Credit: Image courtesy of Argonne National Laboratory
Not surprising, the “north” and “south” magnetic poles that we learned about in elementary school provide a way to use alternating magnetic fields to cause magnetic particles in a mixture to roll (inset). Translating this approach to particles whose diameter is about the same as the width of a human hair, and using very well controlled magnetic fields, the “rolling” particles self-assemble into groups with the same rolling direction. In the snapshot on the left, the color indicates the direction that the individual particles are rolling and same-color patches suggest the formation of “flocks,” groups of particles moving coherently, like birds in flight. On the right, the particles have formed a mixing vortex—a funnel shape with the fastest particles in the center—with the color changes indicating differences of particle velocity.