The black fluid looks like common oil or ink. But suddenly, it leaps from a shallow pool to form a phalanx of rotating spikes. It's somehow soft and sharp at the same time. What on Earth is going on here?
This is a ferrofluid: a liquid that becomes strongly magnetized in the presence of a magnetic field. Ferrofluid is a portmanteau of ferromagnetic—the mechanism that draws certain materials to magnets—and fluid (for obvious reasons). A ferrofluid contains nanoscale particles like magnetite, hematite, or another compound containing iron—all of which are attracted to magnets. The particles have to be small enough to randomly distribute throughout the fluid.
A key feature of a ferrofluid is that it's a colloidal suspension. That means the insoluble particles are suspended in the fluid, so it has two states of matter in one solution. (A more common colloidal is milk, which is butterfat globules suspended in a water solution.)
That dual state is important, because a ferrofluid acts like any other liquid until the particles get near a magnet. Then they force the fluid to behave in ways that are stunning to watch:
Those peaks and valleys reflect the magnetic field, as well as the effects of surface tension and gravity.
The ferrofluid was invented by NASA scientist Steve Papell in 1963. He had hoped to transform rocket fuel into a ferrofluid so it could be magnetically drawn toward a pump inlet in a weightless environment, imposing a sort of artificial gravity. That didn't come to fruition, but today ferrofluids are used in a wide variety of applications, including electronics, engineering, medicine, and art. You can even make your own ferrofluid at home! Mix powdered iron fillings into corn oil, grab a magnet, and let the fun begin. Pro-tip: don't get the ferrofluid too close to the magnet or it will leap out of the container—and splatter everywhere.