How does a suction cup work? A suction cup works by creating a partial vacuum and using air pressure to keep the cup stuck to a wall.
A suction cup stays on a wall because of the weight of the air. Suction cups are made of rubber, silicone, or plastic that is flexible enough to deform, but strong enough to return to its original shape. This ability to change shape and then spring back again is called elasticity, and materials that do this well are called elastomers. The long chains of polymers that make up the rubber or plastic are all tangled up. When the elastomer is deformed, the polymers straighten out and become more ordered. Energy has to be put into the material to do this. The polymers would much rather stay in a state of disorder than be neatly arranged, so the material releases that energy and returns to its original tangled shape. That springing back is a large part of what makes the suction cup work.
When a suction cup is placed on a wall, it is pushed down. This compresses the cup and squeezes some of the air out from underneath it, or through a one-way valve in larger industrial cups. When the pressure is released, the material tries to return to its original shape and the cup springs back. When this happens, the inside of the cup returns to almost the same volume it had before, but now there is less air trapped inside it. This creates an area of low pressure, which is a partial vacuum.
As with a vacuum cleaner, it is easy to think that the low pressure inside the cup is sucking it to the wall, but that is not really what is happening. It is actually the heavier air outside the cup pushing it against the wall. The same is true of a vacuum cleaner. The air outside pushes dust and dirt into the nozzle because the pressure inside is lower. With the suction cup, the air around it presses the cup into the wall and, because there is much less air inside the cup to push back with equal force, it stays stuck. Air molecules inside the cup are still bouncing around and pushing outward, because it is only a partial vacuum, not a perfect one. But there are fewer of them, so the force they produce is weaker than the force of the air outside. That difference in pressure is what holds the cup in place. This is why suction cups work best on smooth glass, metal, or tile, where a good seal can be made.
The only real requirements are that the cup is not porous and the surface is smooth. If the cup is porous, or the wall is rough, air will slowly leak back in and the cup will lose its suction. Dust, grease, or even a tiny scratch can be enough to weaken the seal.
When the cup is removed, all that has to be done is to break the seal. As soon as that happens, outside air floods back in, the pressure equalizes, and the suction disappears. With very large and strong suction cups, that can be difficult to do by hand, so they may have a valve that can be popped to let the air back in. The strength of the cup, and the weight it can bear, depends largely on its size. The bigger the surface area of the cup, the more air is pushing it into the wall, and the more weight it can hold. People have even used suction cups to climb the sides of glass skyscrapers, so they can certainly be strong enough to hold the weight of a person.
Suction cups have many uses other than in bathrooms or kitchens. They are used widely in industry, and many robotic arms use suction cups to pick up items. In those cases, the robot pumps air out of the cup, picks up the item, and then lets air back in to release it. Some industrial suction systems can lift enormous weights, which shows just how powerful air pressure can be when the seal is good.
The earliest patent for a suction cup dates from 1875, but modern plastic and silicone suction cups only appeared after World War II. The 1875 patent was for a method of picking up photographic plates, and it was invented by an American photographer called T. C. Roche. Suction cups have actually been around for much longer than that, though. In the third century BC, there are examples of people using suction cups in medicine to draw blood to the surface of the body. In those cases, the air was removed from the cups with heat, which is a method that is still used today. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Suction_cup
https://physics.stackexchange.com/questions/199721/how-do-suction-cups-work
https://www.atlasobscura.com/articles/suction-cups-history
Photo by Pixabay: https://www.pexels.com/photo/white-android-smartphone-inside-vehicle-33488/