Unraveling the role of funnel effect vs. gutter effect in water permeance and antifouling performance of polyamide nanofiltration membranes

Conventional thin-film composite (TFC) membranes often face the inherent limitation of the funnel effect. The funnel-like transport pathway, arising from water traveling transversely in the polyamide (PA) film to reach substrate pores, greatly increases the effective transport length and impairs the available water permeance. This curved transport pathway further raises a fundamental challenge for determining PA material's intrinsic water permeability. Herein, for the first time, we present a novel method to calibrate the intrinsic water permeability by accounting for this critical phenomenon. We found that the calibrated value can be several times of the apparent permeability (calculated using PA thickness without calibration). Furthermore, prompt by the disparate literature reports on the effectiveness of interlayers for mitigating funnel effect, we systematically investigated the interplay of several crucial factors (e.g., porosity and PA thickness) on the gutter/funnel effect through both experimental and modelling works. We demonstrated that the gutter effect is far more effective in enhancing water permeance and antifouling performance for membranes suffering from more severe funnel effect. For the thin-PA membrane with a low-porosity substrate, the water permeance was enhanced by approximately 75 % with the interlayer incorporation, while the flux reduction during the fouling test was significantly mitigated (i.e., from 33 % to 13 %). This work provides critical guidance on the future development of high-permeance and anti-fouling TFC membranes.