Fibrous membranes are widely used in fog water collection, seawater desalination, moisturemanagement textiles, and oilwater separation. However, droplets exhibit distinct wetting and transport behaviors at macroscopic and microscopic scales, and a unified mechanism linking single fibers, local networks, and layered architectures has remained elusive. In a study published in ENG. Chem. Eng., researchers at Nanjing Tech University and collaborators combined in situ visualization with multiphase flow simulations to establish a crossscale framework for understanding and controlling liquid transport in fibrous membranes.
At the singlefiber scale, in situ observations revealed that fiber diameter and curvature regulate droplet nucleation, adhesion, and migration. On hydrophilic PAN fibers, condensed droplets distributed axisymmetrically along the fiber axis. Fine fibers generated higher curvature, lowering the nucleation barrier and producing denser, more uniform droplets. Coarse fibers suppressed early nucleation, promoting droplet coalescence into larger, sparser droplets with more complete spreading. Droplet sphericity increased with droplet volume as surface tension contracted the shape. On hydrophobic PVDF fibers, droplets remained highly spherical and randomly distributed regardless of fiber diameter. Droplet detachment occurred via sliding on hydrophilic fibers and dripping on hydrophobic fibers, with fine fibers exhibiting smaller critical detachment diameters.
At the 2D fiber network scale, interfiber spacing and pore size governed liquidbridge formation and transport pathways. In hydrophilic PAN networks, narrow spacing promoted stable meniscus formation and continuous liquidfilm development, enabling rapid lateral diffusion. Wider spacing prevented stable bridging, causing liquid retraction into discrete droplets and suppressing spreading. COMSOL simulations confirmed that pore size and spacing critically determine capillary continuity. In hydrophobic PVDF networks, droplet growth occurred primarily through coalescence, with minimal spreading.
At the 3D multilayer scale, coupling a wettability gradient with layer stacking drove the transition from surface spreading to throughthickness penetration. In a hydrophobichydrophilic bilayer, droplets on the hydrophobic layer remained spherical until contacting the hydrophilic layer, where the wetting gradient triggered rapid unidirectional transport. As condensation continued, repeated droplet transfers and capillary bridge formation reduced interlayer resistance, enabling rapid migration.
Based on these insights, the team designed Janus membranes with hydrophobic PVDF top layers and hydrophilic PAN bottom layers of varying fiber diameters. As PAN fiber diameter increased, pore size and gas permeability increased, and droplet penetration time decreased dramatically from 14.8 s to 0.4 s. Further optimization of the hydrophilic layer architecture, with coarse fibers at the interface and fine fibers below, achieved the best performance: rapid penetration, high water vapor transmission rate, and fogharvesting rate. Inverted finetocoarse gradients created capillary barriers, reducing performance.
This work provides a general design strategy for constructing fibrous membranes tailored for diverse applications requiring rapid liquid uptake, directional transport, and stable mass transfer.
DOI
10.1007/s11705-026-2683-1