Swarms of bacteria and the cytoskeletons of living cells move collectively through an environment by maintaining a nonequilibrium state in which they gain energy from an external source (see the article by Robert Evans, Daan Frenkel, and Marjolein Dijkstra, Physics Today, February 2019, page 38). Researchers have begun to develop synthetic counterparts to mimic that active matter. One approach uses magnetic colloids—microscale particles that rotate and move when a local magnetic torque is applied by an external magnetic field. Usually suspensions of the colloids form vortices only near their boundaries. But now Alexey Snezhko of Argonne National Laboratory and his colleagues have observed dynamic vortices far from the boundaries, which suggests that researchers could manipulate vortices over large scales.
Snezhko and his colleagues put about 10 000 ferromagnetic nickel spheres with diameters of 125–150 µm in a flat, water-filled petri dish and applied a uniaxial oscillating magnetic field...
