice of pizza, you probably wouldn't consider first chilling it in the fridge. But a theoretical study suggests that cooling, as a first step before heating, may be the fastest way to warm up certain materials. In fact, such precooling could lead sometimes to exponentially faster heating, two physicists calculate in a study accepted in Physical Review Letters.
The concept is similar to the Mpemba effect, the counterintuitive—and controversial—observation that hot water sometimes freezes faster than cold water (SN: 1/6/17). Scientists still don't agree on why the Mpemba effect occurs, and it's difficult to reproduce the effect consistently. The new study is "a way of thinking of effects like the Mpemba effect from a different perspective," says physicist Andrés Santos of Universidad de Extremadura in Badajoz, Spain, who was not involved with the research.
This potential for faster heating doesn't actually apply to pizza slices, but to certain simplified theoretical models of materials, which scientists use to make calculations that help them understand real materials. Physicists Amit Gal and Oren Raz of the Weizmann Institute of Science in Rehovot, Israel, studied a theoretical system called the Ising model, a 2-D grid of atoms which have magnetic poles that point either up or down. In particular, they considered a version of the Ising model in which neighboring atoms tended to point their poles in opposite directions, behavior which is called antiferromagnetic. In that system, heating could occur faster after a precooling phase.
For the new effect to occur, there must be some relevant property of the system other than a uniform temperature that is affected by the precooling. Otherwise, there'd be no difference between a system that had been precooled and rewarmed, and one that hadn't. "The temperature cannot really tell the whole story," Gal says.
In the case of the antiferromagnetic Ising model, the researchers considered the total magnetization produced from all the atoms, as well as how many magnets pointed in the opposite direction of their neighbors. Cooling the material could change the ratio between those two properties in a way that would allow heating to proceed more quickly.
Raz hopes that physicists might look for the effect in real materials next, such as magnetic alloys.
"The prospects are exciting," says physicist Adolfo del Campo of the Donostia International Physics Center in Spain. Scientists have been searching for ways to speed up heating in tiny machines that follow the rules of quantum mechanics and can bypass some of the limits of standard machines (SN: 4/1/19). If the effect can be exploited in such minute machines, he says, "it would [be] quite handy."
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More information:
A. Gal et al. Precooling Strategy Allows Exponentially Faster Heating, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.060602
https://phys.org/news/2020-03-mpemba-effect-fastest-materials-cool.html