A systematic study on the impact of feed composition and substrate wettability on wetting and fouling of omniphobic and janus membranes in membrane distillation

Abstract The design and fabrication of omniphobic and Janus membranes has been already established and reported in the literature. However, the reports on their fouling and wetting performance in membrane distillation (MD) are contradicting, mainly due to a lack of comparable membrane design and operating feed conditions. In this study, we systematically evaluate, under the same conditions, how the wettability of Janus membranes and other special membranes can affect their wetting and fouling performance when tested with different complex feeds during MD. We measure their in-air water, in-air oil, and under-water oil contact angles to produce a comprehensive understanding of why they behave differently. We study the membrane's physical and chemical properties in combination with their MD performance to provide a mechanism for their wetting and fouling behavior. The results of this work suggest that wetting and fouling mechanisms in MD do not depend solely on the membrane or the feed composition, but rather on the combination of both. We show that omniphobic membranes performs differently in regard to oil fouling depending on the presence of surfactants in the feed. Surfactant-containing oily feeds are treated efficiently by omniphobic membranes while those without surfactants result in severe fouling, which was also confirmed by a static oil-fouling test in the two feeds. We finally conclude that Janus membranes are still susceptible to wetting by surfactants-containing feeds if their substrate is hydrophobic, while only a Janus membrane with hydrophilic surface coating layer and an omniphobic substrate will be able to simultaneously resist both fouling and wetting during MD operation with complex feeds containing organics and low surface tension components, demonstrating its potential towards the treatment of industrial wastewaters. This study offers a roadmap for researchers in the membrane community for proper selection of omniphobic and Janus membranes based on feed composition and substrate wettability.