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What is viscosity?
Liquids have different properties and viscosity is one of them. Viscosity is a measure of how 'gloopy' or sticky a liquid is.
To be precise, it tells us how resistant the liquid is to flowing. In other words, if a liquid flows easily, it has a low viscosity, and if it struggles to flow, it must have a high viscosity. Here are some examples:
We use oil to reduce the friction between surfaces, but it must be thick, or sticky, enough to stay on these surfaces. If liquid soap was as runny as water, it would run off our hands before we had time to wash them. Oil and liquid soap are two fluids with high viscosity.
How can you compare viscosities?
You can do this very easily in a laboratory by 'racing' marbles!
Let's say you wanted to test the viscosity of five different fluids. You begin by placing a sample of each of them in test tubes, as shown.
To make this a fair test, you would have to ensure that each sample had the same temperature and same volume in each tube. (As you can see in the photograph, this particular student hasn't measured out their liquids evenly!)
All you then need to do is drop a marble into your test tubes and time how long it takes to reach the bottom of the tube with a stopwatch.
Gravity will pull the marble down through through the tube, while friction will act against it, pushing upwards as it falls.
The faster the marble sinks through the fluid, the less viscous it must be. This is because the particles in that liquid will be further apart, and therefore it is easier for the marble to fall through it.
The opposite would be true for a more viscous liquid - the marble would sink more slowly and take longer to reach the bottom of the tube. The particles in a more viscous fluid are closer together, making it more difficult for the marble to sink.
Now that you have seen the ideas about viscosity, have a go at the activity!
