[Tom]

Tom  asked the Naked Scientists:
   
I've noticed this strange phenomenon since my late teens but no one has been able to explain it, and most people don't believe me until they try it:

Take a ceramic or glass coffee mug. Fill it with hot water (or just stick it in a microwave for 2 minutes). Stir in one heaping teaspoon of instant coffee. Then, with a metal spoon, start tapping the bottom of the cup rythymically (from within the liquid, not from the outside), about 2 taps per second or so.

You will notice the sound of the taps will increase in tone with each tap, and there seems to be no limit as to how high the sound will go.

If you stop and stir the mixture and then resume tapping, the sound will start out lower than when you stopped tapping, and then continue to increase.

It works best if you hold the cup in one hand and tap with the other; but it also works if the cup is on the table or counter.

Why is this?

I think it must have something to do with the coffee going into solution over time. But that really doesn't explain it well enough. Some people have suggested it might have something to do with the cooling of the liquid but I don't think so.

Tom Ninkovich
Watsonville, California

What do you think?

[LeeE]

I think that heating water in a microwave oven is very dangerous.  It is possible for the water to become super-heated without boiling.  If this happens, the water will just seem to be very hot until it's either jarred or something is dropped into it, like a teaspoon of coffee for example, at which point it will suddenly and vigorously (and even almost 'explosively') boil over.

[Geezer]

Have you tried doing the same experiment without the coffee?

It's possible what you are hearing is just an effect of the stress in the container caused by the thermal gradient between the inside and the outside. The gradient will change as the temperature near the outside of the container gets closer to the internal temperature.

[RD]

If the pitch change is reversible by stirring then it can't be due to evaporation reducing the volume of liquid.

If this phenomenon is as described then I think it could be due to the solute (coffee/sugar) settling out,
creating a concentration gradient, with a more concentrated, and denser, solution forming in the base of the cup.

When the cup is stirred concentration gradient is temporarily removed which alters the distribution of the mass which in turn alters the resonant frequency of the cup.


[BTW there is an allegedly musical instrument called the waterphone whose note changes pitch when water moves about in it].    

[sharkeyandgeorge]

I rather suspect that your simply resonating the cup which build frequency and changes the tone. The time taken to stir allows the cup to stop vibrating and hence starts on a lower tone once resumed.

[schenectandy]

I just noticed this the other day with hot chocolate and was hoping the internet could do my "homework" for me 
One thing I noticed the first time is that if I stopped tapping for a few seconds it would still be at a high pitch and getting higher - so it probably wasn't a resonance from regular tapping.

I did some more variations tonight:

1) Cold, still ceramic mug of water - constant tone
2) Boiled water in ceramic mug in the microwave - constant tone
3) Stirred the hot water (is the motion of water or shape of water surface doing it?)- constant tone
4) Stirred in powdered chai tea - higher pitch with each tap
5) Stirred chai again - higher pitch with each tap
6) Repeated stirring - 'resetting' effect diminishes, and I'm not sure, but the highest pitch reached seemed lower than in 4)

So far RD's hypothesis seems to fit the evidence best.. more experiments to follow (yum!).

[chris]

This is a question we looked at a little while ago; it's to do with the damping effect of bubbles within the body of the liquid. Bubbles act as shock-absorbers within the fluid, slowing down the rate at which vibrations can travel. Consequently the higher pitches are lost when the bubbles are present. As the bubbles rise and disappear, the liquid becomes "stiffer" and so the higher frequencies dominate.

We looked at this a little while back in the context of frothing milk, for which the principle is the same:

http://www.thenakedscientists.com/HTML/content/qotw/question/2239/

Chris

[Don_1]

I wonder if this can also be the explanaition for the change in pitch when stirring a cup of tea/coffee. The 'ringing' sound the spoon makes as it contacts the side of the cup changes pitch as the liquid goes from still to swirling around.

I think I'll just go check on that. I could do with a nice cuppa!