Phase Transitions and Ferrofluid

When I was younger, I came across a tutorial that described a simple way to make thermite entirely from homemade ingredients. The crux of the instructions was that iron oxide, the key ingredient in thermite from which molten iron is created, can be isolated from common sand simply by repeatedly dragging a magnet through a container full of it. At the time, my family happened to live near a beach, and so I resolved to gather as much iron oxide as possible and test out the recipe.

In order to collect the iron oxide, I would¬†drag a bag full of magnets behind me every time I went to the beach. After about two weeks of regular collection missions, I obtained enough iron oxide in the form of magnetite (a black, crystalline solid) that I was able to successfully synthesize thermite, using a recipe I’ve described in previous post.

I eventually moved on to using purified, store-bought reagents for safer reactions, but I still had a large amount of magnetite leftover. Eventually another use of it occurred to me when I read this tutorial, which outlines the unusual properties of a ferrofluid, or magnetic liquid. Ferrofluids consist of ordinary solvents, like gasoline or acetone, that have been mixed with a high concentration of nanoscopic iron particles. The tiny bits of iron essentially act as bar magnets, and so they align in unison with an applied magnetic field just as the magnetic needles of a collection of compasses would. But because the iron particles are so small, Brownian motion (the “mixing” that constantly occurs in liquids due to the chaotic thermal motion of their constituent particles) keeps them suspended within the fluid. As a result, the liquid can shift from behaving like the solvent in the absence of the applied field to behaving as a solid when a magnet is brought near the liquid.

I managed to make a very rough ferrofluid by finely grinding up my leftover magnetite and then using the recipe found on this website. The store-bought ferrofluid used in the video at the top of this post was made using precise industrial methods, and so it naturally behaves in a much more elegant manner because the iron particles inside it are much more uniform. But my ferrofluid still exhibits two key behaviors of ferrofluids: it solidifies in response to an applied field, and it tends to form small, clumped structures rather than a single lump:

The erratic behavior of the ferrofluid can be seen as a simple type of phase transition, in which a system subjected to a smoothly varying stimulus (the proximity of the magnet to the fluid) undergoes a discontinuous change in behavior (the sudden appearance of peaks and lumps in the fluid). Phase transitions are crucial in biological systems in which many autonomous parts (like blood cells or the individual members of a school of fish) must behave as a collective entity for mutual benefits. In the ferrofluid example, the mutual benefit is energetic efficiency—the fluid tends to arrange itself in such a way as to minimize its internal energy. The individual particles of iron are initially independent and diffuse freely through the solvent, but when the external field is applied it becomes energetically favorable for the particles of iron to align and congeal into collective aggregates. In the first video, when the magnet is far from the fluid, large lumps tend to form that have well-defined peaks and arrangements. But when the magnet is brought much closer, these peaks tend to disassemble into many small, hairlike filaments because such structures contain less internal energy. The reason for this behavior is that the energy of the fluid is mostly stored in its surface tension–the collective attractions between different magnetized iron particles on the surface of the liquid hold energy in the same manner as distended springs. When the magnet is brought much closer to the liquid, it becomes necessary for the system to offset the excess energy by further increasing its surface area, resulting in a greater number of small structures.