Why are suction cups ineffective at high altitudes?

Straws are a common item and seem easy to use, just suck from one end and the liquid will flow. But in reality, the way straws work is more complicated than many people think. For example, not everyone knows that suction cups are more difficult to use at high altitudes and cannot be used in a vacuum environment.

First, suction is essentially the result of pressure differentials that cause liquids, gases, and solids to move from one area to another. Suction, like centrifugal force, is not a real force.

When holding the tube and trying to suck, the user creates a closed pipe from the mouth to the drink, separating it from the outside. Next, the user needs to reduce the pressure in the mouth by increasing the volume inside. They do this unconsciously by lowering their tongue or pulling it back in their mouth.

This creates a pressure difference between the inside of the mouth and the outside environment, with the pressure inside the mouth being lower. Outside, atmospheric pressure presses on the drink, and at an altitude equivalent to sea level, the atmospheric pressure reaches about 101 kilopascals. The pressure in the straw is lower, so the liquid is pushed up the straw and into the mouth – also a low-pressure environment.

Because suction cups are dependent on atmospheric pressure, they operate inefficiently at high altitudes. There is less pressure on the drink, so the force pushing the liquid into the mouth is less. This means that outside a vacuum environment, the straw will not work because there is no pressure acting on the drink to push the liquid into the mouth.

At an altitude equivalent to sea level, the limit of height at which the liquid can rise through the suction pipe is about 10 m. This means that even when creating a perfect vacuum inside the suction tube, the external pressure at sea level is only enough to push the water up a maximum of 10 m. If a 15 m high suction pipe is used, the water cannot rise higher than that.