## 24 September 2020

### The Hot Chocolate Effect

No time to blog today, but I wanted to share this video of the Hot Chocolate Effect.  Very cool, unexpected, and interesting.
The hot chocolate effect, also known as the allassonic effect, is a phenomenon of wave mechanics first documented in 1982 by Frank Crawford, where the pitch heard from tapping a cup of hot liquid rises after the addition of a soluble powder. It was first observed in the making of hot chocolate or instant coffee, but also occurs in other situations such as adding salt to supersaturated hot water or cold beer. Recent research has found many more substances which create the effect, even in initially non-supersaturated liquids.

It can be observed by pouring hot milk into a mug, stirring in chocolate powder, and tapping the bottom of the mug with a spoon while the milk is still in motion. The pitch of the taps will increase progressively with no relation to the speed or force of tapping. Subsequent stirring of the same solution (without adding more chocolate powder) will gradually decrease the pitch again, followed by another increase. This process can be repeated a number of times, until equilibrium has been reached. Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.
Try this in your own kitchen (impress your kids).  You learn something every day.

1. WOW! Very cool... and easily done.

2. when i was a kid, the big fun was drinking tea in glasses. the fun started when you poured the milk into the glass of tea, and watched it swirl.

I-)

3. I make coffee by pouring boiling water onto dry grounds in a mug, and stirring till most of them have sunk, and I have known this phenomenon for decades. The note goes Tonk tunk tank tenk tink tink tink. My colleagues are underwhelmed, but I love it. In my case it is repeatable, just give the mug a stir and note goes back down. I always thought it was the shortening of the column of liquid as the grounds settle, but perhaps the allassonic effect is in the mix.

1. Interesting. I noticed that in the video there wasn't a lot of froth or overly-vigorous stirring, so I'm guessing that the dry chocolate powder (and your coffee grounds) carry some air into the liquid on the surface of the particles.

4. Then what do you suppose it means, when you vigorously stir a low level of coffee in a thick-walled ceramic cup, it says, "WOIN-ya, WOIN-ya, WOIN-ya."

1. I think that has to do with the spoon's hollow, sharp and round sides alternately scraping against the cup's inner wall.

5. The size of the effect is amazing: it rises through multiple octaves.