Cash prize offered for solving Mpemba effect

[Mark M]

The Usenet FAQ has a very good entry about the Mpemba effect:

http://math.ucr.edu/home/baez/physics/General/hot_water.html

 

[DragonPetter]

No, but he's trying to repeat an effect that has already been observed.

The whole Mpemba combination - and I think they have prizes for a whole bunch of similar problems. It's not just that should be able to repeat it, but you also need a theory to make it repeatable.

I'm really confused as to how a theory makes an observation any more repeatable. Anyway, I was impressed he went to the effort to try to reproduce the observations, with or without a theory.

 

[pgardn]

I have done a sloppy version of what Andre did in the past, same as Watters, and nope.

I tried to do a better test, not as well done as Andre. Nope.

If anyone can actually get this to work chime in please. I have read the papers, but a simple procedure would be nice. I have never observed liquid CO2 either even though I know others have been able to reproduce the pressures, etc... needed. Is this going to be as difficult for the layman?

* I love to watch the flash freezing. Freaks me out every time I see it. On the other hand, I also rather enjoy being shocked by a Van de Graff...

 

[russ_watters]

The Usenet FAQ has a very good entry about the Mpemba effect:

http://math.ucr.edu/home/baez/physics/General/hot_water.html

From the article:

Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real...

It is still not known exactly why this happens...

Why hasn't modern science answered this seemingly simple question about cooling water? The main problem is that the time it takes water to freeze is highly sensitive to a number of details in the experimental setup, such as the shape and size of the container, the shape and size of the refrigeration unit, the gas and impurity content of the water, how the time of freezing is defined, and so on. Because of this sensitivity, while experiments have generally agreed that the Mpemba effect occurs, they disagree over the conditions under which it occurs, and thus about why it occurs.

My problem with this is that there is an implied repeat-ability problem here and if the experiment is highly dependent on initial conditions, but isn't repeatable, then you can't with confidence say that it is real and not, for example, a product of experimental error. See: Cold Fusion.

 

[russ_watters]

Here's the experiment I did a few years ago: https://www.physicsforums.com/showthread.php?t=205920&page=2

 

[Nessdude14]

You know when you get wet, you feel cold?...That's because the thin layer of warm air is no longer over your skin. So, your body cools quicker. But that principle is not a factor in the Mpemba effect.

If that's the reason you feel cold when you get wet, how do you explain why it feels cold just after you step out of a warm shower? The water you're covered with is warmer than the air in the room, so why should you still feel cold when the "layer of warm air" is just replaced with warm water?

The real reason you feel cold when you're covered in water is because of the latent heat of evaporation of the water. As the water sits on your skin, a small amount of the water molecules gain a kinetic energy that is much higher than the average energy of the other water molecules, and then they turn to gas, carrying thermal energy away from your body and into the room.

 

[feyn]

I think i have solved it ! Before anybody thinks he could send it into get the 1000 pound before me,they do not accept any more entries ;)

First of all this works not in every case, it depends on the container (you will later see why). The container should be 4-10 cm diameter and at least 5 times as high, max 10 times as high. One must be nearly boiling, the other lukewarm.
Well if we have say 25° Celsius in one batch and say 98° Celsius in the other batch, round container 6cm diameter 42 cm high (filler till 40 cm) The hotter water evaporates faster and this evaporation leads to a loss of energy (quite a bit energy). Going from a liquid to a gas simply needs much energy. So this water cools quite quick. Now when it cools it gets heavier and sinks down. Now the next layer of hot water is on top, and the whole thing repeats with the next layer. It is pretty much the same principle as a lava lamp. There you also see that the difference in temperature doesn't need to be big, 1-2° is enough ^^.
Now the reason it outpaces the other liquid once the temperatures get closer and closer is that the water moves much faster in one container. You can also observe that in a lava lamp, it will go on for quite some time after you quit putting more energy in the container. So the one that started hot simply has constantly the hottest layer on top and exchanges the layers much faster. That way it has more evaporation at equal temperature then the other one. So while both may have the same average temperature, the hottest layer is hotter and the coolest is cooler in the container that started with hot water. So you still have more evaporation here

sorry for my bad english, i hope it is still clear what i mean

 

[Studiot]

Well feyn, that certainly answers my comment in post 16,

If the two test vessels have a different temperature distribution they can have the same average temperature and still coll at different rate. It may be debatable whether the liquid can be represented by one temperture if it is not uniform but that would seem a good way forward.

 

[feyn]

Well feyn, that certainly answers my comment in post 16,

If the two test vessels have a different temperature distribution they can have the same average temperature and still coll at different rate. It may be debatable whether the liquid can be represented by one temperture if it is not uniform but that would seem a good way forward.

Hi studiot

well after thinking everything through that is the only explanation i can think of that makes any sense whatsoever. If you observe lava lamps,and meassure the temp difference there,you realize you get quite a current going with just little temperature differences ( 3° are already enough ). Once you have the current going it keeps going and going for quite long. But you need a relatively high start temperature to get it going in the first place, which is why i think either only the hot one has that effect at all, and the lukewarm has no inner current, or the inner current in that one is only very slow, that would have to be tested. That way the one with the current should be able to loose temperature faster when both are of equal average temperature.

 

[K^2]

Lava lamp's heater has quite a bit of heat still in it when you disconnect the power. That's what allows a lava lamp to keep going for a while. The inertia of water in a container is absolutely minimal. Nowhere near enough for one fluid to pass the other in terms of cooling time.

You can verify this by keeping track of temperature in both containers, and when the warmer container gets close to catching up, drop some food coloring in both. You'll see nearly identical rates of convection.

I'm starting to think that the issue might simply be volume loss due to evaporation. Has anyone actually checked how much fluid remains afterwards? Even a small defect in mass would make a huge difference in freezing time.

 

[feyn]

I am starting to suspect it is not a single explanation at all, but the sum of various effects that summed up lead to this puzzling phenomenon ;) I will say more about that when i have more time