Engineering antiwetting hydrophobic surfaces for membrane distillation: A review

Membrane distillation (MD) is an emerging membrane separation technology with great potential for desalination, wastewater treatment and volatile resource recovery. It becomes even more attractive as it can utilize low-grade heat or renewable energy, and treat high-salinity waste liquids towards zero liquid discharge. However, the performance of MD is often limited by the wetting of hydrophobic porous membranes during operation, leading to reduced flux and efficiency. To overcome this challenge, the development of antiwetting hydrophobic MD membranes has gained increasing attention in recent years. In this review, we examine the liquid entry pressure (LEP) and its influencing factors (e.g. the maximum pore size, surface chemistry/free energy and surface roughness/architecture) of an MD membrane, which determine the antiwetting performance of the porous MD membrane. From enhancing the LEP point of view, we propose two key strategies for engineering antiwetting surfaces: (1) reducing the membrane pore size, and (2) increasing the liquid contact angle by minimizing the surface free energy and the liquid/solid contact area through enhancing the surface roughness and/or creating hierarchical/re-entrant structures. These strategies include various specific fabrication techniques, such as surface coating, vapor deposition, layer-by-layer assembly, surface fluorination, and surface functionalization. Green surface modification materials and methods are also discussed to reduce the application of less environmentally friendly fluoride-containing compounds. Furthermore, we provide insights and future directions for the design and engineering of high-performance antiwetting hydrophobic MD membranes. Overall, this review offers a comprehensive analysis of the current state-of-the-art research in engineering antiwetting hydrophobic MD membranes, and highlights the potential for the development of next-generation MD membranes with improved performance and efficiency.