Fun with Ferrofluid

After seeing some cool ferrofluid videos, ( especially this one and this one), and chatting to Pete Mühlenkamp at dorkbotlondon who was using FF in an art project for a college degree show, I decided to dig out the bottle of the stuff I bought from Ebay a couple of years ago & never got round to playing with seriousy.

Pete said that the stuff he was using was particularly developed for long life and good spike growth, and he'd had to buy a fairly large quantity for his project as the maker, Ferrotec, wouldn't deal in small quantities.  At the end of his Dorkbot presentation he commented that he'd be selling some of the excess to recover his costs - I was first in the queue, but was disappointed that I would have to wait until his project had finished. I was aware that there are various grades of FF available, and I had seen that the stuff I had was rather prone to evaporating after being exposed to air for few days, however I was feeling in an experimenting mood so decided to see what could be done with it...

Some initial experiments showed that a major problem with doing anything with ferrofluid inside a transparent container is that the fluid leaves a trail on the surface, spoiling the view. I had also been wondering how oil-based ferrofluid behaved when mixed with water - imagining a sort of magnetic lava-lamp type of thing.
Initial experiments were done with a plastic tank ( Ferrero Rocher box), filling with water and dripping a few drops of FF into it.
Initial observations were that the presence of the water did significantly reduce the tendency to stain the bottom and sides of the container, and the fluid took a more blobby shape when not in the presence of a magnetic field. When a magnet was held near the tank, the traditional spike effect tended to appear broken into several seperate spikes, as opposed to the traditional 'hedgehog' look of ferrofluid in air.

The first problem was that dropping FF into water instantly created a nasty brown   oily film on the surface. This can be removed by picking some off with a magnet and carefully dragging paper  over the surface to remove the residual film, however I found a more effective way was to start with fairly shallow water, have a strong magnet under the bottom, and drip the fluid in over the magnet - the fluid then tended to shoot straight for the magnet without leaving a surface film. Mostly.

Now that I could make nice blobs of FF in the tank, and move them around and make interesting shapes with a magnet, I started thinking about the sticking/staining issue some more. I figured that a smooth surface would be less prone to sticking, so glass would probably be better than plastic for the container. I also had a vague recollection of reading something about using borosilicate glass with ferrofluid, and also a mention of surface issues in Big Clive's Lava Lamp project page

As I didn't have a suitable glass container, I just cut a rectangle from the window of an old scanner, cleaned it throughly with meths, and placed it in the bottom of the plastic tank I'd been using before.

This worked really well - the blobs would move around the bottom nicely without staining. If blobs were left in the same place for tens of minures, some staining would occur, and this was aggravated if a powerful magnets was used close to the bottom, presumably as this was forcing the fluid against the glass. However even these stains were at least initially in the form of small dots, and could sometimes be removed by dragging a blob of fluid over them.  
Fluid blobs left in place for extended periods ( several hours) did start to make larger brown stains. It may be possible to prime the glass somehow to prevent this altogether, but my chemistry knowledge isn't up to this - I was just happy that I had found a solution that was good enough to do some serious playing.

One of the videos that had particularly impressed me was this one from MIT, which combined a rotating axial magnetic field with a vertical one to create some amazing spontaneous patterns. This used FF contained in a thin space in a glass shell.

Now I'd cracked the staining issue, I decided to see what effects I could produce by containing the FF into a thin layer like this. I simply placed a bent piece of tinned-copper wire around the FF blob on the first glass sheet in the bottom of the tank to act as a spacer, and droppped another piece of glass on top. 
Seeing how this behaved made me realise that the effects shown in the MIT video were basically the result of squashing the normal FF spikes sideways into threads, and as they had the same magnetic orienation, the threads repelled each other, creating regular-ish spacings as the fluid spread out as much as it could within the constraints of the magnetic field.

Moving a 12.5mm cylindrical Neodymium magnet by hand from below.

Similar setup as above, but with white paper under tank to increase contrast.

This sequence used a larger flat magnet of the type found in hard disk drives. Here you can see that as the threads get spread out, they revert to the spike type formation when there is no longer enough fluid to make continuous threads.

 The MIT video appeared to use what looked like the stator of an induction motor to create the rotating axial field. AFter having a look through my pile of assorted motors and mechanical things, I failed to find anything suitable. I thought about maybe usuing a stepper motor, but couldn't really be bothered to build up something to control it.
Then I thought - " If you want a rotating field, why not just rotate a permanent magnet..?"
So I grabbed a DC motor which had a nice smooth action to allow easy speed control using a variable bench PSU, and stuck a long neodymium magnet on the end with Blu-Tak. This was then held under the tank, below the ferrofluid blob.

The static horizontal magnet below elongated the blob into a sort of slug-shape, which rotated with the magnet, but as speed was increased, it returned to a circular blob as its viscosity prevented it from following the magnet.

One problem with this setup is that to get effects similar to the MIT setup, the vertical field had to be applied by holding a second magnet from above. This is obviously a lot less stable and predictable, plus you get some wobble from the attraction of the rotating magnet below. The fact that the rotating magnet was only secured by Blu-Tak also meant it had a habit of being moved by the other magnet, or flying off and sticking to something...!

Another problem was that the overhead magnet made it hard to get a good view and lighting for recording, not helped by the used of an extremely cheap webcam..

However despite these limitations, some features can be seen that are similar to those seen on the MIT video. Considering this was all done in a few hours, I think the results look promising, and further investigation should be worthwhile as & when I get time..

Note that there is some wobble visible in these sequences due to the strobing effect of the camera and rotation speed of the magnet.

I think a major limitation is having the (relatively short) magnet below the fluid, and I think it would be probably better with rotating magnets either side, in the same plane as the fluid cell, so the fluid is being pulled evenly and  horizontally, instead of the rather non-uniform field strength and angle from the short magnet below.

The sketch below shows how I think it could work better - using a cylindrical vessel with 2 glass discs, and a C shaped rotor to place the two rotating magnets in the same plane as the ferrofluid.

The vertical magnet could in principle be above or below - below makes viewing easier but makes it harder to adjust, however some adjustment could be done by moving the whole container vertically relative to the rotor assembly.

One surprising thing I did notice that was above a critcal rotational speed where the viscosity of the fluid was enough to keep it as a round blob, the rotation speed made almost no difference at all to shape of the patterns produced. This means that if using an induction-motor type coil assembly instead of a rotating magnet, the drive frequency shoudn't be too critical, and it may even be possible to use mains frequency, and just a variac to control the amplitude.
If it turns out to need a higher frequency, a sinewave signal generator driving an audio power amplifer  (possibly followed by a step-up transformer) should do the job. I have no idea of how powerful an amplifier would be needed though - this would depend significantly on the characteristics of the coils. However I do have a 1000W amp available if need be.....!  

Another avenue I feel would be worth exploring is the use of two perpendicular electromagnets, driven from two sinewaves with independently adjustable frequency and phase, to see what effect this has on the shapes produced.

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