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Red_AtNight t1_j9kbil7 wrote

It's not terribly complicated. The contents of the can are under pressure. There's a valve on top of the can. When you open the valve, some of the contents come out because fluids flow from high pressure to low pressure.

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-domi- t1_j9kis6d wrote

One aspect that hasn't been covered by comments thus far is that the nozzle which you operate with your finger has a straw attached to it. That straw is dipped in the liquid content. That's why if you start running low on paint, oil, hairspray, etc, you have to keep the can upright in order to get the intended content out.

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valeyard89 t1_j9mlc74 wrote

if you blow across the top of a straw dipped in water, it creates low pressure in the top part of the straw, which draws up the liquid from below.

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-domi- t1_j9mot82 wrote

The venturi effect you're describing is not how spray cans work. It is how (a lot of) spray guns work for airbrishing and stuff. But typical aerosol cans are literally just a substance that's kept liquid by the pressure it's under, with a straw dipped in it leading to a nozzle you can open. That's why once a can gets somewhat empty, you can only get a decent amount of paint/oil/hairspray/etc out if you hold it upright.

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Meechgalhuquot t1_j9ke554 wrote

There are chemicals called “aerosol” that are a liquid when contained in pressure. When you release the pressure they expand into a gas and accelerate out of the can, pushing the other contents out with them. If you flip a can upside down and spray it you may see frost forming on a surface you aim it at, or you'll feel a can get cold the longer you spray cause it's the exact same mechanic that allows your air conditioning to work. This is compared to using compressed air which would need a much sturdier container, it's why the walls of air compressors are so thick

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manofredgables t1_j9lk2u4 wrote

Many gases become liquids at relatively modest pressures. Air, for example, does not. If you were to pressurize an aerosol can simply with air, the pressure would dissipate rather quickly. But something like Butane turns into a liquid at room temperature at a practical 6-10 atmospheres, and if you try to reduce the pressure it will boil and restore the pressure, until all of it has boiled off. This provides a lot more volume. This boiling action requires heat, just like boiling water does. It will absorb heat, and that's why you may have noticed that aerosol bottles become cold when used a lot.

That's also why many aerosols are very flammable. There are alternatives that aren't flammable, but they're often needlessly expensive. Butane is dirt cheap.

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