– Why does water help with suction cups?
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In the space like between the rubber cup and the surface it’s stuck to, water is held in by capillary attraction (due to the surface tension). Because that space is extremely thin it’s held in very tightly, preventing air getting behind the cup. Without air getting in the cup can’t come off.
Because suction cup materials are usually very soft and flexible, but also have tiny, microscopic imperfections in them. On their own, those imperfections would allow air to pass the edge and get into the cup and well, then you wouldn’t have a suction cup any more.
Water has two important properties, it’s both *adhesive* and *cohesive.* This means it both likes to stick to itself, and it also likes to stick to other materials. So a little bit of water will stick to the tiny imperfections around the lip of the suction cup and form a barrier that the air can’t get past. Then when you press the suction cup down you push the air inside the cup part out creating a partial vacuum. The vacuum means that the pressure of the air outside the suction cup will push it down onto the material forming a seal.
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