FERRO FLUID

[Dave Chippy]

Has anyone got any ideas on this please
http://www.physicsforums.com/showpost.p ... ostcount=1

[jim_mich]

Imagine a container made of two cilinders communicated with a tube. We fill this container with ferrofluid with the same density than water (for example). Now the gravity and the pressure will be in equilibrium, so the ferrofluid is equally high in both cilinders.

Now, we put a big magnet in one of the sides, so the ferrofluid level in one side goes higher than on the other side.

Now gravity, pressure and magnetic field forces are in equilibrium for the fluid.

Now, introduce a chain of buoys (every buoy weights 1Kg and has 2 Litres of volume).

Then when in ferrofluid, buoys will float, and in air, they will go down by gravity.

For the buoys, the effects of gravity and floatation will be different on one cilinder or another (force direction).

If we have different ferrofluid levels on the cilinders, the buoys will be affected in a way to rotate the chain in a continuous way!

The ferrofluid is affected by 3 fields (gravity, pressure and magnetic forces), but the buoys are only affected by gravity and floatation... so they see an imbalance, and feels different forces on one cilinder or another.

At first look it seems this should work. Usually in this type of device the force needed to push the buoy into the liquid at the higher pressure at the bottom causes the device to balance. But in this case the pressure would be the same at the bottom entrance as the top exit because of the magnets. So any buoyancy is caused by the pressure difference between the liquid pressing on the top of a buoy relative to the liquid pressing on the bottom of the buoy as it rises in the column. Up near the column top near the magnet the pressure difference might be reversed. The buoys might tend to sink instead of rise near the magnet at the top of the column. We should remember that a buoy rises because the pressure on the bottom of a bouy is higher than the pressure on the top. It is this pressure difference that makes the bouy rise and this pressure difference is caused by gravity. If the magnet lifts the ferrofluid then there is may be a reverse pressure difference near the magnet.

So the bottom line is I expect it will balance. Or I could be wrong.

[Fletcher]

Hi Dave .. I'm just guessing here.

As far as I know Ferro Fluids have particles with magnetic attraction in suspension in the medium. This would mean that the suspension itself had to have an average density, say equal to water as is suggested.

When not in the field of a magnet I suspect the density of the fluid is relatively uniform (but would settle over time unless in oil), but when placed near a magnet the fluid separates to a degree which could lead to a greater density near the magnet (top of cylinder) perhaps.

I would think that as with a bar magnet that the material in suspension would attract each other & slowly all the Ferro material would migrate towards the magnet. This would leave a high density area near the magnet & the rest of the fluid with less density than it started with.

As Jim said the buoy has buoyancy because of the relative difference in pressures beneath & above the float. Buoyancy force is equal to the volume of the float displaced in water less the weight of the float construction material used, giving a net upthrust.

This means the float would rise with the same upthrust regardless of depth until it reached a denser level of fluid (density changes usually thought of as temperature or salinity related).

In this case the much denser fluid will be near the top of the cylinder so the float will have to transition between density levels. The greater weight of the Ferro density above the float is partially supported by the magnetic force but the physical density of close packed Ferro materials (with probably less viscosity than the remainder carrying medium) might be a real problem to break thru as it effectively produces a reverse buoyancy gradient IINM.

Think of an air bubble trapped underneath an ice flow perhaps as an extreme example.

Now I could be completely off the mark here but those are my initial thoughts about it, not dissimilar to Jim's.

[rlortie]

We discussed this Ferro fluid topic a few months ago. While doing some browsing at that time, I ran across a recipe for home made ferro fluid.

Grind up ceramic magnets, the finer the better! mix with motor oil (ATF is of a higher detergent and cleaner to work with) Stir frequently.

I would think that the grade or weight of oil would be relevant to the % of suspension obtained.

Ralph

[Fletcher]

The last time around I think it was the idea where a buoy is allowed to enter the bottom of the vessel via a one-way valve (the valve theoretically didn't allow any fluid to leak out). Supposedly it then would rise to the top of the fluid.

Problem was that it could not enter thru the valve because the pressure of the fluid from its depth & density was greater than the air pressure it was coming from beneath the vessel. The reverse gradient.

In this case the magnet is supposed to artificially lift the level of the fluid in one container by the magnetic attraction while the other containers fluid level remains lower by comparison, so that the buoy can fall back down again & use potential energy from gravity ?!

Thanks for the info ralph.

[Vic Hays]

Perhaps if the ferro fluid was enclosed in a circular ring and a magnet affected one side there would be an unbalanced force on an array of floats within thus producing rotation.

If the magnet affected density or pressure this would seem to work. The rotation of the float array would keep the elements of the ferro fluid dispersed.

[Dave Chippy]

Thank you for all the replies they where very informative.