If you’ve ever used an aerosol can in cosmetics, food, or blowing the dust off your keyboard, you know how quickly the can turns chilly. Even a small blast can cause frost to form. Significantly! Because the can does not appear to warm up after standing still, this reaction does not revert and may be permanent.
The average pressure in the aerosol cans cannot change when shaken. It’s also interesting how you increase energy by shaking only to have more energy lost by cooling. Shaking the can, of course, causes the cold liquid to pick up some heat from the warmer metal that was above the level of the cold liquid, so warming the liquid (a little) and chilling the metal. So, let’s solve this mystery and explain this weird conduct.
Reasons Behind Aerosol Cans Getting Cold After Shaking:
When shaken, spray cans don’t become colder. Shaking an aerosol can, however, causes it to feel colder because it increases the heat transferred from your hand to the can. Thermoreceptors, located beneath the skin and accurately measure the rate at which body heat is lost through the skin, allow humans to sense temperature indirectly rather than directly.
For this reason, the temperature is the same; contacting a metal surface will feel substantially colder than a wooden or plastic surface. An aerosol can is typically much cooler than body temperature when you pick it up, meaning it is at room temperature.
Because the aerosol can is a narrow metal cylinder, it conducts heat well yet has a low heat capacity. So, the amount of heat loss from your hands (and thus the impression of cooling) is mostly determined by the pace at which whatever is inside the can absorbs heat from the surface.
A general aerosol can contain a propellant that is slightly over its boiling point at room temperature and pressure but is maintained liquid by the increased pressure inside the can. Some propellant fluid boils when the spray valve is opened, raising the pressure once again until the liquid and gas phases are balanced.
This means that in a spray can that is half full, the top half will be filled with gaseous propellant (which, as a gas, will not absorb heat very effectively), and the bottom half will be filled with a liquid propellant that has been mixed with the payload fluid and is just below the boiling point.
Because it is denser (and therefore has higher heat conductance and capacity), and heating it will cause some of it to boil, some of the absorbed heat will be converted into its latent heat of vaporization. This liquid will absorb heat much more effectively than the gas at the top.
So you can notice that a spray can’s bottom end will feel cooler to your hand than its top end, even if you don’t shake it.
Now, instead of just coating the bottom of the can when you shake it, the propellant liquid coats the entire interior surface when you shake the can. The warmer liquid near the can’s surface mixing with the colder liquid inside the can increases the heat transferred from the aerosol cans to the liquid due to sloshing.
Similar to why moving air feels cooler on your skin than still air, this is the main cause of that feeling. So, an aerosol can that has been shaken will generally feel cooler to the touch than either end of one that hasn’t been shaken.