Tea leaf paradox

He correctly connected the appearance of secondary flow (both Earth atmosphere and tea cup) with "friction on the bottom".

[2] The formation of secondary flows in an annular channel was theoretically treated by Joseph Valentin Boussinesq as early as in 1868.

[1] The solution first came from Albert Einstein in a 1926 paper in which he explained the erosion of river banks and repudiated Baer's law.

[4][5] The stirring makes the water spin in the cup, causing a centrifugal force outwards.

[5] The phenomenon has been used to develop a new technique to separate red blood cells from blood plasma,[6][7] to understand atmospheric pressure systems,[8] and in the process of brewing beer to separate out coagulated trub in the whirlpool.

The tea leaves collect in the middle and the bottom, instead of along the rim.
The blue line is the secondary flow that pushes the tea leaves to the middle of the bottom.
Visualization of secondary flow in river bend model (A. Ya. Milovich, 1913, [ 1 ] flow from right to left). Near-bottom streamlines are marked with dye injected by a pipette.