Question

If you stir a cup of leaves with a few loose leaves free to move in it, the leaves
commonly accumulate in the center (axially) of the bottom of the cup.
a. Why, since leaves are more dense than the tea, do they not end up peripherally
located as a result of centrifugal action?
b. How might this effect be put to good use by and organism as part of a feeding
strategy?

Answer 1

The surface of the tea is concave with the depth lower in the center in both cases. Thus the pressure in the center cannot be higher than at the edge. The tea leaves drown, so they are heavier than water and should not move to where pressure is lower.

When you are stirring the tea in your cup you are trying to give rotational motion to the fluid layers but the layers in contact with the cup surfaces remain at zero speeds. The velocity gradient radially (as the viscous drag between the layers reduces the speed) picks up the leaves but it can not maintain the circular path in absence of required centripetal acceleration and slowly collects at the centre.

For moving along with the fluid they need necessary centripetal force and that is not being provided by the layers of fluid.

b)

The solvent clearly mixes with the solute and you can have a good fixture of the compound.