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

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.