predicted outcome of the growing procedure of LY-Janus. Moreover, the cross-sectional SEM image of the LY-Janus membrane reveals distinct morphologies for the upper and the lower parts and their merging in the middle (Figure ), further confirming that LY-Janus is formed by the coalescence of the two different membranes. The cross-section of LY-Janus was further investigated by energy dispersive spectroscopy (EDS). The elemental mapping demonstrates uniform distribution of C, N, and O across the whole cross-section. In contrast, a gradient decline in the content of chlorine was observed from the bottom surface to middle section (Figure ). This could be explained by the fact that the upper part mostly comes from LY-QH, whereas the lower parts is mainly consist of LY-SH, and the middle part is generated by the formation of COF solid solution from copolymerization of Cl-TPA, O-TPA, and TAPB. Since dialdehydes Cl-TPA and O-TPA simultaneously diffused into the aqueous phase in opposite direction, it is reasonable that they will coexist in the middle part, and the closer to the middle between the two interfaces, the more balanced distribution of Cl-TPA and O-TPA. This result also suggests the Janus COF membrane is integrated, with gradual transition of the two distinct COFs. On the other hand, this result also indicates a gradual change in wettability (or polarity) across the membrane thickness, that is, gradually shifting from hydrophilic to hydrophobic (or reverse). Such a hydrophilicity gradient feature was found to play a crucial role in improving performance of membrane distillation. Asymmetric wettability of the Janus COF membrane was demonstrated by water contact angle (WCA) measurements (Figure ). The Janus membrane exhibited a remarkable difference in WCA between its upper surface (0 o ) and lower surface (133.3 o ). In contrast, this phenomenon was not observed for the two-component COF membrane of LY-QH, which showed WCA of 49.9 o and 59.9 o for the upper and lower surfaces, respectively. It should be noted that the water contact angles of the upper and lower surfaces of LY-SH were measured to be 73.7 o and 120.4 o , respectively, indicating a certain degree of asymmetric wettability. This was obtained by serendipity, which could not be designed and predicted because both the upper and lower surfaces of this membrane have the same chemical constitution, especially when compared with the result of LY-QH. The different outcomes of LY-QH and LY-SH again manifests that the conventional two-phase interfacial polymerization method cannot provide precise control over the wettability of the membranes in preparation, and is hard to design and predict asymmetric wettability of a membrane. Moreover, underwater oil contact angle (UOCA) measurements conducted for the upper surface of LY-Janus further corroborate the asymmetric wettability of the Janus membrane, which showed UOCA of 124.5 o ± 0.4 o for five different types of oils (Figure ). WCA under varying pH conditions was further measured for the lower surface. Angles of 132.9 o ± 0.7 o were obtained at 7 different pH values (Figure ). These results clearly demonstrate the Janus feature of the membrane and its universal asymmetric wettability. Next, antifouling capability of the Janus membrane was assessed by ejecting water (or oil) to its surfaces when the membrane was immersed in oil (or water). It was observed that water droplets (dyed with Direct Red 28 for naked-eye observation) readily slid off the hydrophobic surface immersed in dichloromethane, while toluene droplets (dyed with Solvent Blue 104) quickly slid away from the hydrophilic surface immersed in water (Figure and videos S1-2), showcasing an excellent antifouling property. Moreover, the opposite wettability between the two sides of the Janus membrane should contribute to a unique unidirectional water transportation characteristic (Figure ), which was proposed to be a result of response to multiple pressure. To examine it, LY-Janus was affixed in the middle of an H-type cell which contains a small amount of Direct Red 28 dye in the left part and water in the right (Figure ). In contrast to the COF membranes LY-QH and LY-SH for which water could free pass from both the upper and lower sides, LY-Janus exclusively permits water to transport from the hydrophobic surface (lower side) to the hydrophilic surface (upper side), while effectively blocks the reverse direction, demonstrating a behavior similar to the function of a diode. The above processes were also monitored by videos, which clearly showed dramatic difference between the membranes LY-Janus and LY-QH and LY-SH, as well as the unidirectional water transportation (videos S3-8). Furthermore, the Janus membrane exhibited a high durability. It could maintain its performance on the unidirectional water transport over 20 cycles without any deterioration (Figure ). The longevity underscores potential of the membrane for sustained and repeated use during practical applications, thanks to its monolithic structure. In summary, by developing a multiphase interfacial polymerization strategy, we successfully fabricated a three-component Janus COF membrane with opposite wettability on its two surfaces. The membrane is monolithic and free-standing, exhibiting robust antipollution capability. Furthermore, in comparison with the two-component COF membranes constructed via traditional two-phase interfacial polymerization method, this Janus COF membrane showcases a unique unidirectional water transportation characteristic, which efficiently transports water from the hydrophobic surface to the hydrophilic surface and maintains stable performance even after many cycles of testing. The method developed in this study provides a facile and general way for designable, predictable, and controllable fabrication of COF membranes with asymmetric wettability, opening new possibilities for their diverse structures and applications. It can also serve as a valuable reference for the construction of heterogeneous/asymmetric membranes from different types of materials.