Capillary ratchets activated by interfacial flows for versatile torque generation and microassembly

Chiral microstructures exhibit distinctive mechanical, electrical and optical properties, but reliable methods to generate unidirectional rotation with precise control remain limited. Previously, Zeng et al. developed "capillary machines," macroscopic machines with hollow channels that braid microscale wires into specific topologies using interfacial capillary forces. Here, we report a versatile ratcheting mechanism that is flow rate dependent. Under high-flow rate conditions, robust unidirectional rotation of the floating object is generated by the interplay between interfacial flows and capillary forces. Simulations reveal that interfacial flows depend on the actuation direction, leading to the observed hysteresis. Leveraging this principle, we successfully braid multiple microwires into a hierarchically twisted bundle without destructive torsion. By coupling interfacial hydrodynamics with geometric design, this approach establishes a scalable strategy for fabricating delicate chiral architectures, opening transformative paradigms in interface-mediated microassembly.