Gradient-driven sandwich-like nanofibrous membrane for high-flux antifouling emulsion separation

Membrane separation for nanoemulsion treatment offers advantages including superior effluent quality and low resource consumption. However, it faces challenges of low flux and membrane fouling. To address these, we fabricated a sandwich-like spindle-knot nanofibrous membrane (SSK-NM) to enhance water permeation and mitigate fouling. The SSK-NM features a thin fiber layer as the skin, a spindle-knot layer in the middle, and a thick fiber layer as the base. Fabricated via controlled spinning solution concentration and deposition sequence, the SSK-NM incorporates three gradients including hydrophilic wettability (water contact angle from 38.6° to 21.3°), inverse pore size (from 3.1μm to 0.604μm), and curvature. For oil-in-water emulsions, a 6μm thick skin layer dramatically boosted flux by up to 153%, achieving a 39,912 LMH/bar with >99% oil rejection. Concurrently, this skin layer shifted dominant fouling model away from cake filtration, reducing fouling indices by 41.9%. Rapid vertical water droplet penetration, evidenced by a 45% increase in penetration rate compared to a skin layer-free membrane, was driven by a substantial positive Laplace pressure difference (∼369.53 kPa) generated through the combined pore size and wettability gradients. Crucially, the 6μm skin layer maintained optimal hydrophilicity without significant loss of anti-fouling, shown by the maximum contrast in water adhesion force and oil adhesion force (ΔF=96.2 μN). Thinner layers were insufficient to drive droplets, while thicker layers formed dense surfaces prone to pore blocking. The SSK-NM exhibits exceptional separation efficiency and universality across diverse emulsions and industrial oily wastewater, offering a streamlined, single-material strategy for designing high-flux, antifouling membranes.