Catching Particles Floating on the Surface of Water

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What type of experiment is this?

Experimental procedure and explanation:

• First, we use a spoon to scoop up scraps of paper floating on the surface of the water. Water flows into the spoon and the scraps of paper come into the spoon with the water.
• If the spoon is turned upside down, the scraps of paper cannot be scooped up because water flows along the back of the spoon, carrying away the scraps of paper.
• Next, we float small pieces of Styrofoam (expanded polystyrene) on the surface of the water, and catch the pieces with a cup.
• With the mouth of the cup turned upwards, many particles are caught. The water close to the rim of the cup is pushed outwards, and only the particles near the edge are carried away by this flow to escape.
• However, when the cup is turned upside down, a lot of the particles escape. Most of the water between the bottom of the cup and the surface of the water is pushed outwards, and many of the particles are carried away by this flow to escape. For this experiment, the cup must have a flat bottom for this experiment; if the bottom is dented, water will stay inside, and therefore more particles will remain trapped by the cup.
• Small, light objects have a tendency to get carried by a flow; this is referred to as having good entrainability.

[Keywords]	Entrainability, Viscous resistance
[Related items]	Catching Particles at the Bottom of a Volume of Water, Catching Particles Floating in Water
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P30・・1.