Ten Thousand Colloids Temporarily Break Newton’s Third Law in an Electric Field

Japanese researchers suspended thousands of colloidal particles in water between microfabricated electrodes and applied an alternating electric field, which caused mixed-size particles to form self-propelled pairs and chase one another, effectively breaking action–reaction symmetry for about an hour. When all particles were the same size, interactions remained reciprocal and the system crystallized; with mixed sizes, particles clustered and then split repeatedly. The finding could inform programmable materials and microrobotics.