A suction cup, also sometimes known as a sucker, is an object that uses negative fluid pressure of air or water to adhere to nonporous surfaces and in the process creates a partial vacuum. They exist both as artificially created devices, and as anatomical traits of some animals such as octopuses and squid.
The working face of the suction cup has a curved surface. When the center of the suction cup is pressed against a flat, non-porous surface, the volume of the space between the suction cup and the flat surface is reduced, which causes the fluid between the cup and the surface to be expelled past the rim of the circular cup. When the user ceases to apply physical pressure to the centre of the outside of the cup, the elastic substance of which the cup is made tends to resume its original, curved shape. Because most of the fluid has already been forced out of the inside of the cup, the cavity which tends to develop between the cup and the flat surface has little to no air or water in it, and therefore lacks pressure. The pressure difference between the atmosphere on the outside of the cup, and the low-pressure cavity on the inside of the cup, is what keeps the cup adhered to the surface.
The length of time for which the suction effect can be maintained depends mainly on how long it takes for fluid to leak back into the cavity between the cup and the surface, equalizing the pressure with the surrounding atmosphere. This depends on the porosity and flatness of the surface and the rim of the cup.
Artificial suction cups are believed to have first been used in the 3rd century, B.C., and were made out of gourds. They were used to suction "bad blood" from internal organs to the surface. Hippocrates is believed to have invented this procedure.
The first modern suction cup patents were issued by the United States Patent and Trademark Office during the 1860s. TC Roche was awarded U.S. Patent No. 52,748 in 1866 for a "Photographic Developer Dipping Stick"; the patent discloses a primitive suction cup means for handling photographic plates during developing procedures. In 1868 Orwell Needham patented a more refined suction cup design, U.S. Patent No. 82,629, calling his invention an "Atmospheric Knob" purposed for general use as a handle and drawer opening means.
Notable Uses: May 25, 1981, Dan Goodwin, a.k.a. SpiderDan, scaled the Sears Tower, the former world's tallest building with a pair of suction cups. After which, Dan Goodwin went onto scale the Renaissance Center in Dallas, the Bonaventure Hotel in Los Angeles, the World Trade Center in New York City, Parque Central Tower in Caracas, the Nippon TV station in Tokyo, and the Millennium Tower in San Francisco.   
To calculate the force of a suction cup, use the formula
- where F = force, A = area, P = pressure.
This is derived from the definition of pressure, which is
For example, a suction cup of radius 2.0 cm has an area of (0.020 m)2 = 0.0013 square meters. Using the force formula (F = AP), the result is F = (0.0013 m2)(100,000 Pa) = about 130 newtons, assuming the pressure inside the suction cup is negligible when compared to atmospheric pressure (about 101,000 Pa).
Suction cups have a number of commercial and industrial applications. In office and household settings, they are commonly used to affix objects (ranging from signs to mugs) to nonporous vertical surfaces such as refrigerator doors and tiled walls. Large suction cups are used to temporarily hold and move large panes of glass and are often utilized when fitting large smooth objects such as automobile windscreens and raised floor tiles. Suction cups are also found on Nerf darts and toilet plungers. Specialized suction cups have also been utilized by urban climbers to scale buildings with smooth exterior surfaces.
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- "United States Patent 52,748".
- "United States Patent 82,629".
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