What became known as the Hot Chocolate Effect was the result of a US physicist, Frank Crawford in 1982, noticing a pitch change in the sound of his spoon hitting the bottom of a mug of newly-made drinking chocolate.
There have been a couple of papers describing the phenomenon but it does not appear to be widely known. Crawford’s analysis explains the effect as being due to the creation of bubbles in the liquid followed by their gradual collapse as they reach the top. Associated with this is a change of the speed of sound in the chocolate and hence the resonance frequency of the column of liquid. It is possible that other means of producing such a change in the speed of sound in a liquid might be possible which could be controlled.
The change produced in a cup of hot chocolate is dramatic and we hear the resonance frequency of the column of liquid sweep over a large range so it is a major effect, one which we would hope has scope for potentially useful applications.
Undergraduate
This project is first to create an experimental rig to investigate the phenomenon and then reproduce Crawford’s results in detail. This would provide the basis for refining his theory and determining the controlling factors/features.
The next stage will be to find how this pitch change could be realised as a permanent repeatable effect rather than a transient phenomenon (limited by the time taken for bubbles to collapse). For example this could be by replacing the bubbles with other permanent intrusions by which the bulk modulus of the liquid (and hence the speed of sound) could be changed.
Finally the project would suggest and assess potential uses.
Refs:
1. Crawford F S American Journal of Physics 50, 398 (1982); doi: 10.1119/1.13080 The Hot Chocolate Effect
2. Crawford F S American Journal of Physics 58, 1033 (1990); https://doi.org/10.1119/1.16268 Hot water, fresh beer, and salt
3. The Physics Teacher 50, 458 (2012); https://doi.org/10.1119/1.4758141 Bubbles that Change the Speed of Sound
4. American Journal of Physics 76, 975 (2008); https://doi.org/10.1119/1.2907773 An audible demonstration of the speed of sound in bubbly liquids
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Acoustics Lab (City 422.154, Lab)