Collecting Tea Leaves

Let's take a look!

Watch the video on YouTube.

What kind of experiment is this?

Experimental procedure and explanation:

• Let us try to gather tea leaves at the bottom of a mug.
• Stir the water using a spoon and lift the spoon up. Soon, tea leaves will accumulate at the center of the bottom.
• Because of viscosity, peripheral speed of water is slower at bottom of the mug. Therefore, the centrifugal force at the bottom is smaller. (In fluid dynamics, we often observe this phenomenon along with rotating fluid particles (a rotating coordinate system). With this in mind, and with the help of apparent and centrifugal forces, we will explain the current phenomenon.)
• However, the effect of viscosity is weaker in the upper part of water, and thus the peripheral speed is faster. Therefore, the centrifugal force in water near the top is larger than that in water at the bottom.
• Owing to the centrifugal force, water pressure is greater near the walls of the mug. However, at the bottom, the centrifugal force is smaller, and a center-bound flow is created by the higher pressure near the walls. This flow forms a rising current at the center, an outward flow at the top, and a downward flow along the walls, creating a circulation.
• Though the overall flow (primary flow) is rotation, a circulatory flow perpendicular to the rotation is also formed. This flow is called a secondary flow. Tea leaves accumulate at the center because of this secondary flow.

(An explanation was added as Mr. Takuya Matsuda (honorary professor, Kobe University) pointed out that it was misleading.)

[Keywords]	Forced vortex, Secondary flow
[Related items]	Secondary flow inside a basin, Centrifugal force
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs pp. 60-61