SFU researchers demystify how hot water can cool faster than warm water

CONTACT:

John Bechhoefer; Department of Physics, 604.872.2132, johnb@sfu.ca

Shradhha Sharma; University Communications and Marketing, 604.202.2504, shradhha_sharma@sfu.ca

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Video: https://youtu.be/HXDHqOk7SLw

SFU researchers have identified a simpler way to demonstrate the “Mpemba effect”—a phenomenon in which hot water can sometimes cool and start to freeze faster than warm water.

Research by SFU physics professor John Bechhoefer and his PhD student Avinash Kumar, published this week in the journal Nature, draws inspiration from an observation by Aristotle about 2,300 years ago that, “to cool hot water quickly, begin by putting it in the sun.”

The Mpemba effect is named after Tanzanian teen Erasto Mpemba, who performed the first systematic, scientific experiments on this effect in the 1960s.

Until now, the effect has been difficult to prove, partly because of the long time it takes to cool large volumes of water and because water has properties that can be much more complicated than evident at first.

Bechhoefer and Kumar devised a way to speed up the cooling process by inserting a microscopic glass bead in water and then subjecting it to carefully designed forces, as well as random thermal forces from surrounding water molecules.

They credit their system of using the microscopic bead, which cools in less than 1/10th of a second, for allowing them to conduct the large number of trials needed to understand the effect.

The pair determined that, based on the variation of forces on the bead in the water, some things can cool much faster than normal if the “landscape” of these forces is “properly” designed. These design principles include shaping the free-energy landscape so that hot systems have a more direct path to “cooler” states.

Bechhoefer says this work does not explain all the particulars of why hot water placed in a freezer can start to freeze more quickly than cold, but it does provide an important insight into this curious phenomenon and suggests that analogues could exist in many other settings and materials.

About Simon Fraser University: 

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