After stirring the tea, a small pile of tea leaves forms in the middle of the cup. Albert Einstein already dealt with this teacup effect – today, it is even used technically. Why do tea leaves collect in the middle of the cup?

Anyone who likes to drink tea has probably observed that tea leaves collect in the middle of the cup many times. After stirring, tea leaves collect at the bottom of the cup and not around the edges. In order to understand this phenomenon known as the TEACUP EFFECT, we have to take a closer look at the working and balance of power in the cup.

The stirring initially causes the tea to start a horizontal rotation. This so-called primary circulation gives the surface of the liquid a dent. The water level at the edge is higher than in the middle of the cup. This form results from the interaction of two forces. The first force is the outward centrifugal force caused by the inertia of matter. The second force is the downward pressure force caused by the gravity of the liquid column.

Why Do Tea Leaves Collect In The Middle Of The Cup?
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The molecules of the tea are slowed down by friction at the edge and bottom of the cup. This reduces the centrifugal force at these points. The pressure force, however, remains unchanged. As a result, the forces, both the compressive force and the centrifugal force, are no longer balanced there. To restore the balance between the forces in the cup, the movement of water in the cup changes. The so-called secondary circulation overlays the primary circulation caused by the stirring. The tea sinks to the edge of the cup, then flows to the center of the cup base, and from there, it flows up again. The tea then flows outwards on the surface and dives down again at the edge of the cup.

This secondary circulation continues until the tea has come to rest and has a smooth surface again. Until then, the secondary circulation will transport tea leaves that have sunk to the bottom into the center of the cup. Since the specific density of the leaves is slightly higher than that of water, they do not usually move up together with the circulating tea. Instead, the tea leaves stay in place and form the characteristic little heap at the bottom of the cup.

Albert Einstein even took up this phenomenon in one of his papers. In a paper published in 1926, he illustrated the physical processes through which river loops are formed using the teacup effect. In fact, similar effects occur in many areas. For example, in our atmosphere, the air rises in a low-pressure area and decreases in a high-pressure area. This is because the friction of the air on the ground triggers a secondary circulation.

The teacup effect is also suitable for technical applications. In beer brewing, for example, the unwanted suspended matter can be removed more easily if the disruptive particles – just like in the teacup – collect in the middle of a container. And in medical technology, the teacup effect is used to separate blood plasma from red blood cells and other microscopic particles.

Who would have thought that a common yet unnoticed thing like Why do tea leaves collect in the middle may have such incredible applications. It is true that if Einstein had not discussed it in one of his papers, it would not have been noticed. That is how the brains of the most intelligent people work.

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