Engineering beads-on-string structural electrospun nanofiber Janus membrane with multi-level roughness for membrane distillation

A novel Janus membrane was fabricated by constructing a thin hydrophobic layer with beads-on-string structure on the surface of superhydrophilic polyacrylonitrile (PAN) electrospun nanofiber (ENs) substrate through hybrid electrospinning and electrospraying techniques. The electrosprayed polystyrene/poly(dimethylsiloxane) (PS/PDMS) beads were interlinked by nanofibers and further anchored by 3-D network formed by electrospinning polyvinylidene fluoride-co-hexafluoropropylene (PH). Surface roughness and morphology of formed beads and relevant hydrophobic layers could be highly affected by weight ratios and concentrations of PS and PDMS. The hydrophobic layer (PH-7PS/7PDMS) prepared by electrospraying PS/PDMS (7 wt%/7 wt%) mixed solution showed the highest water contact angle (WCA) value of 148.5o and the largest surface roughness of 812 nm. The optimized Janus membrane could exhibit a water flux of 27.7 L/m2h and a salt rejection efficiency close to 100 % at the temperature difference of 40 °C when 3.5 wt% sodium chloride (NaCl) was used as the feed solution. The addition of PDMS could endow the Janus membrane with better performance stability and anti-fouling ability due to the enhanced surface hydrophobicity and formation of unique multi-level roughness on the hydrophobic layer. The prepared Janus membrane could be used as a promising candidate for the direct contact membrane distillation (DCMD) process.