Does stirring accelerate cooling?

January 9, 2007 1:10 PM

Does stirring a mug of hot liquid make it cool faster?

Let's say I have a nice, hot mug of coffee. It's too hot to drink at the moment because I got it from McDonald's. Would stirring the coffee with a spoon make it cool down faster? I'm just talking about a gentle stir. I think that stirring it fast enough to create a vortex would accelerate cooling because the surface area of the liquid would increase, but I can't decide if a gentle stir would affect the cooling process. What do you think?

posted by flyingcowofdoom to science & nature (19 comments total)

put it in a bowl, stir, then sip with a spoon from the edge of the bowl not the center, it's the cooler liquid
posted by matteo at 1:12 PM on January 9, 2007

Yes. The greater the temperature differential between the surface of the liquid (both the top and the sides) and the surrounding environment the faster heat will escape.

If the liquid is just sitting, then the liquid at the surface cools a bit, and the air at the surface heats a bit so the temperature differential shrinks and the rate of heat exchange slows. Stirring even a little should mean that the liquid at the surface is closer to the average temperature of the liquid, and also should cause some small bit of air circulation.

So thats the theoretical basis. In the real world, I'm not sure the effect would be enough to be noticable.
posted by Good Brain at 1:15 PM on January 9, 2007 [1 favorite]

Yes. Otherwise, since a mug is pretty good insulation, a mini-thermocline may result. Circulation!
posted by cowbellemoo at 1:16 PM on January 9, 2007

Yes, absolutely. And if you are using a spoon and want to cool it down quicker, use the spoon to take out a tiny spoonful of coffee and then dip it back in - and repeat.
posted by vacapinta at 1:23 PM on January 9, 2007

I would also think a metal spoon would help wick away heat better than a wooden or plastic spoon.
posted by JonnyRotten at 1:26 PM on January 9, 2007

Yes, both because it increases the surface area and because it causes warmer molecules from the middle of the liquid to come to the surface to replace the warmer ones that evaporated from the surface. (And it reduces the partial pressure of evaporated water near the surface, but that effect will be smaller.)
posted by raf at 1:26 PM on January 9, 2007

The rate of cooling is proportional to the temperature difference between the liquid and the surrounding area. Stirring arguably does what convection does; it provides momentum for the hotter liquid to come to surface and make contact with the air to cool more quickly - although the contact time is reduced. Obviously the shape of the mug must allow convection, and not be finned or prevent liquid flow.

However it must be recognized that you are also putting energy into the liquid by stirring it. A miniscule amount to be sure, but be aware of it. Stirring more vigourously would result in a temperature increase.

I would argue that yes, stirring the liquid does cause it to cool more quickly than a quiescent liquid. How much more? I can't say as there are many factors that determine the cooling; stir rate, temperature difference, mug thickness and material, and spoon size/material.
posted by fox_terrier_guy at 1:28 PM on January 9, 2007

Since the spoon warms up and has considerable thermal mass, obviously the liquid must cool down. I'm not sure I believe that any of the other effects described here are significant, however.
posted by Steven C. Den Beste at 1:34 PM on January 9, 2007

According to the master of empiricism, agitating a coke makes it chill faster; the same principle should apply here, just with higher temperatures.
posted by solotoro at 1:40 PM on January 9, 2007

Stirring more vigourously would result in a temperature increase.

So you can boil water by stirring fast enough?
posted by oxford blue at 2:15 PM on January 9, 2007

In theory yes. Is it effective and a good use of your time? Who knows until someone provides measurements.
posted by MonkeySaltedNuts at 2:20 PM on January 9, 2007

When I have had a sterling silver spoon to stir my tea, the spoon conducts heat very well, and acts a s a radiator, cooling the tea faster. I was born with it my mouth; don't know how it got there...
posted by theora55 at 2:22 PM on January 9, 2007

The stirring motion could also move around the air on top of the coffee, increasing the convective heat transfer. This effect is probably very small, but perhaps significant.

Another way to possibly increase the cooling rate would be to prop up the cup on some wooden stirring sticks. If there is a space for air to pass under the cup, heat will most likely be carried away faster by air convection than if it had to conduct heat into the table.

A bit more on modes of heat transfer here.
posted by SBMike at 2:22 PM on January 9, 2007

So you can boil water by stirring fast enough?

Theoretically, yes, but the water would have to be extremely well-insulated (as the water approaches the boiling point, the heat transfer rates increase rapidly). As long as you can pump in mechanical energy at a greater rate than it leaves the system, the water will heat up and boil.

Of course, the actual boiling takes extra energy on top of the energy required to bring it to the proper temperature, but still no reason it can't happen in theory.
posted by SBMike at 2:25 PM on January 9, 2007

Oxford blue:

Stirring is similar to microwaving; it is an agitation of the molecular structure. Hard to believe, and difficult to do, but yes, you could boil water by stirring. Modest temperature increases can be observed in the lab through the use of a calorimeter.

You could definitely boil water by stirring if you reduced the vapour pressure of the surrounding region and stirring, thereby effectively increasing the vapour pressure of the liquid (when vapour pressure of a liquid equals atmospheric pressure, the liquid is said to be boiling.) By reducing the vapour pressure of the atmosphere, you could boil water at about 15 degrees C (60F).
posted by fox_terrier_guy at 2:28 PM on January 9, 2007

Ack. Scooped by SBMike by 3 minutes.
posted by fox_terrier_guy at 2:29 PM on January 9, 2007

So you can boil water by stirring fast enough?
The Vitamix blender actually heats soup, just by blending the cold ingredients long enough.
posted by davar at 3:16 PM on January 9, 2007

What an age we live in!

I hear they even have a way of transferring moving pictures through the air!
posted by oxford blue at 3:56 PM on January 9, 2007

The Vitamix blender actually heats soup, just by blending the cold ingredients long enough.

It's likely that most of the temperature increase in this case is the heat output of the electric motor.
posted by chrisamiller at 4:56 PM on January 9, 2007