Birds of a feather flock together, but what causes these flocks to form and move in a particular way has always been a fundamental question in nature. Now, however, a method using “living crystals” that behave in near-lifelike ways when activated by blue light, could provide the answer. Self-organization is seen everywhere in nature, from schools of fish to colonies of insects and bacteria. Understanding driven colloidal self-organization and harnessing these particles’ power could provide scientists with a way to create new and enhanced materials — possibilities now largely untapped. New York University physicists have developed a method to prompt microscopic self-propelled colloid particles to move and assemble — much like birds flock and move together in flight — using blue light. Courtesy of Science/AAAS. Now, physicists at New York University have created a form of self-organization in the lab using self-propelled colloid particles that move with the flick of a light switch. When exposed to blue light, the microscopic particles, made of iron oxide mineral cubes encapsulated in a spherical polymer coating, move and assemble much like birds flock and move together in flight. With the light on, the self-propelled random “swimmers” not only collide and cluster, but also trigger a slight chemical reaction that leads the clusters to crystallize and grow until they turn in separate directions and splinter the crystals. These living crystals continually form, swirl and split. When the light is extinguished, the swimmers stop, and the structures dissolve into individual diffusing colloidal particles. Using the slight magnetism of the particles allows direction of the individual swimmers as well as the crystals. With control of light, magnets and chemical attraction, these active particles bring biological organization to the materials world and pave the way for the design of a range of industrial products, including the architecture of electronics, and for the production of novel moving structures. The research appears in Science (doi: 10.1126/science.1230020). For more information, visit: www.nyu.edu