[EvaH (OP)]

Jonathan recently got in touch with this excellent question:

"When you stir a bucket of water I know the water is pushed to the outside, however why do any particles end up in the centre after the water has finished spinning?"

We'll be answering the question on this week's show, but what do you think?

[Kryptid]

I'm going to go with a tornado analogy and propose that it's due to there being less pressure in the center. It's just a guess, I'll admit.

[evan_au]

why do any particles end up in the center after the water has finished spinning?

Guess #1: On the surface, this sounds like the operation of a "cyclone separator", which in the past was found in industrial settings, but has now found its way into many homes in the form of a bagless vacuum cleaner.
- These separate dust from a rotating vortex of air
- But they also work with particles suspended in liquids
- In this case, the particles are denser than the fluid

The theory of this seems to be that the particles of dust have too much inertia to follow the tight turns of the air, and strike the outside of the cylinder. Unfortunately, this is the opposite of what the OP describes, where the particles end up in the center.
See: https://en.wikipedia.org/wiki/Cyclonic_separation

Guess #2:If the particles are less dense than the liquid, they will tend to float on the surface.
If there is a vortex, most of the surface area will be near the center.
When the water stops spinning, most of the floating particles will remain near the center.

[Bored chemist]

It's sufficiently complicated that Einstein wondered about it.
Fortunately, he was bright enough to solve it.
https://en.wikipedia.org/wiki/Tea_leaf_paradox

[Halc]

I experimented myself with a bucket half full of rain water and assorted dense tree particles at the bottom.  I stirred it quickly and almost immediately the dense stuff at the bottom forms a sort of tornado in the center rather than being flung to the edges as one would intuitively expect of particles more dense than the water.  Einstein did indeed explain the phenomenon.

It seems that friction with the bottom causes the lowest layers of water to spin slower than the water further up. This creates the most outward pressure at the top where it spins the fastest, and results in a current that goes out at the top, down the sides, in at the bottom where it is slowest, and up in the center to complete the circuit of water motion.
The particles at the bottom are carried by this convecting current to the center as the force of the current is stronger than the pressure exerted by the dense particles in the slowly moving water.  As the current slows, the dense particles settle in the center where they've been dragged.

[nudephil]

...and answered on the show! By fluid dynamicist Dan Nickstrom, and at a simple level mind. https://www.thenakedscientists.com/podcasts/question-week/why-do-particles-gather-buckets-centre

[science12]

I would say it's because water always moves a little bit, but not enough that we can see it, due to the movement of the earth but wants to stay at the same place. So it starts cicling and because of centrifugal force it goes to the middle.

[mikahawkins]

I think this has less to do with the properties of water as a liquid and more with physics. When we are moving a bucket of water in a circular way, the water inside will be facing a force which tends to make it move towards the center of the bottom of the bucket.