Designing Janus Two-Dimensional Porous Materials for Unidirectional Transport: Principles, Applications, and Challenges

Abstract

Unidirectional liquid transport, facilitated by asymmetric properties or structures, has gained prominence in applications, such as liquid manipulation, water harvesting, and microfluidic systems. Janus two-dimensional (2D) porous materials, known for their unique capacity for unidirectional liquid transport, have attracted significant interest. This phenomenon is driven by the capillary force and Laplace pressure, enabling efficient liquid flow in a preferred manner. Leveraging this process, Janus 2D porous materials have found applications in a series of multiphase processes without additional energy input. Despite a growing body of researches in this area, there is a notable absence of a comprehensive review focusing on the fundamentals of Janus 2D porous materials with unidirectional liquid transport capacity. This Review aims to address this gap by summarizing present hypotheses, factors influencing unidirectional transport, construction strategies, and potential applications. Furthermore, the future of this field is forecast to present challenges and opportunities.

Buoyancy-Assisted Fabrication of Liquid Diode: Janus Nanofibrous Membrane for Efficient Wastewater Treatment

“…Traditional methods such as gravity separation, electro-flocculation, bioremediation, demulsification, air flotation, adsorption, coagulation and flocculation, coalescence, centrifugation, and oil skimming have been used. , However, these approaches have limitations such as low separation efficiency (SE ff ), high costs of operation and maintenance, and the production of secondary pollutants, especially when treating different immiscible oil/water emulsions. ,, In this regard, polymeric membrane separation technology is considered the most effective method for separating oily wastewater, due to its inexpensiveness, high separation efficiency, and convenience of use. , Several polymeric membranes, including polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyurethane (PUR), polysulfone (PSU), and polytetrafluoroethylene (PTFE), have been developed for this purpose. , Among these, PVDF-based nanofibrous membranes play a crucial role, providing a strong and stable foundation for the membrane’s structure and performance. ,, PVDF is chosen for its remarkable chemical resistance, mechanical strength, and thermal stability, making it an ideal material for membrane applications. , The fabrication of PVDF nanofibrous membranes typically involves electrospinning (e-spinning), a technique that uses high voltage to create nanoscale fibers from a polymer solution. , This technique offers numerous advantages in the fabrication of PVDF nanofibrous membranes. It facilitates the creation of ultrafine fibers with a large surface area and enables precise control over morphology and pore structure of the fibers, enabling customization of pore size, porosity, and interconnectivity. , Additionally, the uniform distribution of fibers enhances the membranes’ mechanical strength and stability, providing opportunities for further enhancements in performance through modifications. ,, Despite the advantages of polymeric nanofibrous membrane separation technology, traditional membranes with single wettability face significant challenges in effectively separating complex oil–water emulsions. , Most superwetting membranes with singular wettability are only effective to separate specific oil or water mixtures . Such membranes are known as “oil-removing” or “water-removing” membranes. , This restriction significantly limits the real-world implementation of membranes in complicated wastewater treatment processes.…”

“…In response to the urgent need for efficient method to separate oil and water in the context of oily wastewater, a novel solution has emerged: the Janus nanofibrous membrane. Unlike traditional membranes, the Janus nanofibrous membrane acts as a liquid diode, offering a revolutionary approach to tackle the complex mixtures of w-in-o and o-in-w emulsions. , These Janus nanofibrous membranes with their distinct wetting properties have attracted significant attention for their ability to transport liquids unidirectionally, achieve high flux, and exhibit exceptional separation performance, especially in separating w-in-o and o-in-w emulsions. ,,, Achieving asymmetric wettability in Janus nanofibrous membranes can be accomplished through various means, including surface modification techniques, material selection, structural design, chemical modification, and template-assisted fabrication . For instance, incorporating metal oxides, two-dimensional (2D) or three-dimensional (3D) novel material, or growing special morphologies on the Janus nanofibrous membrane can further enhance its asymmetric wettability. ,, This leads to better performance in separating oil and water in complex emulsions.…”