Self-roughened omniphobic coatings on nanofibrous membrane for membrane distillation

Novel omniphobic electrospun nanofibrous membranes (ENMs) for direct contact membrane distillation (DCMD) were fabricated by electrospinning poly(vinylidene fluoride) (PVDF) nanofibers and followed by directly functionalizing the fiber surface with long-chained fluorododecyltrichlorosilane (FTCS) via solution immersion without any assistance of roughening treatments. The morphologies of polymerized fluorododecyltrichlorosilane (PFTCS) coated PVDF nanofibers evolved from the newly budding willow twigs with tiny bumps, to vigorous buds and then to intertwined fillets with the increase of FTCS concentration, which could serve as a robust barrier to low-surface-tension liquid penetration. By constructing hierarchically re-entrant structures of low-surface-energy PFTCS coating layer onto the PVDF nanofibers, the resultant porous ENMs were endowed with omniphobic property, low water-adhesion property, remarkable mechanical property, and superior durability in harsh circumstances as well as good breathability. The comprehensive investigations on structural attributes (such as the liquid entry pressure, pore size, porosity and gas permeability), long-term MD stability and DCMD performance in presence of surfactants were carried out. Significantly, the resultant omniphobic PFTCS/PVDF membrane exhibited a competitive permeate vapor flux of 36.9 kg/(m² h) in highly salty solution (3.5 wt% NaCl salt feed; ΔT = 40 °C) over 24 h operation and also demonstrated robust DCMD performance in presence of surfactants. Compared with the commercial PVDF MD membranes, this simple and scalable strategy had great potential for MD applications.