Swelled Water Surface

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What type of experiment is this?

Experimental procedure and explanation:

• Fill a teacup with water to the brim. Even when the water surface swells above the rim of the teacup, the water does not spill over.
• Next, when the water surface is contacted with the tip of a toothpick on which a small amount of a neutral detergent (dishwashing detergent) has been applied, the water that exceeds the rim of the teacup flows over the cup.
• The first part of the experiment is able to be performed because of the action of the surface tension of water. With water and other liquids, a mutually attractive force (cohesive force) between the molecules themselves is exhibited. When water is spilled in the air in a space station, the water forms spheres. This is also due to surface tension. Through cohesive force, the molecules try to gather together to form a single mass.
• This type of cohesive force works in the same manner as the film tension (tensile force) that occurs when the rubber membrane of a balloon or the film of a soap bubble is stretched. Therefore, it may be easier to understand this concept if you think of it as a thin rubber film being spread out over the water surface. This is why the force is known as surface tension.
• The first part of the experiment may be easier to understand if you think of it as having a thin rubber film on the surface of the water that has swollen above the rim.
• In the second part of the experiment, the surface of the water that has exceeded the height of the rim flows over the rim because the surface tension is weakened by the neutral detergent. The neutral detergent is a substance known as a surfactant that acts to weaken the surface tension, and by merely contacting the water surface with only a slight amount of this surfactant, the water flows over the rim of the cup.

[Keywords]	surface tension
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[Reference]	Logergist “Physics Promenade (Zoku Butsuri no Sanpochimi),” Iwanami Shoten, pp. 58–66. (Logergist is a name given to a group of seven physicists including Professor Isao Imai, and in the five volumes of “Physics Promenade,” various physical phenomenon and daily events are considered.) The Japan Society of Mechanical Engineers, “The Wonders of Flow (Nagare no Fushigi),” Kodansha Blue Backs, pp. 62–67. Ryozo Ishiwata, “Illustrated Fluid Dynamics Trivia (Zukai Zatsugaku Ryutai Rikigaku),” Natsume Publishing, pp. 48–49.