# Suction cup

The pressure on a suction cup as exerted by collisions of gas molecules holds the suction cup in contact with the surface.
One cup suction lifter.

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.[1]

Suction cups are anatomical traits of some animals such as octopuses and squid, and have been reproduced artificially for numerous purposes.

## Theory

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.

## Calculations

The force required to detach an ideal suction cup by pulling it directly away from the surface is given by the formula:

$F = AP$

where:

F is the force,
A is the area of the surface covered by the cup,
P is the pressure outside the cup (typically atmospheric pressure)

This is derived from the definition of pressure, which is:

$P = F/A$

For example, a suction cup of radius 2.0 cm has an area of $\pi$(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.

The above formula relies on several assumptions:

1. The outer diameter of the cup does not change when the cup is pulled.
2. No air leaks into the gap between the cup and the surface.
3. The pulling force is applied perpendicular to the surface so that the cup does not slide sideways or peel off.

## Artificial use

SatNav devices often ship with suction cup holders for mounting on windscreens

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.[2][3]

On 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. Afterwards he went on to 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.[4] [5] [6]

Suction cups have a number of commercial and industrial applications:

• In office and household settings, to affix objects (ranging from signs to mugs) to nonporous vertical surfaces such as refrigerator doors and tiled walls
• To move large smooth objects such as panes of glass, automobile windscreens and raised floor tiles
• In toys such as Nerf darts
• In toilet plungers[7]
• By urban climbers, to scale buildings with smooth exterior surfaces[8]