Surface Charge Regulated Non-Equilibrium Assembly and Open Living Crystals of Chemically Driven Ring-Shaped...

Active colloidal systems, driven by self-propulsion and collective dynamics, offer a non-equilibrium route to soft materials. Realizing such materials hinges on creating novel active systems and deciphering their collective behavior. Here we present a new type of ring-shaped, partially-coated Pt-SiO₂ colloidal micromotor and report our finding that the sign of the surface charge plays a crucial role in determining the propulsion speed and reversing the living crystallization behavior. Our results show that negatively charged ring-shaped micromotors can achieve high propulsion speeds exceeding 20 µm s^{- 1} at 0.5 wt% H₂O₂ solution, whereas positively charged rings move much more slowly. We further find that negatively and positively charged rings always exhibit opposite living crystallization behaviors: if one type can crystallize, the other cannot. We reveal that this opposite crystallization behavior stems from the consistently opposite EO flow directions, irrespective of whether the flow field is symmetric or asymmetric. Living crystallization occurs only when the inward EO flow region exceeds 50% of the ring circumference. By combining experiments and numerical simulations, we demonstrate that the sharp difference in the flow fields between oppositely charged ring-shaped micromotors originates from the different surface charge signs of the rings and the substrate.