I attempted to make my own ferrofluid. I failed. First I bought some printer toner but it was not magnetic enough. So then I bought more toner, this time magnetized for check printing (MICR toner). I mixed with vegetable oil at different thicknesses. I made a magnetic suspension fluid but it did not form cool shapes quite like I expected. I'm not sure what the problem was, but I made a HUGE mess and spent a week trying to recover my beaker and magnets, etc. I decided that I would break my personal rule and buy the stuff ready-made. I will not share my formulations as they all failed to impress me and made a massive mess. If you have never dealt with printer toner, I don't recommend you start now. It is ink, and simply opening a bottle of it causes and ink cloud to form and settle on everything around it. Someone once joked that if you sneeze, you may have to buy a new house.
What is ferrofluid? Well, as NASA envisioned it, it is a liquid that can be controlled by magnetic fields. So it has properties of two phases, liquid and solid. In order to make ferrofluid, you need very small (10nm or so) particles of magnetite (magnetic iron oxide), polarized in a surfactant (like soap) and suspended in a carrier liquid. The carrier and surfactant can be one substance in the case of thin oils like mine, but NASA's formulation was hopefully more sophisticated.
Since I skimped on cheap toner, maybe my particles were too big. 10nm is really small. Again, I'm not really sure. I know from a job I had once that growing consistently small crystals on that scale is quite hard. You have to stop growth before the crystals are even visible.
Here is a picture of my new purchased fluid above a medium sized rare earth magnet:
It is a big hard to see black against black, so here is a view of the same picture where I edited the light and contrast:
Note the characteristic spikes arranged in a beautiful 360-degree flower shape. The shape corresponds perfectly with the magnetic field, as expertly drawn by me below. Note the field follows this pattern in 3D, not just to the right and left of the magnet as in my drawing.
And here is another picture with a smaller and therefore weaker magnet:
Note the field is weaker, so it can sustain fewer spikes and they are tending to bend down according to gravity and the smaller shape of the magnetic field.
You may be wondering why the spikes form so precisely. I did, so I looked it up. The particles are so small as to move about and bounce off each other randomly in the same way molecules do in liquid. This is known as Brownian motion. Thus the liquid can arrange itself perfectly with force fields according to probability's directions. As we showed in the magnetic field diagram, the spikes correspond to these magnetic field lines in 3D. However, they are limited by gravity and surface tension. The tips of the spikes are the point when the opposing forces are equal. The spikes grow longer with a stronger magnetic field. The direction of gravity can affect them. The largest and sharpest spikes are formed when surface tension of the liquid is minimized.
If you wish to try something similar and not buy the stuff or figure out how to make it, a quick and easy thing to do is to dump some iron filings on paper or glass above a rare-earth magnet. Don't let the filings get on the magnet! The much-bigger filings align with the magnetic field in a similar way, generally showing the magnetic field shape. They cannot reposition themselves ideally because they are not small enough, not suspended in a fluid, and make physical connections with each other (like little magnets stacking themselves upon each other).
One last thing. What is magnetism? I am oversimplifying this because it is kind of complicated and honestly I don't fully understand it yet. For our purposes here, some materials have unpaired electrons. Iron is one such substance. Another is Magnetite (due to a combination of both Fe(ii) and Fe(iii) cations in its molecular makeup). Paired electrons cancel out a magnetic property known as spin. Unpaired electrons retain this property and are attracted to electromagnetic fields. While electromagnetic fields are usually the realm of physics, I guess my point here is it all comes from the chemistry of the substance.
Thanks for reading,
Paul
p.s. I kind of butchered some of the science for simplicity. In reality, what I am calling magnetic is paramagnetic (other than the magnets themselves). Paramagnetic material like iron filings and this ferrofluid can be attracted to magnetic fields spontaneously. Paramagnetic material has one or more unpaired electrons. Paired electrons oppose each other's "spin," and therefore magnetic properties. This means the bonding of molecules and even between molecules in crystal lattices makes a difference in whether electrons are paired, and therefore, whether they are paramagnetic. O2 is paramagnetic but N2 is not. Without getting into Molecular Orbital Theory (MOT), it is easiest to determine the elements and ions. Iron has four unpaired 3d-shell electrons (according the Hund's law). Iron +2 or (or Fe(II)) also has four. Iron +3 has 3 unpaired electrons. In a compound, note iron oxides hematite (diagmagnetic) and magnetite (strongly paramagnetic) are determined by MOT and not just their metal ions by themselves. A common question is why copper, silver, and gold are not paramagnetic. All three line up in group 11, meaning they have one unpaired electron, but the large size of the atoms means the many filled shells' diamagnetic properties overcome the attraction of the one unpaired electron. I believe MOT is beyond AP-level Chemistry.
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