Engineering omniphobic corrugated membranes for scaling mitigation in membrane distillation

Membrane scaling is unavoidable when treating highly concentrated and even supersaturated brine in the continuous membrane distillation (MD) process. Omniphobic membranes with higher nucleation energy barriers could partially mitigate the membrane scaling. Herein, we fabricated omniphobic corrugated membranes to further alleviate membrane scaling via the synergistic effects of omniphobicity and corrugation on the membrane surface. The scaling behaviors of omniphobic flat membrane (#PVDF-FF), omniphobic corrugated membrane (#PVDF-CF) at parallel (PL) and perpendicular (PD) modes were investigated. Different modes represented the relative position between the feed flow direction and the pattern of corrugated membrane. The results showed that the modified corrugated membranes had water contact angles over 150° and anti-wettability against low surface tension liquids. Compared to #PVDF-FF, #PVDF-CF at the PL mode exhibited a better slippery property while the PD-mode #PVDF-CF had higher slippery resistance. The membrane omniphobicity was confirmed to be effective against membrane scaling when treating a 3.5 wt% NaCl feed solution. However, the scaling resistance of #PVDF-CF at both PL and PD modes was much better than that of #PVDF-FF when 25 wt% NaCl or 20 mM supersaturated gypsum solutions were used as feeds. The better anti-scaling properties of PL-mode #PVDF-CF should be attributed to a higher flow rate and shorter residence time of feed flow parallel to the corrugated patterns, as well as the alleviated concentration polarization. The turbulent effect induced by the perpendicular feed flow on the corrugated patterns and the consequent alleviation of concentration polarization endowed the PD-mode #PVDF-CF with the best scaling resistance.