Hot chocolate effect

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== Recipe for the hot chocolate effect ==

You will need: a cup of hot milk, chocolate powder and a spoon. Procedure: Pour hot milk in the mug and stir in the chocolate powder. once stirred, while the milk is still in motion, tap the bottom of the mug with a spoon (constant speed and strength of taps.

Observation

The pitch that the taps produces will increase progressively with no relation to the speed or force of tapping.

Explanation

Upon stirring, some gases get trapped in the liquid, causing the liquid to become less dense. When we start tapping the bottom of the mug, a small amount of these gases is released, slightly increasing the density of the liquid. The taps will create a standing wave in the mug. As the density of the liquid increases, the wave will travel faster and the pitch will increase.


Frank Crawford's 1982 article [1]