The other day we learnt that a gel is a mixture of two different phases: a solid network and a liquid phase. These two share the same volume but they don’t completely mix, in a microscope one can still tell them apart. Such systems where 2 phases are simultaneously present but not completely mixed are called colloids.

The interesting thing is that you can have colloids involving also other phases. For example, you can have a solid phase suspended in a gas like smoke in the air, or a gas dispersed in a liquid like in foams, or a liquid in a gas like in the case of mist and clouds. Or one can have a solid phase dispersed in another liquid phase and this is the case of Miso Soup.

Essentially Miso Soup consists on a stock and some paste (typically bean paste), which is the solid. There might be other ingredients like tofu cubes or sea weed, but lets focus in the two main ingredients. Why does the paste not just dissolve into the broth like sugar in water? The difference with that case is the size of the solid “particles” which in the case of Miso are much bigger than sugar molecules and therefore they don’t dissolve.

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The paste "particles" in Miso Soup are too big to dissolve in the broth.

One curios effect of this big size of the paste particles is that the fluid motion can be traced by the motion of the particles so that one can see the convention movements that the soup undergoes as it cools down. This motion changes with temperature. When the soup is very hot, the immiscible part of miso convects with the broth. At intermediate temperature, the paste forms a sediment layer at the bottom. This layered structure is destroyed regularly by the instability caused by accumulated heat in the miso layer as a bursting and one can then see a “eruptions” in the miso soup. One can also provoke such eruption by hitting slightly the soup bowl after the cloud has settled on the bottom and before it gets too cold (this is when there is still accumulated heat in some points).

The convention patterns are also very interesting, although more technical to explain. If you are interested you can for example consult this paper where they explain the arrangement of holes on the broth, usually arranged in an hexagonal pattern.

In the next series of pictures you can see the first convention pattern in the first row (in this case the hexagonal structure appeared very briefly) and then 2 eruptions caused by instabilities before the “cloud” finally settles at the bottom of the soup bowl.

Miso soup
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