Large-Scale Nanofiber Manufacturing| An intelligent nanofiber membrane withdual-asymmetric wettability enablingSpontaneous directional water and oiltransportation in opposite direction

Prof. Zhou Hua & Prof. Niu Haitao's Team at Qingdao University: "Bidirectional Liquid Transport" Intelligent Nanofiber Membrane Achieves Opposite Spontaneous Directional Transportation of Water and Oil

With continuous socioeconomic development, oil-water separation has become essential in many fields, demanding processes with low energy consumption, high efficiency, and high speed. In recent years, fiber membrane materials with special wettability (e.g., superhydrophobic or superoleophilic) have attracted widespread scientific attention and found broad applications.Directional liquid transport textile materials (also called "unidirectional liquid transport", referring to spontaneous unidirectional liquid transport through membrane thickness) represent emerging smart materials with excellent prospects in oil/water collection and directional fluid transport. However, existing unidirectional materials only transport specific liquids unidirectionally (either water or oil), limiting their use in complex mixtures like emulsions. A material capable of simultaneously transporting water and oil in opposite directions ("bidirectional liquid transport") would enable two-way separation for more complex environments while significantly improving separation speed and efficiency. Due to insufficient fundamental research, such bidirectional materials remain unreported worldwide.

Recently, Prof. Zhou Hua and Prof. Niu Haitao's team at Qingdao University made a breakthrough in bidirectional liquid transport smart nanofiber membranes, published in Separation and Purification Technology as "An intelligent nanofiber membrane with dual-asymmetric wettability enabling spontaneous directional water and oil transportation in opposite direction".By innovatively combining electrospinning/electrospraying with in-situ wet chemical modification, the team precisely controlled nanofiber layer wettability to construct two opposite wettability gradients (hydrophobic-to-hydrophilic and oleophobic-to-oleophilic) through membrane thickness, successfully preparing modified polyacrylonitrile (PAN) nanofiber composite membranes with bidirectional water/oil transport capabilities. This breakthrough opens new possibilities for applications in microfluidic devices, liquid manipulation, and smart porous membranes.

Fig.1: Membrane fabrication and fiber morphology

The research team successfully fabricated dDLT nanofiber membranes (Fig. 1) by sequentially preparing hydrophilic-oleophobic polyacrylonitrile (FS-PAN), hydrophilic-oleophilic polyacrylonitrile (PAN), and hydrophobic-oleophilic polyacrylonitrile (SR-PAN) nanofibers using in-situ wet chemical modification combined with electrospinning methods.Scanning electron microscopy (SEM) images revealed that these three nanofiber membranes with different wettabilities (SR-PAN, PAN, FS-PAN) showed no significant differences in surface morphology, differing only in fiber diameter (average diameters: SR-PAN 469 nm, PAN 330 nm, FS-PAN 757 nm). Notably, the middle hydrophilic-oleophilic PAN layer with finer fiber diameter plays a critical role in facilitating oil transport from the FS-PAN side to the SR-PAN side, as well as reverse water transport from the SR-PAN side.Prior to developing the three-layer dDLT FS-PAN/PAN/SR-PAN nanofiber membrane, the team first prepared double-layer PAN/SR-PAN and FS-PAN/PAN nanofiber membranes, which achieved directional water transport (DWT) and directional oil transport (DOT) functions respectively. These provided important foundational knowledge for deeper understanding of dDLT.DWT performance of PAN/SR-PAN composite membrane: Through systematic investigation of the DWT performance (Fig. 2), the team precisely quantified its water transport capacity and efficiency under various conditions, providing data support for constructing more complex bidirectional transport systems.

Fig. 2: Study of DWT performance in FS-PAN/PAN/SR-PAN nanofiber composite membrane

DOT performance of FS-PAN/PAN composite membrane: When studying the DOT performance (Fig. 3), the team comprehensively analyzed various factors affecting oil transport, revealing the internal mechanisms of oil transport in the composite membrane and further advancing understanding of directional liquid transport.

Fig. 3: Study of DOT performance in FS-PAN/PAN/SR-PAN nanofiber composite membrane

As shown in Fig. 4, the final FS-PAN/PAN/SR-PAN composite membrane demonstrated remarkable performance. Water could transport from the SR-PAN side to the FS-PAN side within 10s without oil transport, while oil transported from the FS-PAN side to the SR-PAN side in about 32s without water transport, showing DOT behavior for multiple oils. The liquid transport times were essentially consistent between the dDLT membrane and the single-function (DWT or DOT) double-layer membranes.

Fig. 4: Investigation of dDLT performance in FS-PAN/PAN/SR-PAN nanofiber composite membrane

This nanofiber membrane, integrating dual-asymmetric wettability with bidirectional water/oil transport capability, represents a completely new smart material concept never before reported in previous research. The prepared composite membrane not only exhibits outstanding directional liquid transport performance but also shows excellent application potential in oil-water separation (Fig. 5).When used for oil-water separation, regardless of which side the water/oil mixture is applied to, water moves toward the hydrophilic surface while oil moves toward the oleophilic surface. For both immiscible oil/water mixtures and emulsions (oil-in-water and water-in-oil), the separation efficiency exceeds 99%. Most importantly, the unique three-layer structure with dual-asymmetric oil/water wettability gradients enables simultaneous separation of water (contacting SR-PAN side) and oil (contacting FS-PAN side) from oil/water mixtures, demonstrating great potential for improving separation efficiency.

Fig. 5: Application of FS-PAN/PAN/SR-PAN nanofiber composite membrane for oil-water separation

This research achievement represents a new breakthrough in materials science and separation technology, promising to drive further development and innovation in related fields.