  Biological systems often involve the self-assembly of basic components into
complex and function- ing structures. Artificial systems that mimic such
processes can provide a well-controlled setting to explore the principles
involved and also synthesize useful micromachines. Our experiments show that
immotile, but active, components self-assemble into two types of structure that
exhibit the fundamental forms of motility: translation and rotation.
Specifically, micron-scale metallic rods are designed to induce extensile
surface flows in the presence of a chemical fuel; these rods interact with each
other and pair up to form either a swimmer or a rotor. Such pairs can
transition reversibly be- tween these two configurations, leading to kinetics
reminiscent of bacterial run-and-tumble motion.
