Study may explain counterintuitive effect of why hotter systems can cool more quickly

Posted: October 23, 2017 by oldbrew in physics, Uncertainty

Good luck grappling with this longstanding physics conundrum. Even the meaning of ‘faster’ in this context could be open to question. For example there’s rate of heat loss, then there’s total time taken.

Ever since the days of Aristotle, people have made the counterintuitive observation that hot water sometimes freezes faster than cold water, says

In modern times, the observation has been named the Mpemba effect after Erasto Mpemba, an elementary school student living in what is now Tanzania in the early ’60s. When making ice cream, Mpemba observed that using warmer milk causes the ice cream to freeze faster than when using colder milk.

In the last few decades, the Mpemba effect has been studied and observed in several physical systems besides water, including carbon nanotube resonators and ice-like water cages called clathrate hydrates. Despite these findings, the causes of the effect are not well-understood.

Proposed explanations include the presence of impurities, hydrogen bonding, and supercooling. Even the mere existence of the Mpemba effect remains controversial, as one recent study found insufficient evidence to replicate a meaningful effect.

Now, their interest rekindled by a recent paper proposing a generic mechanism for similar effects, scientists Antonio Lasanta and co-authors from universities in Spain have returned to the question in a new study published in Physical Review Letters.

In their work, the researchers theoretically demonstrate and investigate the Mpemba effect in granular fluids, such as those made of sand or other small particles. Using simulations of granular systems and a simple kinetic theory approach, the researchers were able to determine that the initial conditions in which the system is prepared play a critical role in determining whether or not the system exhibits the Mpemba effect. Their analysis also enabled them to identify the initial conditions required in order for a granular system to exhibit the Mpemba effect.

“Our work shows that the existence of the Mpemba effect is very sensitive to the initial preparation of the fluid or, in other words, to its previous history,” co-author Andrés Santos at the University of Extremadura in Badajoz, Spain, told “In our opinion, this may explain the elusiveness and controversy of the Mpemba effect in water, as a consequence of the lack of control on the detailed initial preparation of the sample.”

As the researchers showed, if a system is not prepared under certain initial conditions, then the colder system cools down more quickly than the warmer one, as expected, and there is no Mpemba effect.

“We theoretically showed, at least in the case of a gas, that a system’s temperature evolution and thus its cooling and/or heating rate do not depend on initial temperature alone, but also on the previous history of the system that control the initial value of the additional variables,” Santos said. “Therefore, it is perfectly possible that an initially heated system cools down quicker than a colder one with a different history.”

Continued here.

  1. E.M.Smith says:

    Oh Dear!

    So our climate history might control out rate of current heating or cooling… but I’m sure the science is settled… /sarc;

  2. oldbrew says:

    Correct EM 🙂

    Paper: Climate predictability on interannual to decadal time scales: the initial value problem [2002]

    Abstract: Any initial value forecast of climate will be subject to errors originating from poorly known initial conditions…

  3. A C Osborn says:

    I have experience of this affect and along with my brother conclude it has something to do with surface tension.
    If I make Coffee and forget to put the cold milk the coffee cools much quicker than when I have added the cold milk. Which is counter intuitive.
    Perhaps it has to do with slowing evaporation.
    Why using hot milk would make ice cream freeze faster is very interesting.
    There is so much unsettled science about the simplest of things and yet they know all about the big bang, Black Holes, dark matter and how the Sun works and yet very little about gravity.
    I think they kid themselves.

  4. p.g.sharrow says:

    circulation, once set in motion tends to stay in motion.
    even energy has mass/inertia…pg

  5. Dodgy Geezer says:

    …In modern times, the observation has been named the Mpemba effect after Erasto Mpemba, an elementary school student living in what is now Tanzania in the early ’60s. When making ice cream, Mpemba observed that using warmer milk causes the ice cream to freeze faster than when using colder milk….

    No, it hasn’t. It’s been CLAIMED to be called the ‘Mpemba effect’ by a bunch of anti-racism activists who were frantic to find SOME scientific advance that some black person (preferably poor) had made, and publicised for a while by journalists whose education was noticeably lacking in the classics….

  6. oldbrew says:

    It’s true the effect was known centuries before Mpemba, but his name seems to have stuck to it.

