Ferrofluids: do they produce a force?

[amonvi]

Hi, everybody,

I am doing a thesis related to fluids and I have thought about smart fluids (magnetorheological and electrorheological fluids) and ferrofluids. I've read a lot about and I know well their special characteristics. However there's something I can find nowhere: do they produce force by moving?

I mean. I know nowadays dampers formed with magnetorheological fluids are being strongly developed, and of course they produce an important damping force by altering their viscosity. And I have also read, in case of ferrofluids, that the flow of a ferrofluid can be achieved at a certain velocities just by changing the field applied. But, do this movement produce force? In the case that a fluid accelerated by changing fields in an specific ferrofluid devise would be guided into a conduct with a piston, would this piston be displaced?

This information is something I don't get to find and it's really relevant to my thesis. Thank you everybody for your help.

 

[Danger]

They have to exert a force just by moving. Even a wee ripple on the surface has to displace air and fight gravity as it travels along, which is work. You probably have a much better idea than me as to how much work can be extracted from the stuff, but it must bear some relationship to the amount of electricity or magnetic flux applied to it. It might not be linear, though.

 

[Andy Resnick]

I thought this was a dumb question until I thought a little more. What tripped me up was the sentence "...the flow of a ferrofluid can be achieved at a certain velocities just by changing the field applied."

If the fluid is given energy by the field, then the field must lose energy. My understanding is that magnetorheological fluids only change their rheological properties as a function of the field- they do not achieve steady-state flow. I'm picturing something like this:

http://chemistry.about.com/od/demonstrationsexperiments/ss/liquidmagnet.htm

Here, someone has played around with a time-varying field:

http://hackedgadgets.com/2007/04/19/ferrofluid-morpho-towers-two-standing-spirals/

Looks like any induced flow is unsteady, but no details were given...

 

[pinestone]

...My understanding is that magnetorheological fluids only change their rheological properties as a function of the field- they do not achieve steady-state flow...

Ferrofluids behave differently than magnetorheological fluids.
Ferrofluids are Superparamagnetic: http://www.funmat.uio.no/Uorganisk/research_topics/nanotubes_c.html
From Wiki:
"The particles in a ferrofluids primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications".
http://en.wikipedia.org/wiki/Ferrofluid

See if your library has a copy of R E Rosensweigs "Ferrohydrodynmics": https://www.amazon.com/dp/0486678342/?tag=pfamazon01-20