Shouldn't it move towards the outside, due to centrifugal force?
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I found this one fascinating. The ekman pumping doesn't explain it. According to Wikipedia the Ekman pumping is a property of the spherical earth and the imaginary coriolis forces that apply over the large distances of the air stream.
It is a 90 degree shift of the motion of particles compared to the direction of the wind and that 90 degree shift is opposite in the northern and southern hemisphere.
Now I don't claim to know the answer, and perhaps I will be making a fool of myself, but I think the answer given about boundary layers is close.
At the bottom of the cup all the water moves slowly because it is slowed by the surface of the mug.
Higher up the water is moving at a much higher speed so its outward pressure ( centrifugal force) is higher than the outwards pressure at the bottom. ( lower speed lower centrifugal force)
The result is that there will be flow downwards along the outside wall of the mug.
And of course if fluid is moving downwards somewhere then it must be moving upwards somewhere else. i.e the middle of the mug.
Hence there is a flow of fluid. Down the outside, across the bottom to the middle, up the centre, then outwards to the walls.
And as it moves toward the middle at the bottom then the heavier items are dragged to the middle also by the pure friction of the water moving over them. They would flow with the current.
From there they are too heavy to rise much. But it would explain the little pile that is formed. Higher in the middle. i.e some upward force keeping it in a pile rather than letting it form a flat "puddle" in the middle.
Now if we stirred it faster then particles that aren't too heavy should be transported upwards. Perhaps we can see the tea leaves circulate if we are fast enough?
but think about the rest.
The dissolved sugar at the bottom somehow sweetens the whole mug. So the dissolved particles which are light must have been transported upwards.
It is a 90 degree shift of the motion of particles compared to the direction of the wind and that 90 degree shift is opposite in the northern and southern hemisphere.
Now I don't claim to know the answer, and perhaps I will be making a fool of myself, but I think the answer given about boundary layers is close.
At the bottom of the cup all the water moves slowly because it is slowed by the surface of the mug.
Higher up the water is moving at a much higher speed so its outward pressure ( centrifugal force) is higher than the outwards pressure at the bottom. ( lower speed lower centrifugal force)
The result is that there will be flow downwards along the outside wall of the mug.
And of course if fluid is moving downwards somewhere then it must be moving upwards somewhere else. i.e the middle of the mug.
Hence there is a flow of fluid. Down the outside, across the bottom to the middle, up the centre, then outwards to the walls.
And as it moves toward the middle at the bottom then the heavier items are dragged to the middle also by the pure friction of the water moving over them. They would flow with the current.
From there they are too heavy to rise much. But it would explain the little pile that is formed. Higher in the middle. i.e some upward force keeping it in a pile rather than letting it form a flat "puddle" in the middle.
Now if we stirred it faster then particles that aren't too heavy should be transported upwards. Perhaps we can see the tea leaves circulate if we are fast enough?
but think about the rest.
The dissolved sugar at the bottom somehow sweetens the whole mug. So the dissolved particles which are light must have been transported upwards.
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