Anisotropic performance of a superhydrophobic polyvinyl difluoride membrane with corrugated pattern in direct contact membrane distillation

A novel surface-corrugated superhydrophobic polyvinylidene fluoride (PVDF) membrane (C-PVDF) was prepared for direct contact membrane distillation (DCMD) using a micromolding phase separation (μPS) method. The membrane showed a static contact angle of 159.0 ± 4.0°. However, dynamic measurements of the sliding angles revealed a lower value of 9.1 ± 0.8° when a water droplet slides in parallel to the ridge, and a higher value of 14.6 ± 1.6° if perpendicular to the ridge. This anisotropic property was also reflected in the DCMD fluxes for both feed of deionized water and 4.0 wt% NaCl solution: in case the feed flows in parallel to the ridge, a higher flux is resulted than it flows perpendicular to the ridge. Anisotropic MD performance cannot be explained by the Dusty-Gas model because the average characteristics of the membrane in the model are intrinsically the same for both flow modes. Instead, anisotropic wetting and sliding in the parallel and perpendicular orientation revealed that the MD performance has both thermodynamic and hydrodynamic origins.