eli5: how do paint rattlecans work?
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The thing inside the paint can that makes it rattle is a child’s tooth.
Just kidding. It’s a (usually rejected) ball bearing. Its only role is to make noise so you know you’re sufficiently shaking the can. Spray paint really needs to be shaken well for best results. Making the can satisfying to shake by causing it to make sound reduces consumer complaints about crappy spray paint.
The contents of the paint can are under pressure. Some form of gas (usually nitrogen or carbon dioxide) is in the can, as well as liquid paint. The amount of gas greatly outweighs the amount of paint, likely at a 10:1 ratio or greater.
When you shake the can, you’re agitating the paint so it’s suspended among with the gas, creating an aerosol of gas and paint. When you press the nozzle, the gas rapidly escaping the can ‘pulls’ the paint with it, spraying from the tip of the can which directs the spray in a fan pattern.
As the aerosol moves from a high pressure to a low pressure environment, the gas dilutes in the air you sprayed it in, but the paint remains on the surface you aimed it at.
The can is pressurized with a gas and basic physics is that energy wants to move from a position of high energy to low energy (like how water flows downhill with down being lower energy)
The paint is in a liquidish form so when you press the nozzle the gas starts to depressurize from the can but it has to push the paint out first. That’s why you shouldn’t flip the can upside down.
There is some paint and some pressurized gas in the can. The tip of the can has a basic mechanism inside that lets the paint out when you press. A pipe is attached to this tip which reaches the bottom of the can.
When you press the head, the gas pushes on the paint and forces it to go somewhere. The paint has nowhere to go but the nozzle through the pipe.
(Actually the gas always pushes on the pipe. But the paint cannot move out until you press the head and let the paint out. )
Regular paint cans don’t work upside down because in that position the pipe is not in the liquid but in the gas.
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