Controlled Self-Assembly of Vortex in Ensembles of Active Magnetic Rollers

Magnetically driven colloids, an example of active matter, are able to demonstrate complex collective behavior and self-organize into coherent dynamic patterns via short- and long-range interactions. Here we present a method for guided self-assembly of ferromagnetic rolling particles energized by a uniform AC magnetic field into a stationary vortex via magnetic interaction with an additional strongly localized magnetic field. By tuning the parameters of the additional field we effectively control vortex dimensions, internal order, and a number of entrapped rollers. We find that vortex self-organization is assisted by field-induced magnetic steering and controlled by a phase shift between alternating magnetic fields. The presented method for assisted self-organization of rolling colloids into a vortex with on-demand characteristics suggests a new paradigm for active matter control and may lead to the development of new approaches for microscopic transport in active particles systems.