Posted on 10/23/2017 10:29:39 AM PDT by Red Badger
Ever since the days of Aristotle, people have made the counterintuitive observation that hot water sometimes freezes faster than cold water. 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 coauthors 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," coauthor Andrés Santos at the University of Extremadura in Badajoz, Spain, told Phys.org. "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."
As the researchers explained further, the simplicity of the Mpemba effect in granular fluids compared to water and other systems enabled them to reach this conclusion.
"Our results show that the Mpemba effect is a generic non-equilibrium phenomenon that appears if the evolution of temperature depends on other physical quantities that characterize the initial state of the system," Santos said. "In practice, such an initial state can be experimentally achieved if the system is taken by some physical procedure very far away from equilibrium (for instance, by a sudden heating impulse prior to cooling down). Our theoretical and computational work shows that the Mpemba effect is particularly simple in a granular gas, since, in practice, there is one single extra parameter controlling the Mpemba effect. This parameter is the kurtosis, which measures the deviation of the velocity distribution function from a Gaussian distribution."
With this new understanding, the researchers could estimate a range of initial temperatures for which the effect emerges and determine how different the initial values of this parameter must be in order for the Mpemba effect to appear.
The results also support predictions of the existence of an inverse Mpemba effect: when heated, a colder sample may reach a hot target temperature sooner than a warmer sample. The researchers plan to investigate this area and others in the future.
"On the theoretical side, we plan to carry out a similar study in the case of a molecular solute (where collisions are fully elastic) suspended in a solvent that produces a nonlinear drag force on the solute particles," Santos said. "Going back to granular fluids, we also want to analyze the impact of particle roughness and spin on the Mpemba effect. In the latter system, the simplest model would couple the temperature evolution to that of the parameter measuring the non-equipartition of energy between the translational and rotational degrees of freedom.
"On the experimental side, we think that reproducing in a laboratory the Mpemba effect in a granular gas would be a breakthrough. We are currently working on the design of an ad hoc experiment."
Explore further: Mpemba effect: Why hot water can freeze faster than cold
More information: Antonio Lasanta et al. "When the Hotter Cools More Quickly: Mpemba Effect in Granular Fluids." Physical Review Letters. DOI: 10.1103/PhysRevLett.119.148001. Also at arXiv:1611.04948 [cond-mat.soft]
Journal reference: Physical Review Letters
No, you’re supposed to chill it overnight, then freeze it. :)
We didn’t want to wait.................
The girl with Mpemba keeps walkin...
Are we to believe that water molecules have a 'memory' for its 'previous history?'
Nonsense.
It’s very tricky.
And you have to hold your tongue just right.................
So, it’s kind of like momentum for temperature. If it is already out of equilibrium, even if it is in the wrong direction, it is easier to “push” the system to a new equilibrium point.
“who do you think would get to LA first?”
You left out the variable of how fast the driver leaving Denver to LA can drive.
So the warmer earth gets via Global Warming the quicker it will cool down.... COOL!!!!
BWAHAHAHAHA!!
The article referenced sounds like a load of BS.
There is a grain of truth in the old wives tale, however.
Take two ice cube trays. Fill one with just boiled hot water, the other with tap water.
Put them both in the freezer. See which one has ice cubes faster. Surprise!
The just-boiled water filled tray will have almost nearly evaporated away, and have tiny cubes. So this is merely an exercise in evaporation. Fill two sealed containers, hot water bottles, and then things are much different.
If an equal amount of hot water and colder water are placed in a freezer at the same time, the colder water freezes first. Consider this: The hotter water can not freeze first because for it to do so it would at some point have to be colder than the colder water, and then it would not be the hotter water any more! The original colder water would now be the hotter water. It should freeze first! This is inconsistent logically. Hence, colder water freezes quicker than warmer water of an equal amount.
I’m still wanting someone to explain what happens when
you crack an ice tray in the dark and get flashes of light.
Noticed this many years ago.
triboluminescence
You need to normalize for the pressure inside of the sealed container, which will affect the freezing point.
Perhaps a stretchable container such as a hot water bottle would normalize the pressure inside the vessel.
How about:
The hotter molecules have more energy so they hit the sides of the container more often draining their energy?.............
All I can say is if you actually try it, instead of just writing about it, the hot water will make cubes faster than cold water.
Logically this is utter nonsense.
The reason they can’t duplicate it is because at some point the warmer liquid or gas reaches the same temperature as the cooler liquid and by then the cooler substance is ahead in the process while the warmer is just at the starting point.
There is no Momentum in cooling or heating. Heating and cooling is addition or removal of heat, Period. It is measured in BTU’s or Calories or whatever measurement you want. It is really very simple math.
This article is ridiculous.
When I a was in college a roomate’s high school age brother claimed this and we did the experiment. Of coarse cold first.
But you got me thinking...what about evaporation? Jump in Lake Powell when it’s 104 and you’ll stay cool until your dry.
Got me looking for answers. Found the Mpemba effect. Takes in account evaporation, dissolved gases, convection and surroundings. Interesting.
Not a great idea to dismiss Aristotle. LOL!
“but only 50 mph from Denver on ...”
btt
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