What it shows
A thick mix of cornflour and water flows slowly when poured but turns hard when it is hit or squeezed. It is a suspension whose behaviour depends on how fast you push it.
The science
Oobleck is a suspension of tiny starch grains in water. When pushed slowly, the grains slide past each other with water between them, so it flows like a liquid. When struck quickly, the grains jam together and the water cannot flow out of the way, so it behaves like a solid. Fluids that become thicker when pushed fast are called shear thickening, or non-Newtonian, unlike water.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Cornflour, about 400 g
- Water, about 300 ml
- A wide bowl
- A spoon
- A small cup of about 100 ml as the measure
- A tray
- A plastic bag for disposal
Safety
For this project: Use only food-grade cornflour. Do not wash the oobleck down the sink, as it can block the pipe: scrape it into the dustbin. Do not eat it, and wipe the floor, which can become slippery.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Put 2 small cups (about 100 ml each) of cornflour in the bowl.
Add water a little at a time, mixing with your hands or a spoon, until you have about 1 cup of water mixed in. Add it slowly because it is hard to fix a runny mix.
Poke the surface slowly with a finger and note that it sinks in. Then punch it quickly and note that it feels hard.
Pick up a handful and roll it into a ball. Stop rolling and watch it melt and flow through your fingers.
Pour a little from the spoon and observe how it runs, then tap the pouring stream with a finger.
Test mixes with different ratios, for example 2 cups flour to 1 cup water, 2 to 0.75 and 2 to 1.25, and note which one best shows both behaviours.
Place a small coin on the surface and note how fast it sinks. Compare with a quick tap of the surface.
Write a table of the ratio, how it flows and how it feels when struck, and scrape all waste into the dustbin.
What you should see
At about 2 parts cornflour to 1 part water the mix is hard under a quick hit and runny when poured, while a mix with too much water is only a thin liquid.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Is oobleck a solution?
No, it is a suspension of starch grains in water.
02Why does it turn hard when struck?
The grains jam together and the water cannot move out of the way quickly.
03What is the name for such fluids?
Shear thickening or non-Newtonian fluids.
04Why not pour it down the drain?
The starch settles and can block the pipe.
Take it further
With an adult’s help, spread a thin layer on a small battery speaker sealed under cling film and play a low steady tone at a moderate volume to see it form finger-like shapes.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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