Tokyo team shows nonreciprocal colloid pairs drive dynamic clusters of 10,000-plus particles

A Tokyo University of Science team led by Yutaka Sumino demonstrated nonreciprocal interactions among more than 10,000 colloidal particles for more than 1 hour, phys.org reported, citing Physical Review Letters. Polystyrene particles with radii of 1 and 1.5 micrometers were confined between indium-tin oxide electrodes. An alternating electric field created size-dependent electrohydrodynamic flows so larger particles attracted smaller ones more strongly than the reverse, breaking action-reaction symmetry. Different-size pairs formed self-propelled units even though single particles were not self-propelled. Those pairs assembled into clusters that repeatedly fragmented and reformed rather than growing into static aggregates. Same-size suspensions instead formed static crystalline aggregates. Simulations indicated nonreciprocal pair propulsion is enough to sustain the dynamics. The authors say the controllable platform may inform programmable materials and field-driven microrobotic assemblies.