Microscopic particles self-organize into a rolling mob

Denis Bartolo, Antoine Bricard and Nicolas Desreumaux

One of the nearly infinite number of astounding sights in the animal kingdom plays out when creatures get together in large groups. Whether it’s a flock of birds or a school of fish, the group can take on an identity of its own, moving in seemingly perfect coordination. People have long puzzled over how this wonderful choreography works.

The flocking pattern is not directed by some controlling leader; it emerges from the behavior of individuals. Researchers have found that they can simulate virtual swarms by making individuals follow a few simple rules, such as keeping an equal distance from one’s neighbors. This research is no longer limited to the digital realm, though.

Antoine Bricard, Jean-Baptiste Caussin, and three of their colleagues have come up with a unique setup to build on previous studies. They used electrical insulators to create tiny spheres just 5 microns across (less than the width of a human hair) and placed them in a conductive fluid sandwiched between two microscope slides. Applying an electric field results in something called “Quincke rotation” in which uneven, fluctuating charges on the surface of the sphere interact with the field to make it spin. Turn on the field, and all these little particles start rolling around in random directions.