    The effect is named after Tanzanian Erasto Mpemba. He described it in 1963 in Form 3 of Magamba Secondary School, Tanganyika, when freezing ice cream mix that was hot in cookery classes and noticing that it froze before the cold mix. He later became a student at Mkwawa Secondary (formerly High) School in Iringa. The headmaster invited Dr. Denis G. Osborne from the University College in Dar es Salaam to give a lecture on physics. After the lecture, Erasto Mpemba asked him the question “If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?”, only to be ridiculed by his classmates and teacher. After initial consternation, Osborne experimented on the issue back at his workplace and confirmed Mpemba’s finding. They published the results together in 1969, while Mpemba was studying at the College of African Wildlife Management.

    Of course there weren’t any plug-in freezers when Aristotle was a lad.
    – – –
    Paper – The Mpemba effect: When can hot water freeze faster than cold?
    Although the effect might appear impossible, it has been observed in numerous experiments and was discussed by Aristotle, Francis Bacon, Roger Bacon, and Descartes.
    . . .
    The observation that hot water pipes are more likely to burst than cold water pipes is also discussed.

  7. oldbrew says:

    A list of suggested explanations.

    IIRC there was a theory somewhere that water has a sort of barrier at around 4C, and slower-cooling water gets a bit ‘stuck’ there while faster-cooling ex-hot water goes straight through it. That’s probably a poor description, but see ‘convection’ in the above list.

  8. ivan says:

    I first heard about this way back in the early 50s when I attended one of the Royal Institution lectures. I can’t remember exactly what was said but the main thrust was about temperature differential and molecular movement and I think the 4 degree C was also mentioned.

  9. dai davies says:

    Liquid water has a transient nanocrystaline structure that goes through some kind of transition at 4 C then gradually breaks up as temperature rises to 30 C. Above this, around 35 C, it’s specific heat goes through a minimum.

    Re this effect, it’s likely that thermal conductivity would be greatest when water molecules are free and not in nanocrystals. If the water is cooling, as temp drops below 30 C crystals may not form. It would depend on how fast the temp was dropping, I suppose.

    This is a likely explanation for the water thermostat effect. See notes with references at the end of my Energy and Atmosphere article.


  10. Curious George says:

    Please re-post this on April 1.

  11. dai davies says:


    If you’re old enough to remember the original TV science guy, Julius Sumner Miller, back in the 60s, he explained why you should put the milk in your tea/coffee before you answered the door/phone.

    Hot black coffee cools faster because the difference in temp between it and the surroundings is greater than if you’ve added milk first up, so better to drop its temp first rather than later.

    Surface tension is probably relevant. That comes from extra crystaline layer at surface, which probably influences surface infrared emission. See Energy and Atmosphere ref.

    Yes, we have a lot to learn yet.


  12. dai davies says:


    The circulation suggestion seems plausible, too.


  13. tom0mason says:

    Ref EM above,
    Indeed water a liquid has may of the properties that are quite different from those expected from knowledge gained about other liquids.
    Water is just so anomalous — see

    As an aside I wonder if these effects have been incorporated into those climate models?

  14. Damian says:

    Putting milk in your coffee makes it lighter which should decrease its emissivity?

  15. oldbrew says:

    Surface tension tricks…

  16. oldbrew says:

    Rare ‘hole-punch clouds’ shaped like UFOs stun holidaymakers in Dorset

    While they can be found worldwide, a ‘skypunch’ is rather rare and the phenomenon is hard to explain.

    Read more:

  17. oldbrew says:

    Why Hot Water Freezes Faster Than Cold Water

    Published on November 16, 2017
    Written by Universidad Carlos III de Madrid

    A team of researchers from Universidad Carlos III de Madrid, the Universidad de Extremadura and the Universidad de Sevilla have defined a theoretical framework that could explain the Mpemba effect, a counterintuitive physical phenomenon revealed when hot water freezes faster than cold water.

    The researchers, who have recently published the findings in Physical Review Letters, have confirmed how this phenomenon occurs in granular fluids, that is, those composed of particles that are very small and interact among those that lose part of their kinetic energy. Thanks to this theoretical characterization, “we can simulate on a computer and make analytical calculations to know how and when the Mpemba effect will occur,” said Antonio Lasanta. Lasanta is from the UC3M Gregorio Millán Barbany University Institute for Modeling and Simulation on Fluid Dynamics, Nanoscience and Industrial Mathematics. “In fact,” he said, “we find not only that the hottest can cool faster but also the opposite effect: the coldest can heat faster, which would be called the inverse Mpemba effect.”

  18. p.g.sharrow says:

    It would seem that energy flow has inertia. Just like electrical current…pg