fluid dynamics - Why does the sound pitch increase on every consecutive tick at the bottom of a filled cup of coffee?

Since I don't know the proper physical terms for this, I describe it in everyday English. The following has kept me wondering for quite some time and so far I haven't found a reasonable explanation.

When you fill a ceramic cup with coffee and you click with the spoon at the bottom (from the top, through the coffee), each following tick, even when you pause for some seconds, will have a higher pitch. The following I've observed so far:

• works better with coffee than with tea (works hardly at all with tea)


• works better with cappuccino than with normal coffee


• doesn't work with just cold water


• works best with ceramic cups, but some plastic cups seem to have the same, yet weaker, behavior


• doesn't work on all types of cups, taller cups seem to work better


• must have a substantive amount of liquid (just a drop doesn't make it sing).

It must be something with the type of fluid, or the milk. I just poured water in a cup that had only a little bit fluffy left from a previous cappuccino, and it still worked. Then I cleaned it and filled it again with tap water and now it didn't work anymore.

Can someone explain this behavior?

Answer

I think you are observing "the hot chocolate effect" or something similar. See Crawford, Am. J. Phys. 50, 398 (1982). I have to confess I haven't read through the paper in enough detail to adequately summarize it.

Abstract:

The ’’hot chocolate effect’’ was investigated quantitatively, using water. If a tall glass cylinder is filled nearly completely with water and tapped on the bottom with a softened mallet one can detect the lowest longitudinal mode of the water column, for which the height of the water column is one‐quarter wavelength. If the cylinder is rapidly filled with hot tap water containing dissolved air the pitch of that mode may descend by nearly three octaves during the first few seconds as the air comes out of solution and forms bubbles. Then the pitch gradually rises as the bubbles float to the top. A simple theoretical expression for the pitch ratio is derived and compared with experiment. The agreement is good to within the 10% accuracy of the experiments.