One-step spraying preparation of omniphobic membrane with anti-wetting behavior for membrane distillation

Membrane distillation (MD) with the application of omniphobic membranes shows much promise for desalinating saline wastewater containing low surface tension contaminants. However, the complicated membrane production steps and the wetting of the membranes have hindered its industrial application. This study reported a simple and highly efficient air spraying strategy to fabricate an omniphobic membrane by spraying a coating solution comprising fluorinated multi-walled carbon nanotubes (F-MWCNTs) onto a commercial polytetrafluoroethylene (PTFE) membrane. The contact angles for water and hexadecane on the surface reached 173.0° and 161.2°, and the water flux remained stable for 12 h when tested in DCMD with low surface tension fluids (SDS and STAC). The omniphobic membrane showed higher wetting resistance in direct membrane distillation (DCMD) experiments than the PTFE membrane when feeding low surface tension feed solutions. During spraying, F-MWCNTs randomly accumulated, and solvent evaporation resulted in micron-level mounds and surface roughness. The presence of air gaps on the membrane surface prevented the surfactant molecules from directly contacting the membrane surface and enabled the membrane to operate generally in the liquid with surfactant for a long time. The membrane was not only resistant to wetting of liquids containing low surface tension but was also easy to prepare and was expected to be used for large-scale desalination of low surface tension wastewater.