Tillandsia-Inspired Asymmetric Covalent Organic Framework Membranes for Unidirectional Low-Friction Water Collection

Friction plays a pivotal role in many phenomena of physical chemistry and has long been in the focus of research thereof. As a crucial parameter, friction in membranes' inner and/or outer surface can be minimized to reduce solvent inlet resistance and enlarge inner pore fluid flux, ideally reaching near frictionless transport of water at nanoscale. Inspired by the leaf structure of Tillandsia, a porous membrane with a rough surface and a hydrophilic inlet together with hydrophobic pore channels was designed and fabricated, based on covalent organic frameworks (COFs). Combined with COFs' inherent highly oriented pore structures, the as-made asymmetric membranes through chemical etching can minimize the solvent critical intrusion pressure and enable inner pore low friction water transport. Ultimately, obtained COF membranes succeeded in trapping fog from air and achieved a water harvesting rate (WHR) of 1570 mg cm^-2 h^-1, together with small molecular pollutants filtrated off in the meantime. Intriguingly, the synthesized asymmetric COF membranes illustrated unidirectional low friction water collecting and transporting features, the successful imitation of T. macdougallii. This work presents a practical strategy to construct functional porous membranes for low friction water collection and transport, and created a model paradigm to design fluid transporting pore channels.