Water that Won't Fall

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Fill a glass completely with water, put a lid over it, and turn it upside down. The lid will stay in place and the water will stay inside. This is a well-known experiment.
• Many people explain that the water will not fall because “surface tension is supporting it,” but this is not quite true.
• The majority of the force that is preventing the water from falling is not the surface tension but the pressure difference between the water in the top part of the glass and that in the bottom part. The pressure right above the lid is almost at the atmospheric level (slightly lower than atmospheric pressure, to be accurate), and the pressure at the bottom of the glass (upper part) is lower by “water density × gravitational acceleration × height” and is at equilibrium.
• What role does the “surface tension” have in this phenomenon then?
• The surface tension is actually not significant and is not capable of supporting the weight of the water, as shown by the fact that it was not capable of supporting the pressure difference between the top and bottom points in the “Plastic bottle with Holes” experiment.
• The surface tension supports the weight of the lid. There is atmospheric pressure underneath the lid (air), and the pressure above the lid (water) is lower by the amount that is enough to support the lid. This slight pressure difference is supported by the surface tension acting in the slight gap (water film) between the glass and the lid.
• However, the lid can no longer be supported when a neutral detergent (the detergent weakens the surface tension by acting as a surface active agent) is applied, because it lowers the surface tension.

[Keywords]	surface tension, depth and pressure
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 62-67 “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 48-49, 18-19