I could happily sit on the floor playing with magnets for a couple of hours. I have never outgrown the delight of feeling invisible forces at work, all the more because magnets almost seem to possess some agency and a playful nature. Like baby monkeys, they snatch things that get too close and the next minute freak out and run away. If you look back a couple of posts at the one called ‘magnets sorting themselves out’ they move like highly trained but bumbling soldiers racing to stand at attention, and in formation. They show what feels like certainty about their destination and they seem to clamber over each other to get there.

The thing about invisible forces is how difficult it is to properly imagine what is taking place. If we never saw air moving through smoke or mist, the wind would be a similar mystery to our imagination. 

The only way to see the shape and force of a magnetic field is similar to seeing the wind act on smoke. We can’t see the force in a pure, abstract way… we need to see it acting upon something. We need a material the magnet will engage with, but not a large, lumpy object like another magnet. We need something like a cloud, made of tiny, reactive particles.  To the right is a familiar image of light iron filings scattered like sand on paper lying atop a horseshoe magnet. A little gentle tapping on the paper and they line up cleanly along the magnetic field lines. If you want a better look, it’s linked to a full-size version.

This is a rather static vision of the effect though. Below is a slightly more dynamic way to see it. Rather than paper, it’s a clear acrylic box with a liquid suspension of iron filings.

It’s better, we can even get a little taste of the three dimensionalities of the effect.  Remember though, we never see a magnetic field acting in a vacuum. Iron filings don’t show the truth of magnetic fields, they show that truth as applied to iron filings.

Ferrofluid

Ferrofluid shows that truth acting on a rather dense liquid. There’s an earlier post on the composition and history of Ferrofluid. Take a look if you need a touch more foundation. Basically, it’s a colloidal suspension of magnetic nanoparticles. Ferrofluid breaks the “fourth wall” of being demurely flat and passive. It’s often in a bowl or other container, open and available to us. The very different physics of a liquid and a powder are instantly apparent. Ferrofluid makes one think of an alien life form or some stylish evil entity. It undulates, it climbs up and down surfaces like sentient oil. It even leaps. It appears to have moods, and rather peculiar ones.

While this view of magnetic fields is not truer than the behavior of iron filings, it’s more dynamic and thrilling. It’s truer perhaps in that way. From the point of view of magnetic energy, peaks and valleys are energetically favorable. In the corrugated configuration, the magnetic field is concentrated in the peaks; since the fluid is more easily magnetized than the air, this lowers the magnetic energy. In consequence, the spikes of fluid ride the field lines out into space until there is a balance of the forces involved.

At the same time, the formation of peaks and valleys is resisted by gravity and surface tension. It requires energy both to move fluid out of the valleys and up into the spikes and to increase the surface area of the fluid. In summary, the formation of the corrugations increases the surface free energy and the gravitational energy of the liquid but reduces the magnetic energy.

Just for fun, ferrofluid mixed with glow-stick liquid:

And finally, with some awesome coloration in the mix.