Self-assembly of binary rotors*

Rotors are common in various aspects of life - from rotating proteins in a membrane to point vortices in superfluids. They often form distinct structures. Yet, little is known about the behavior of rotors in viscous fluids when the Reynolds number is small but not negligible, and complexities arise due to inertial effects. We experimentally study the nonequilibrium self-assembly of a binary mixture of rotors, in which some rotate clockwise and others counter-clockwise. We engineered small rotating cylindrical motors that float on the surface of a viscous fluid. Critically, our project utilizes mechanical rotors that enable us to control spin direction and eliminate interference from inner forces such as electromagnetic effects. We find that rotors with the same rotation repel and orbit each other, while oppositely spinning rotors attract and propagate together as a bound pair. Starting from a random organization, with half of the motors spinning clockwise and half spinning counter-clockwise, all with the same angular velocity, the rotors rearrange and create long chains driven solely by fluid motion and steric interactions. Our findings indicate that these chains are the stable form of the system.