Reports

Self-organization often develops in thermal equilibrium as a conse-quence of entropy and potential interactions. However, there are a grow-ing number of phenomena where order arises in driven, dissipative systems, far from equilibrium. Examples include “random organization” of sheared colloidal suspensions (1) and rods (2), nematic order from giant number fluctuations in vibrated rods (3) and phase separation from self-induced diffusion gradients (4). Biological (5–7) and artificial active particles (8–11), also exhibit swarm patterns that result from their inter-actions (12–15). In order to study active, driven, collective phenomena we created a system of self-propelled particles where the propulsion can be turned on and off with a blue light. This switch provides rapid control of particle propulsion and a convenient means to distinguish non-equilibrium ac-tivity from thermal Brownian motion. Further, the particles are slightly magnetic and can be stabilized and steered by application of a modes