I can’t believe I haven’t done this before because it is quite a cool experiment and all you need can be found in most kitchens: water and cornflour. (Cornflour is called cornstarch in some countries. In Spanish we call it maicena). I also added a bit of red colorant, but that is completely optional.

Ingredients for goo

Ingredients for goo

All you have to do is put some cornflour in a bowl and add water to it slowly while mixing it with your hands until it has a creamy consistency. The feeling is very weird: it seems like you won’t be able to mix them because when you touch the flour it feels quite hard, however they do mix very quickly. And then, very strange things happen.

If you put pressure on the mix by punching it or grabbing it with your fingers it becomes solid, but as soon as you release the pressure it goes back to liquid! For example, you can make a ball with the liquid by moving the mix fast between your hands or closing your hand around it strongly, but as soon as you stop moving them, the ball liquifies and slips through your fingers.

Making a ball of liquid

Making a ball of liquid

It is not easy to explain it with words and the best thing to understand the feeling I am trying to describe is to experience it yourself, but I uploaded a couple of videos so that you can get an idea of what is going on:

Why does cornflour behave like that? This mix is again a colloid. Like in the case of miso soup we have a solid (the corn starch) suspended in a liquid (water). The molecules of corn starch are quite big and, when moving slowly they are able to pass by each other and flow. However, when pressure is applied, they come together making movement more difficult and trapping water in between a few starch molecules. In a small scale, the structure in this case is similar to that of a gel: water trapped between entangled starch molecules. This is actually just another example of a phase transition, only that the transition is not provoked by a change in temperature but rather a change in pressure.

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.

IMG_3454

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