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Build your own airbrush - Applying Bernoulli
Find out how to build an airbrush to produce beautifully smooth paint finishes, and what this has to do with a car engine.
What you need
| A straw |
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| Scissors |
What to Do
Cut the straw into two halves.
Hold one of half-straw in your mouth.
Put the other half into the water, and hold, or tape it so that it sits vertically, with about 1-2cm emerging from the surface.
Use the straw in your mouth to blow quite hard, across the top of the other straw.
If nothing happens, try lowering the straw in the water.
What may Happen
You should find that when you blow hard enough, you produce a spray of tiny water droplets, just like an airbrush.
If you put some colour or paint into the waterm you can use it to paint with (though this could get very messy, very quickly, so make sure you do it outside).
What is going on?
Rapidly moving air will be at a lower pressure than when it is stopped. This is called Bernoulli's principle. So when you blow across the top of the straw, the rapidly moving air is at a lower pressure than the air above the water in your glass.
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One straw is set up to blow over the top of the other one. | The fast moving air is at a lower pressure than the air above the water, so the water is pushed up the straw. |
If you blow hard enough, the air above the glass will push the water all the way up the straw, and out of the top, where the rapidly moving air breaks the water up into tiny droplets, which are good for painting with.
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Eventually the water gets to the top of the tube, and is broken up into small drops by the fast moving air. |
Commercial airbrushes work exactly the same way, they just use higher pressures and better designed nozzles and produce smaller droplets.
This is also how fuel is mixed with air in an engine's carburettor: Air moves across a nozzle, and fuel is sucked up, and forms a fine, flammable mist, which is then burnt in the engine.
Why is fast moving air at a lower pressure?
Air, being a gas, can only be accelerated by a change in pressure. If the air is slowing down, then it must be going from an area of low pressure to a higher pressure, so the rapidly moving air must be at a lower pressure than the slower moving air.
I don't get it. If you spray a stream of air from a high pressure tank it slows down, but it's moving from high pressure to the lower pressure atmosphere?
As a good scientist (retired) I can provide evidence, and the following links should warm the heart of any kitchen scientist. The first is a teaching demonstration on the website of the Department of Physics at the University of Maryland, USA.--
http://www.physics.umd.edu/lecdem/services/demos/demosf5/f5-12.htm
This next set of demonstrations are very neat. They are in the process of being created by John Welch, the Physics Lab director at Cabrillo College in California, USA. The videos are on a link on this page--
http://www.cabrillo.edu/~jwelch/bernoulli/bernoulli-coanda.html
I can help explain how these demonstrations, how the incorrect ones on the web, and shamefully in some textbooks, are actually working if need-be. Steve
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