Kids

The strange behaviour of sand

This site shows you photos of a cutter suction dredger. It loosens up the seabed or riverbed with its cutter head and at the same time sucks up the loosened material. When you cut into sand under water, something strange happens. We can demonstrate that strange behaviour with a simple experiment.

Here is a picture of a plastic bottle filled with sand and water. A drinking straw has been inserted through the cap so that the water can flow out of the bottle. When you squeeze the bottle, you expect the water to squirt out through the straw. But it doesn't! In fact, the water level in the drinking straw drops. The bottle actually sucks in water!

That space is full of water. When you squeeze the bottle, you change the bottle's shape. The grains of sand then have to roll past one another. They can only do this if the space between them increases (this is shown here very simply). On the left, you can see the grains of sand before you squeeze the bottle. On the right, you see what happens when you squeeze it. Extra water has to get in between the grains of sand. That's why the water level in the straw drops.

This picture shows a half-litre plastic bottle, but a small bottle works just as well. Drill a hole through the middle of the cap so that the straw fits tightly. Push the straw into the cap. It's important that there is a good seal, with no space between the straw and the cap (so that nothing leaks). You can seal the hole with glue or sticky tape.

Half fill the bottle with water and then add sand. You can use any kind of sand, as long as it isn't too fine (in other words the grains mustn't be too small). "Masonry sand" (the sand used by bricklayers) is great, but sand taken from a children's sand pit ("play sand") is probably too fine.

Fill the bottle with sand up to about one centimetre below the top. Then fill it right up to the top with water and screw on the cap.

Have fun!