From liquid to solid in a punch

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

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

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.