Corn Flour Magic

Exploring the Differences Between Fluids and Solids

In this experiment, we will investigate some of the differences between a fluid and a solid, by looking at a remarkable goop that sometimes seems to be liquid and sometimes solid!

What you will need:

Corn flour (the more the better – several full boxes is ideal)

Water (roughly as much water as corn flour)

A bowl to mix in and a spoon to stir with

What to do:

This can get messy, so do this somewhere it is easy to clean up

1) Empty the corn flour into the bowl.

2) Slowly add water and mix it through the corn flour. As you stir, the mixture will get quite thick, but keep going. When you have added roughly as much water as you have corn flour (by volume), you will have a thick, slimy looking goop.

3) Now we need to experiment with the goop – is it liquid or solid? To test this out, first try slowly submerging your hand into the goop. What happens? For comparison, think about what happens when you try to submerge your hand into a bowl full of water, or what happens when you try to submerge your hands into the surface of a table!

4) Next, take your bowl of corn flour goop and smash your fist down onto the surface of the goop as fast as you can. What happens now? How does this compare to a bowl full of water or the surface of a table?

5) Perhaps you can come up with some experiments of your own to test out this remarkable substance – try molding it like you would clay, then watching what happens when you release it.

6) Try submerging you fingers slowly again, until your hand is completely submerged, then try to yank it out as fast as you can.

What is Happening:

When you slowly submerged your hand in the corn flour, the goop flowed around your fingers like water (well, a very thick, viscous fluid), but when you hit it hard with your fist (or tried to yank your hand out), the goop became solid!

On a microscopic level, the corn flour goop consists of small starch particles packed close together. Separating the particles is a thin layer of water that acts like grease – allowing the particles to slide across each other and move around, as long as they move slowly. So, when you slowly push your fingers into the goop, the starch slides out of the way, allowing you to slide in easily. In this situation, the fluid applies viscous drag to the grains gently slowing their motion. However, if you try to smash your fist in quickly, the starch tries to move faster than the water can accommodate and grains come into contact. Now, the much stronger force of static friction acts between the grains – as long as they are being pushed together, there is force preventing them from sliding across each other - and the harder they are pushed together, the stronger the friction force is!

Almost instantly, long columns of starch grains are pushed together – a chain reaction of jammed particles that are held together by the stress you are applying (the force downwards from your hand) and the frictional forces that stop them slipping sideways out from under your hand like they did when you moved slowly. This "jamming" leads to "force chains" through the goop. While the stress is applied these force chains can last essentially forever, because of the static friction. Releasing the stress allows the structures to break down, returning to its fluid-like state.

This is something like Roman arches that hold together without mortar. Gravity pushing down on the archway causes friction between the heavy stone blocks that stops them from sliding across each other and collapsing. In the case of our corn flour goop, the structure is random and "self assembled", occurring when there is a very high concentration of starch granules (leaving only a small volume for the lubricating water to fill), otherwise the chain reactions of jamming do not occur and force chains cannot form.