Tic tic toc
October 8, 2006 11:30 AM   Subscribe

If I put a glass of cocoa or milk in the microwave and get it out when it's hot, hold it in my hand and tap on the bottom of the glass (or mug) with a spoon - so with the spoon in the drink, not under the glass - the sounds it makes starts out as a really low "toc" but becomes higher with each tap. What is happening? It's not as if the glass becomes much hotter or colder in the first 10 seconds after I get it from the microwave.
posted by Skyanth to Science & Nature (10 answers total) 1 user marked this as a favorite
The Straight Dope says it's air bubbles being released.
posted by stefanie at 11:37 AM on October 8, 2006

You can see this phenomenon clearly, if you microwave liquid in the same cup for the same time, and then quickly pour granulated sugar into the cup. The sugar both carries small bubbles of entrained air with it when it drops into the coffee, and can provide a large amount of surface area for superheated steam to use as a temporary "nudge" to come out of suspension. Get the liquid hot enough, and the rising foam from a couple teaspoons of simple sugar can be pretty impressive.
posted by paulsc at 11:48 AM on October 8, 2006

Wow! Thanks for bringing this question up, Skyanth! I've noticed the same thing but I always forget to investigate. Yay!
posted by cadge at 3:21 PM on October 8, 2006

Wow. I was going to post this -exact- question yesterday. The season for hot chocolate has arrived!

I don't think it could be air bubbles being released - the phenomenon is repeatable, by simply stirring the contents of the glass and tapping again. The sound goes from low to high over and over again, with no change in the lowest tone despite time having passed since it came out of the microwave - I continued to have the same results for 7 or 8 minutes straight, which I think is far longer than any remaining air would still be escaping.

That said, I have no actual answers, sorry. ;)
posted by po at 3:22 PM on October 8, 2006

I agree the phenomenon is due to trapped air bubbles, but the further explanation offered by the site stefanie linked to:

Sound waves move a lot more slowly through air than they do through liquid. Ergo, the frequency of the standing wave set up inside the cup is lowered, and the sound you hear at first is low in pitch. As the bubbles burst, the pitch rises.

which the author of the quoted text attributes to Jearl Walker, strikes me as sheer nonsense. If that were true, tapping an empty cup, in which the frequency of the standing wave is therefore at its lowest, would produce the lowest sound. It doesn't. In fact, if you try it as I just did, I think you'll hear that an empty cup give a higher pitched tone than a cup full of liquid even with no bubbles in it.

The sounds you hear when tapping a cup are coming from the cup itself (and possibly the spoon, depending), not standing waves from the resonant cavity formed by the cup. The reason the bubble filled liquid makes the sounds lower is that foam is a more efficient absorber of sound energy than solid or liquid ( because of the the tremendous surface area of the internal bubbles and because the sound causes those bubbles to vibrate, thereby increasing their surface area and absorbing energy, I think). Higher frequencies are more readily absorbed by lossy media than lower (think how thunder is lower in pitch the farther away the lightning), and in this case the foam in the cup is absorbing the high frequency vibrations of the cup and spoon. As stirring breaks up the foam, that absorption becomes less efficient and the pitch rises.

Po, I can only explain your report by arguing that the initial stirring you do each repetition entrains air in the cocoa which gradually comes back out during the time you are tapping.
posted by jamjam at 4:24 PM on October 8, 2006

but stirring aerates your beverage!
posted by sonofslim at 4:24 PM on October 8, 2006

I could have stated the cause of the phenomenon better, above. It's nothing to do with the minor amount of air entrained by either stirring or the introduction of a granulated compound.

The phenomenon is that you drive parts of the liquid to superheated status, and then, through additional turbulence, the introduction of a physical object, or some other substance with lots of surface area, the superheated liquid readily supports the creation of little steam bubbles, through phase changes, changing the relative density of the liquid, and therefore its conduction of sound waves, changing as it cools. The spoon, the turbulence wave, or the sugar/sweetner/salt just provide molecular points of concentration where the bubbles can first nucleate, and they'll continue to form as long as the liquid remains above superheated temperature, although as they form and rise out of solution, they effectively cool the remaining liquid.

You can stop this from happening by not getting the liquid as hot, or by inciting sufficient turbulence as you do heat it, to prevent or forestall superheating. Superheating of certian areas of a beverage is just an easier phenomenon to produce in microwaves, where the liquid remains quiescent through the heating phase, and where regions of the liquid can readily become superheated. But you can see the same thing happen on a stove top, in any pan of water where the liquid is suffienctly deep with respect to volume and height, that tiny bubble first form evenly all over the bottom of the pan, before the liquid breaks into rapid boil.

And, you can hear the same kinds of pre-boil sound effects you get from your nuked coffee.
posted by paulsc at 5:51 PM on October 8, 2006

Be careful doing this. Under some conditions, microwaving a cup of liquid will leave the liquid in a state where it will boil violently upon adding sugar or a spoon. If you're not expecting it, you could get badly burned. There's even a crazy chain email going around about this, no doubt started by someone who didn't know about superheating.
posted by Mr. Gunn at 7:00 PM on October 8, 2006

Superheated liquids from microwave ovens are indeed common, but when you stir them or, say, add a teabag to them, the result is very commonly eruptive and can be explosive.

Here we are by no means certain the OP has heated his or her milk or cocoa to the point of superheating. In the case of the cocoa, which presumably contains many, many grains of cocoa powder along with their associated entrained air, all of which can serve effectively as centers of nucleation already without any need for stirring, it's not very likely, in fact.

But both milk and water (or milk) for cocoa do contain dissolved air which tends to come out of solution as they are heated, and both milk and cocoa are viscous enough to prevent these bubbles of air from rising to the surface and breaking as readily as they would in water, or if they were on the stove instead of in the microwave, where heating from the bottom establishes a regime of convection which will carry them to the surface. I think these bubbles are more likely to be the source of the OP's phenomenon than bubbles of superheated water vapor.

It's true that in a deep pot water at the bottom will superheat and form bubbles, but these bubbles will tend to blow up suddenly, rise a very short distance and collapse (making a sound) as the cooler water above turns the vapor of which they are composed back into liquid. I too have seen a uniform layer of tiny bubbles all over the bottom of the pan, but as I recall, they precede the stage of superheating considerably, and I continue to think they are bubbles of formerly dissolved gases
posted by jamjam at 7:28 PM on October 8, 2006

I don't think super heating has anything to do with this. The same thing happens when I pour a cup of coffee from the pot, which isn't boiling. As I stir in my sugar, the tinkle of the spoon increases in pitch.

I can't verify that this will happen a second time. I've always assumed it was due to the cup getting warmer. Possibly it is about air being stirred into the liquid, but I wouldn't have thought so, as I don't feel I stir that vigorously.
posted by Goofyy at 12:34 AM on October 9, 2006

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