Why Do My Shaker Cans Get Sucked Together?

You’ve no doubt noticed that when you properly shake an alcoholic drink, the parts of your shaker are drawn tightly together by an internal vacuum force. Why does this happen and how big is the force?

A couple of cool things are happening when you shake. First, the air that’s in your shaker starts off at room temperature. As you are shaking, this air gets cooled just like your drink does. Cooling the air causes the pressure to go down, which causes a vacuum. That isn’t all that’s happening, though. Ice is less dense than water. When ice melts, it actually contracts in volume. When the volume of liquid plus ice in the shaker contracts, the volume of air in the shaker increases. Since you aren’t adding more air molecules, increasing the volume decreases pressure, causing more vacuum. Third, as your liquor gets colder, its density increases, again shrinking in volume and creating more vacuum. A third factor may be a small amount of expelled air when the bartender slams down on the cans before the shake. The last effect is hard to calculate mathematically. [...]

Plug in all the numbers, and the inside of the shaker has a negative pressure of about 1.7 psi. Since the small shaker measures 3 3/16 inches across, the force to pull the cans apart should be about 13.6 pounds.

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