Physics Coffee cup?

Question

when I make instant coffee (or hot chocolate), stir the milk and sugar in and tap the spoon on the rim of the mug, for about 10-20 seconds, the note that the tapping makes rises in pitch.

If I then restir the it, the resonant note deepens, and again rises when I tap on the rim.

this behaviour does not happen with:
(a) a cup of plain cold water,
(b) a cup of plain hot water,
(c) nor even a mug of cold water containing instant-coffee solids
(d) nor a mug of hot tea.

it happens only when it is both hot and contains instant-coffee or cocoa (hot chocolate) solids.

can anyone explain this phenomenon?

Answer 1

From the time dependence, It sounds like it is the *rotation* of the liquid, not so much the dissolved contents (which causes the surface to become paraboloidal) that changes the principle frequency. While it's spinning, the liquid level at the edge is higher, causing the resonance to be effectively that of glass with more liquid. That would give it a higher frequency. The fact that you are mixing in another substance is incidental. Try it with regular water after stirring.