Did you know cornflour slime is a very special fluid? Follow this simple recipe to make slime without borax and find out why this fluid can behave like a solid. A great kids science activity!

You will need

  • cornflour
  • food colouring
  • small mixing bowl
  • plastic spoon
  • water.

What to do

  1. Pour some cornflour into a mixing bowl.
  2. Stir in small amounts of water until the cornflour has become a very thick paste.
  3. To make the slime the colour of your choice, thoroughly stir about five drops of food colouring into the mixture.
  4. Stir your slime really slowly. This shouldn't be hard to do.
  5. Stir your slime really fast. This should be almost impossible.
  6. Now punch your slime really hard and fast. It should feel like you're punching a solid.
  7. You can keep your cornflour and water mixture covered in a fridge for several days. If the cornflour settles, you need to stir it to make it work well again.

What's happening

Anything that flows is called a fluid. This means that both gases and liquids are fluids. Fluids like water which flow easily are said to have low viscosity, whereas fluids like cold honey which do not flow so easily are said to have high viscosity.

Cornflour slime is a special type of fluid that doesn't follow the usual rules of fluid behaviour. When a pressure is applied to slime, its viscosity increases and the cornflour slime becomes thicker. At a certain point, slime actually seems to lose its flow and behave like a solid. Cornflour slime is an example of a shear-thickening fluid.

The opposite happens in shear-thinning fluids; they get runnier when you stir them or shake them up. For example, when toothpaste is sitting on a toothbrush it is pretty thick, so you can turn the toothbrush upside down and the toothpaste doesn't fall off. But if it was that thick when you tried to squeeze it out of the tube, there is no way you could manage it. Fortunately, toothpaste gets runnier when you are squeezing it out of the tube.

Other shear-thinning fluids include:

  • blood
  • paint
  • ballpoint pen ink
  • nail polish.

Although there are lots of shear-thinning and shear-thickening fluids, nobody has a really good idea why they behave the way they do. The interactions between atoms in the fluids are so complicated that even the world's most powerful supercomputers can not model what is happening. This can be a real problem for people who design machinery that involves shear-thinning fluids, because it makes it hard to be sure if they will work.

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