The mysteries (and physics) of miso soup

In my Feb. 21 column in The Japan Times, I wrote about strange phenomena in miso soup, sent in by an observant reader. He noticed that his soup seemed to be moving around in the bowl of its own accord. And then as it cooled it formed a mysterious ball in the center. The photo in the paper did a fair job of capturing the ball, but there’s nothing like moving pictures to catch miso in motion! Click on the videos below to see for yourself.

First, a video of the moya moya モヤモヤ cloudy bits churning around in the soup:

The video above was posted by a physicist who said he was so fascinated to find an example of convection in his lunch that he forgot to eat his soup. Bundles of hot molecules (called “convection cells”) are rising out of the interior of the soup to let off heat at the surface, then fall back down into the soup to be reheated so they can rise again to release more heat. If the liquid was clear, you wouldn’t be able to see this movement, but the suspended solids make it possible to track the movement. In Japanese, “convection” is tairyū 対流.

Here’s another video of the same phenomenon:

Finally, in the course of my research, I came across another miso mystery that I couldn’t include in my article for lack of space. Watch this bowl of miso trying to escape across the table! (Warning: turn down your volume before you click — Japanese restaurants in the U.S. must be loud!)

Can you explain how this happened? It’s hard to tell in this image, but the bowls used for miso have an indentation under the bowl. Air trapped in this cavity is heated by the soup in the bowl above, and starts to expand, lifting the whole bowl upwards. The bottom of the bowl or the surface of the table must be wet, reducing the friction between the bowl and the table, allowing the bowl to slide as it’s elevated.

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