Tag Archives: Mpemba Effect

Mpemba on steroids: the surprising way to heat some things faster

The effect doesn’t just apply to water.

In 1963, 13-year old Erasto Bartholomeo Mpemba from Tanzania came across a bizarre phenomenon. He was freezing ice cream mix that was hot in a cookery class and noticed that it froze faster than the cold mix. He never really understood why.

A few years later, Dr. Denis G. Osborne from the University College in Dar es Salaam gave a lecture at Mpemba’s highschool. After the lecture, Mpemba asked Osborne why hot water seems to freeze faster than cold water. He was ridiculed by his classmates and teacher.

But after initial consternation, Osborne gave the experiment a go. He was shocked to see that the young Mpemba was right: water at 100 °C (212 °F) can freeze faster than water at 35 °C (95 °F).

The two published the results together, and the Mpemba effect became known as one of the weirdest phenomena in modern physics.

The phenomenon is weird because it’s parameter dependent. In other words, simply saying that “hot water freezes faster than cold water” is incorrect — or rather, imprecise. Rather, there are some pairs of temperatures at which, all other things being equal, hotter water freezes faster than colder water.

Several published papers have replicated the results and attempted to explain the theoretical reason why this happens. In 2017, two research groups independently and simultaneously found theoretical evidence of the Mpemba effect and also predicted a new “inverse” Mpemba effect in which some materials might be heated faster if they are cooled first. Now, a new study adds more weight to that theory.

Physicists Amit Gal and Oren Raz of the Weizmann Institute of Science in Rehovot, Israel studied a theoretical system called an Ising model — a mathematical model of magnetism (ferromagnetism, to be precise) in statistical mechanics.

The Ising model involves a 2D grid of atoms which have magnetic poles that point either up or down. In their version of this study, neighboring atoms tended to point their poles in opposite directions. In this setup, researchers say, heating could occur faster after a pre-cooling phase.

“The prospects are exciting,” says physicist Adolfo del Campo of the Donostia International Physics Center in Spain.

While this is unlikely to affect our day to day life, this could enable scientists to speed up heating in some quantum setups and bypass some of the limits of standard physical machines.

In order for the effect to take place, this particular magnetic property needs to be achieved, otherwise, there would be no difference between a system that had been pre-cooled and rewarmed and one that hadn’t (which is still intriguing, but holds no practical potential).

“The temperature cannot really tell the whole story,” Amit Gal explains.

However, researchers suspect that a similar effect might also happen in different scenarios. Next, they will turn their attention to real materials, such as magnetic alloys.

Journal Reference: A. Gal et al. Precooling Strategy Allows Exponentially Faster Heating, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.060602

Ice.

Why does hot water freezes faster than cold water? Enter the Mpemba effect

A Spanish team of researchers has developed a framework theory that could explain the Mpemba effect — the mysterious physical phenomenon that makes hot water freeze faster than cold water.

Ice.

Image via Pixabay.

Preheated liquids freeze faster than cold ones. There’s some evidence that Aristotle first observed this effect in the 4th century AD, and it later piqued the curiosity of intellectual heavyweights such as Francis Bacon and René Descartes.

Hot ice cream

In 1960, the phenomenon took its first steps toward theory when Tanzanian student Erasto Mpemba observed that the hottest mixture of ice cream froze faster than the cold one during cookery class. Later, as a student at Mkwawa Secondary School in Iringa attending a physics lecture, he inquired about the causes of this phenomenon and was ridiculed by classmates and his teacher. However, the lecturer, Dr. Denis Osborne from the University College in Dar es Salaam, tested and confirmed Mpemba’s observations. The two published a paper in 1969 detailing the findings.

The strange effect later made it into both educational and science outlets, but its causes have been poorly understood until now. That’s why a team of Spanish researchers set out to determine why the seemingly counter-intuitive Mpemba effect exists.

“It is an effect that, historically, has not been addressed in a rigorous manner but merely as an anomaly and a didactic curiosity,” said Antonio Prados, paper co-author and researcher at the Universidad de Sevilla Department of Theoretical Physics. “From our perspective, it was important to study it in a system with the minimum ingredients to be able to control and understand its behavior.”

Certain “ingredients” have to come together for the effect to occur in a given system, the team reports. They studied the Mpemba effect in granular fluids, substances that contain hard inelastic spheres but behave as liquids. This environment was selected so that the team could simulate interactions between particles and “make analytical calculations to know how and when the Mpemba effect will occur,” said Antonio Lasanta, study co-author and a researcher at the UC3M, Universidad Carlos III de Madrid.

When these particles collide they shed energy. Since higher temperatures mean more motion at the molecular levels, the Mpemba effect will take place faster in warmer liquids. The study also confirmed the existence of a ‘reverse-Mpemba’ effect generated by the same interactions, where the coldest system will heat up faster than the hottest one. This reverse was first predicted about a year ago by Oren Raz, now at the Weizmann Institute in Israel, and Zhiyue Lu from the University of Chicago.

“The scenario that the effect will most easily occur in is when the velocities of the particles before heating or cooling have a specific disposition — for example, with a high dispersion around the mean value,” the athors add. This way, the evolution of the temperature of the fluid can be significantly affected if the state of the particles is prepared before the cooling, they explain.

A better understanding of the Mpemba effect won’t just advance our understanding of basic science but could have practical applications in the mid to long term. If the team’s theory is verified, it could lead the way to electronics that cool down faster, for example.

There’s still a ways to go until then, however. Like Raz and Lu’s paper before, the study garnered some criticism for the very simplified model used to study the effect, compared to water’s more complicated behavior. Taken together, however, the studies support each other and lend a great deal of confidence that such particular interactions fit in the wider mechanisms of the Mpemba effect.

At the same time, Mpemba himself first observed the effect in milk, which has many large particles suspended in water. The granular liquid models could also apply to water samples: large solute particles in impure water samples could contribute to the overall Mpemba effect.

The paper “When the Hotter Cools More Quickly: Mpemba Effect in Granular Fluids” has been published in the journal Physical Review Letters.