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The Physics of Why Hot Water Sometimes Freezes Faster Than Cold Water

For decades, physicists have debated whether the phenomenon exists and how to study it

Physicist Avinash Kumar of Simon Fraser University stands beside the Mpemba effect apparatus. (Photo by Prithviraj Basak)
smithsonianmag.com

The story goes that in 1963, Tanzanian high school student Erasto Mpemba was making ice cream with his class when he impatiently put his sugar and milk concoction into the ice cream churner when it was still hot, instead of letting it cool first. To his surprise, the confection cooled faster than his classmates’ had.

With the help of a physics professor, Mpemba performed additional experiments by putting two glasses of water, one just-boiled and one warm, in a freezer, and seeing which one reached the freezing finish line first. Often, the water with a higher starting temperature was the first to freeze. Their observations set off a decades-long discussion over the existence and details of the counterintuitive phenomenon, now called the Mpemba effect.

Now, new research published on August 5 in the journal Nature not only shows that the Mpemba effect does exist, but also sheds light on how it occurs, Emily Conover reports for Science News.

Rather than experiment on freezing water, which is surprisingly complicated to study, physicists Avinash Kumar and John Bechhofer of Simon Fraser University focused their sights—and lasers—on microscopic glass beads. They measured how the glass beads moved under very specific conditions in water and saw that in some circumstances, beads that started off very hot cooled faster than those that didn’t.

“It’s one of these very simple setups, and it already is rich enough to show this effect.” University of Virginia theoretical physicist Marija Vucelja tells Science News. The experiment also suggests that the effect might show up in materials other than water and glass beads. Vucelja says, “I would imagine that this effect appears quite generically in nature elsewhere, just we haven’t paid attention to it.”

If the freezing point is the finish line, then the initial temperature is like the starting point. So it would make sense if a lower initial temperature, with less distance to the finish line, is always the first to reach it. With the Mpemba effect, sometimes the hotter water reaches the finish line first.

But it gets more complicated. For one thing, water usually has other stuff, like minerals, mixed in. And physicists have disagreed over the what exactly the finish line is: is it when the water in a container reaches the freezing temperature, begins to solidify, or completely solidifies? These details make the phenomenon hard to study directly, Anna Demming writes for Physics World.

The new experiment does away with the details that make the Mpemba effect so murky. In each test, they dropped one microscopic glass bead into a small well of water. There, they used a laser to exert controlled forces on the bead, and they measured the bead’s temperature, per Science News. They repeated the test over 1,000 times, dropping the beads in different wells and starting at different temperatures.

Under certain forces from the laser, the hottest beads cooled faster than the lower temperature beads. The research suggests that the longer path from a higher temperature to the freezing point might create shortcuts so that the hot bead’s temperature can reach the finish line before the cooler bead.

Bechhoefer describes the experimental system as an “abstract” and “almost geometrical” way to picture the Mpemba effect to Physics World. But using the system, he and Kumar identified the optimal “initial temperatures” for a Mpemba cooling effect.

“It sort of suggested that all the peculiarities of water and ice – all the things that made the original effect so hard to study – might be in a way peripheral,” Bechhoefer tells Physics World.

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