I googled this question, and all I got was stuff about defeating the degeneration of the body in weightlessness, so I'm hoping I can get some insight from people who know about geology.

What happens to a fluid in a centrifuge in zero g?

If you had a sphere or a cylinder that was half full of a fluid in a weightless environment, and you were to spin that fluid in the container, what would happen?

In a cylinder, would the fluid force itself to the sides of the container under centrifugal force, leaving a hollow center? What about in a sphere? In a sphere, would there be air at the poles of it, as well as air in the center?

I'm wondering because I was reading one of those hollow earth threads, and I was thinking, if the fluid magma in the core of the earth was moving at a sufficient rate, wouldn't it concentrate itself to the outside of the sphere, possibly leaving a hollow center, and possibly hollow poles, assuming the earth was not completely full of magma?

To add: I understand what I've learned thus far about the composition of the center of the Earth, with a solid iron core, and that has never really made sense to me because it seems that the closer you got to the center of the earth, the less effect of gravity would have, leaving the center of the earth essentially a zero g condition, with all of the mass of the earth pulling outward in gravitation equally in all directions. Am I missing the mark there?

As soon as you started spinning a mass in a centrifuge, you'd be creating 'artificial' gravity in respect to that mass and that is the principle under consideration for long term human space voyages IE spin a cylinder at the correct speed in relation to its diameter and everything inside experiences a force like gravity as they're pushed against the outer wall. In the centre they'd experience only the ambient external gravity which would be weightlessness in a zero gravity environment.

True laboratory centrifuges can generate huge gravitational effects, sufficient to virtually separate blood cells from plasma by pushing them all to the 'bottom' of their container for example.

I agree that there would be a point at the centre of mass where all directional gravitational directional forces would be in balance producing an effective zero-g point but that zone would have incalculable tons of mass crushing down on it from all directions so there'd be nothing 'floating' there. Earth's rotational speed isn't sufficient to produce any really noticable centrifugal effects even on the surface EG something like less than 0.3% loss of measured weight for an object on the equator.