Superhydrophobic and self-cleaning membranes, inspired by natural non-wetting surfaces like lotus leaves, are gaining significant attention in wastewater treatment due to their water resistance, high rejection rates, low fouling, and long-term stability. These membranes find applications in oil-water separation and membrane distillation (MD), where water vapor permeates a hydrophobic membrane. Conventional fabrication methods often involve complex surface modifications and the use of nanomaterials or organic solvents, raising concerns about cost, environmental impact, and potential toxicity. Existing techniques, including depositing nanomaterials (e.g., TiO2 nanoparticles), employing delayed nonsolvent induced phase separation (NIPS) or vapor induced phase separation (VIPS), and using electrospinning, often involve drawbacks such as time-consuming processes, high solvent usage, and limited control over surface roughness and contact angle hysteresis. This study proposes a facile and environmentally friendly SANIPS method to overcome these limitations. The SANIPS method manipulates the morphology of semicrystalline polymer membranes by controlling the phase inversion speed and crystallization growth rate during the membrane formation process, eliminating the need for post-treatment and using water as the sole coagulation medium. The study aims to investigate the underlying mechanisms and systematically evaluate the effects of key parameters on membrane properties, demonstrating the efficacy of the fabricated membranes in direct contact membrane distillation (DCMD) tests with high-salinity dye wastewater.

The literature extensively explores the creation of superhydrophobic membranes. Methods reviewed include depositing nanomaterials like TiO2 or carbon nanotubes onto polymer substrates, although this can be time-consuming and chemically intensive. Delayed NIPS or VIPS processes offer alternatives by controlling phase inversion rates, but often rely on significant organic solvent use. Electrospinning produces porous membranes, but often requires additional modifications to achieve superhydrophobicity. Fluorinated materials are commonly used to lower surface energy, but their cost and biodegradability are limiting factors. This work contrasts with these methods by presenting a more sustainable and efficient technique.

The SANIPS method involves casting a PVDF solution onto a glass plate, followed by spraying a chosen material (air, ethanol, or water) onto the nascent membrane using an airbrush. This spraying step is crucial, initiating a rapid phase separation. The membrane is then immersed in water for phase inversion completion and subsequently freeze-dried. A control NIPS membrane (without spraying) was also prepared for comparison. Several characterization techniques were employed: Field Emission Scanning Electron Microscopy (FESEM) to examine surface and cross-sectional morphology; polarized light microscopy (PLM) to monitor phase inversion; atomic force microscopy (AFM) to analyze surface topology and roughness; calculations to determine membrane porosity; Instron tensiometer for mechanical property measurements (tensile stress and Young's modulus); optical contact angle measurement system (OCA) for contact and sliding angle determination; a home-made setup for liquid entry pressure (LEP) measurement; and a lab-scale DCMD setup to assess membrane performance using a feed solution of 10% NaCl and 2000 ppm Rose Bengal dye. The effects of different spraying materials and PVDF concentrations were investigated. The SANIPS method was also applied to PVDF-HFP and PAN polymers to assess its versatility.

The SANIPS membranes exhibited significantly higher water contact angles (>150°) and lower sliding angles compared to the control NIPS membrane (75.8°). The membranes demonstrated excellent repellence towards various aqueous solutions (SDS, Rose Bengal, GO, ethanol). The spraying process, particularly air spraying, led to a highly porous, multilevel rough surface and a macrovoid-free structure, which is superior to the conventional NIPS membrane. Different spraying materials resulted in distinct surface morphologies; air spraying produced a structure with clusters of nanoscale polymer crystals, while ethanol and water spraying yielded coralloidal and volcano-like structures, respectively. SANIPS-W had the highest tensile stress and Young's modulus due to its dense structure and small pore size. SANIPS-E showed the highest LEP (3.58 bar). Short-term DCMD tests showed stable fluxes for SANIPS membranes, whereas the NIPS membrane's flux decreased rapidly. Long-term DCMD tests (100 h) demonstrated stable fluxes (up to 36.0 kg m⁻² h⁻¹) and salt rejections over 99.9% for optimized SANIPS membranes. The SANIPS method proved applicable to other semicrystalline polymers (PVDF-HFP and PAN), enhancing their hydrophobicity or hydrophilicity depending on the polymer's inherent properties.

The SANIPS method successfully addresses the limitations of conventional membrane fabrication techniques. The combination of spraying-induced rapid surface solidification and subsequent delayed demixing in the water bath creates membranes with a unique hierarchical structure and excellent superhydrophobic properties. The tunability of the method, achieved by altering the spraying material, allows for optimization of membrane properties for various applications. The superior performance in DCMD tests underscores the potential of SANIPS membranes for efficient and sustainable water treatment. The ability to extend the SANIPS method to other semicrystalline polymers highlights its general applicability and versatility.

This study demonstrates a facile, scalable, and environmentally friendly SANIPS method to fabricate high-performance superhydrophobic and self-cleaning membranes. The method's versatility extends beyond PVDF, and its tunability offers control over membrane properties. Future research could focus on optimizing spraying parameters (nozzle type, droplet size), developing theoretical correlations between spraying parameters and membrane properties, and exploring applications beyond water treatment. The SANIPS method holds substantial promise for large-scale production of advanced membranes for various applications.

While the SANIPS method demonstrates significant advantages, further investigation is needed to fully optimize the spraying parameters. The current study focused on specific spraying materials and conditions; a broader exploration of various parameters could reveal further improvements. Long-term fouling studies under diverse wastewater compositions are necessary to assess the robustness of the membranes in real-world scenarios. A comprehensive cost-benefit analysis compared to existing methods is also warranted.