Secret of Suction Cups

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What type of experiment is this?

Experimental procedure and explanation:

• Suction cups can stick to all kinds of objects. We are going to discover the secret behind suction pads.
• Stick a suction pad with a diameter of 6 cm firmly to a table. It does not move at all even when pulled with a force of approximately 5 kg.
• When we stuck a suction cup on the floor and pulled it upwards using a spring scale, we found that the suction cup could withstand a force of up to approximately 19 kg.
• There are two characteristics that enable a suction cup to withstand such a large force: (1) the suction cup is made of a soft material such as rubber or vinyl, and (2) its shape is indented in the center (concave).
• Because the suction cup is made from a soft material, when it is pushed from above, the air in the center of the concave area is pushed out, significantly decreasing the pressure in that region.
• The soft material also has another function. When the suction cup is pushed strongly, the surface that is in contact with the table yields and sticks to the table by contact adhesion. This keeps it airtight, blocking the hollow inside from the atmospheric air outside, and therefore the pressure inside the hollow stays extremely low. If a hard material were used, the surface would not yield, and therefore there would be gaps between the suction cup and the table, and the seal would not be airtight (like the plastic sheet in “Sticking a Plastic Sheet to a Table”).
• The shape is concave to create a hollow cavity in the center. First, by pushing out the air, the pressure becomes very low (ideally, the pressure would be close to a vacuum). Because of the difference between this low pressure and the atmospheric pressure on the outside, a large adsorption force is generated. When the cavity is at low pressure, this increases the contact pressure of the surface where the suction pad is in contact with the table; the increased contact pressure suppresses the unevenness of the surface to make the seal more airtight.
• The combined effect of the suction cup’s characteristics induces a large adsorption force. With the 6-cm-diameter suction cup used in this experiment, the force due to atmospheric pressure acting on the outside is approximately 28 kg (approximately 1 kg per 1 cm² area). In the experiment, this suction cup withstood a force of approximately 19 kg, showing that it was functioning efficiently. (If the underside were a complete vacuum, the theoretical maximum value of approximately 28 kg could be achieved.)

[Keywords]	Atmospheric pressure
[Related items]	Sticking a Plastic Sheet to a Table, Sticking a Plastic Sheet to the Ceiling
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P16－17. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P34－37.