3D Capillary Liquid Diode for Selective Liquid Steering and Smart Lubrication

ABSTRACT As a new concept emerged in recent years, a liquid diode is described as a device that allows liquid to flow only in one direction and has broad application potential in microfluidics. Conventional liquid diodes are generally confined to two‐dimensional open surfaces, thus limiting their application scenarios. Inspired by cilia in biology, tilted micropillars were arranged inside a microtube to construct a 3D capillary liquid diode. Geometrical characteristics of straight and curved tilted micropillars were optimized to improve the unidirectional infiltration property. The maximum forward‐to‐backward ratio of infiltration height for the optimized 3D micro‐liquid capillary diode is 5.4. Additional experiments using liquids with different surface tension, viscosity, and chemical complexity further demonstrate that the diode effect is maintained over a broad liquid range. The integration of the optimized structures into branched semi‐microchannels enabled selective liquid steering, where liquid routing was governed by wettability. In addition, these 3D capillary diodes were arranged on a frictional interface for perpendicular lubricant transport to form a sustained oil film, thus resulting in favorable tribological performance with low friction and wear. Long‐term friction tests further confirmed the improved durability and stability of the friction coefficient for the micropillar self‐lubricating structures.