Interaction Engineering Enables Reconfigurable Flocking in Chemical Micromotors

A flock of chemical micromotors is potentially useful for drug delivery and environmental sensing, but realizing this potential is often hindered by uncontrolled clustering due to phoresis/osmosis. Here, we decouple the propulsion and head-to-head repulsion in chemically powered micromotors, enabling highly ordered flocking. The micromotors are Janus SiO₂ microspheres of 10 µm in diameter with a Ni/PtO bilayer cap, where the PtO layer propels the motor toward its cap in H₂O₂ and the Ni layer generates in-plane magnetic dipolar repulsion between the caps. This head-to-head repulsion creates the alignment critical for flocking. Under a perpendicular magnetic field (B_⊥) of ∼20 mT, micromotors moving at 10-20 µm/s form coherent flocks spanning up to 20 particle diameters, with a flocking order parameter P_local approaching unity. Because propulsion and alignment are independently tunable, the system exhibits reconfigurable transitions among disordered gas, cluster, polar cluster, and flocking states as functions of magnetic field strength, particle density, and propulsion speed, consistent with Brownian dynamics simulations. Weak in-plane magnetic fields B∥ further enable the steering of the flock and reconfigure it as microfluidic valves. This work establishes a framework for engineering emergent collective behaviors in chemically powered micromotors by engineering their pairwise interactions.