Unlocking the Mpemba Effect

A new research study at Simon Fraser University in Canada may lead the way to exploiting the mysterious Mpemba Effect – the process by which hot water freezes faster than cold water.

The Mpemba Effect is named after Tanzanian school student Erasto Bartholomeo Mpemba who, legend has it, was making ice-cream in a food tech class when he noticed that warm mixture froze faster than cold mixture. Although at first he was ridiculed by his classmates for his observation, later in life he teamed up with Denis Gordon Osborne and wrote a paper on the phenomenon. Indeed, the effect has been observed throughout the ages; more than 2,000 years ago, Aristotle noted that “to cool hot water quickly, begin by putting it in the sun”.

But there is still widespread debate about what actually causes the Mpemba effect, and how to reliably demonstrate it across a range of conditions. The unusual physical properties of water and the difficulty in measuring its temperature across its entire volume have further complicated experiments.

In the new study, graduate student Avinash Kumar (pictured) and Professor John Bechhoefer found a way to better analyse the cooling process by inserting a microscopic glass bead in a colder beaker of water. The experiment was repeated thousands of times, allowing them to replicate the Mpemba Effect, and define the conditions required for it to occur.

“Our results are reproducible and agree quantitatively with calculations based on a recently proposed theoretical framework,” say the researchers. “By carefully choosing parameters, we observe cooling that is exponentially faster than that observed using typical parameters, in accord with the recently predicted strong Mpemba effect.

“Our experiments outline the generic conditions needed to accelerate heat removal and relaxation to thermal equilibrium, and support the idea that the Mpemba effect is not simply a scientific curiosity concerning how water freezes into ice – one of the many anomalous features of water – but rather the prototype for a wide range of anomalous relaxation phenomena of broad technological importance.”

It is hoped that once the effect is better understood, it could be reproduced in different settings and even different materials. This could lead to advances in heat-transfer systems, and may also be exploited in reverse, to provide more efficient heating as well as faster cooling.

To read more about the study, click here.

Featured image courtesy of Simon Fraser University.