Why do particles gather in a bucket's centre?

Phil Sansom made a splash answering this question, with the help of fluid dynamics expert Dan Nickström...

Phil - Jonathan, I don’t blame your dad for not knowing - the thing you describe is actually a phenomenon that confused physicists for ages. They called it the ‘tea leaf paradox’ because they saw it happening when they stirred their tea. Then, about 150 years ago, they managed to solve it. And now, Dan Nickstrom from Maynooth University is here to solve it again for us.

Dan - It's a very interesting phenomenon! It's all got to do with how the water moves after you stop spinning and the fact that it is dragging the particles along with it. If you look at a clear bowl full of water from the side, you'll notice that when you spin it the sides go up really high and the centre goes down low. This is because the centrifugal force from you stirring pushes the water out towards the edge, and it is pushed upwards when it hits the side of the container.

Phil - Centrifugal force is a force that pushes a spinning thing outwards, and its often called a fake force because what’s really going on is the water wants to move in a straight line, and it hits the edge of the bucket. That just makes it look like its being forced outwards, and the result is this whirlpool with high edges.

Dan - When you stop stirring, the interesting part happens! The water at the edge (and the particles it's dragging with it) gets pushed downwards by gravity. The water in the centre in turn goes back upwards. This means the water at the edge flows down to the bottom, inwards to the centre of the bucket, and back up to the top again. This loop repeats over and over as it’s all slowing down. The particles are dragged along like this too! It's all very complicated!

Dan - As it gets slower and slower the force pulling the particles gets weaker and weaker until it's not able to fight against gravity and can't pull them up at the centre. This means they get dropped at the centre! One after the other, until they're almost all there.

Phil - I just did this myself in my kitchen, and Jonathan is 100% right - the little bits of schmutz do all end up in the centre. All thanks to the pattern the water flows in. Thanks Dan Nickstrom. Next time, we’re leaving water behind, and talking about an entirely different liquid, thanks to this question from Charlie...

Charlie - Maybe this is just me, but it dawned on me that whenever I have to hold in a pee, the need to go increases exponentially when I know that relief is close. Why is this?