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The hot chocolate effect

Hot Chocolate Effect in a glass of water

The dramatic influence of bubbles on the acoustic properties of a liquid is well illustrated by the "hot chocolate" effect.

When an instant drink is prepared, the air captured in the powder is released into the liquid in the form of many bubbles: the beverage is bubbly. When one knocks the bottom of the cup with a spoon, the pitch of the note that is heard increases with time. The video shows this effect, here with a glass of bubbly water.

A simple argument for explaining this effect consists in considering that the note corresponds to the fundamental mode of the column of liquid, whose frequency is proportional to the speed of sound. As the speed of sound is low when the concentration of bubbles is high, the note increases when the bubbles are disappearing.

This effect can be used for chemical compound characterization [3].

- [1] F. S. Crawford, "The hot chocolate effect", Am. J. Phys. 50 398 (1982)
- [2] P. S. Wilson, R. A. Roy, "An audible demonstration of the speed of sound in bubbly liquids", Am. J. Phys. 76 975 (2008)
- [3] D. Fitzpatrick et al. "Principles and Applications of Broadband Acoustic Resonance Dissolution Spectroscopy (BARDS): A Sound Approach for the Analysis of Compounds", Anal. Chem. 84, 2202 (2012)


Dynamics of Out-of-Equilibrium Systems, Acoustics

Contact : Published on / Publié le 4 November 2010