Key Takeaways & Executive Findings
- •• • Millimeter-scale channel width (few millimeters) between hydrophilic and hydrophobic membranes enhances emulsion droplet enrichment and collision, yielding substantial increases in oil and water recovery rates compared to conventional Janus membranes with micro/nanoscale thickness. • • The JCM achieves concurrent recovery of both water and oil from surfactant-stabilized emulsions, overcoming the fundamental limitation of uniform-wettability membranes that recover only the permeating phase while failing to retrieve retained components. • • The hydrophilic membrane exhibits underwater superoleophobicity, enabling rapid water permeation, while the hydrophobic membrane concurrently recovers oil, achieving synchronous dual-phase separation from high-concentration-surfactant-stabilized emulsions. • • Multistage device configurations incorporating JCMs enable multi-level strengthening of separation and recovery efficiency, providing scalable potential for real industrial emulsion separation and recovery operations.
Abstract
Conventional oil-water separation technologies, including centrifugation and flocculation, suffer from density-dependent limitations and secondary pollution, while membrane filtration with uniform wettability selectively recovers only one phase, leaving retained components unrecovered. Janus membranes with asymmetric wettability enable on-demand separation but their micro/nanoscale thickness restricts interfacial separation to microscopic domains. This work presents a Janus-channel-membrane (JCM) architecture that scales asymmetric wettability from micro/nanoscale to millimeter-scale spatial channels, constructed from a hydrophilic membrane and a hydrophobic membrane separated by a slit of a few millimeters. When surfactant-stabilized emulsions are delivered to the slit, purified water permeates rapidly through the hydrophilic membrane while oil is concurrently recovered at the hydrophobic membrane. The millimeter-scale channel width enhances enrichment and collision behavior of emulsion droplets via interference interactions between the membrane pair, producing substantial increases in oil and water recovery rates. The JCM demonstrates distinctive advantages for separating and recovering both phases from high-concentration-surfactant-stabilized emulsions. Multistage device configurations incorporating JCMs can further strengthen separation and recovery efficiency at multiple levels, offering transformative potential for industrial emulsion separation and recovery. This spatial configuration of membranes with contrary wettability at the millimeter scale addresses key bottlenecks in concurrent oil and water recovery from stable emulsions.
1. Introduction
Oily wastewater represents a global water pollutant, while trace water in transportation fuels and lubricants threatens vehicle safety. Conventional separation methods such as centrifugation, which is density-dependent, and flocculation, which requires chemical additives and generates secondary pollution, have inherent limitations. Membrane filtration is widely used for oil-water separation, relying on interface separation between immiscible phases. The wettability of filtration membranes, reflecting liquid-surface interactions, is critical for emulsion separation. Single superlyophobic membranes, including superhydrophobic and underwater superoleophobic variants, can only separate water-in-oil or oil-in-water emulsions, respectively. Integrating superhydrophobicity and underwater-superoleophobicity on the same membrane via heterogeneous surface chemistry enables switchable permeability for on-demand separation. Janus membranes with asymmetric wettability have emerged as promising candidates for separating intractable oil-water emulsions that challenge conventional uniform-wettability membranes. Nevertheless, conventional separation systems fundamentally limit synchronous dual-phase recovery from stable oil-water emulsions, as they selectively recover permeating liquid phases while failing to retrieve retained components.
Regulating asymmetric wettability, layer thickness, and pore microstructure endows Janus membranes with unusual superiority in interface separation of oil and water from emulsions. However, due to the limitation of thickness scale of Janus membranes, their asymmetric wettability only affects interface separation in the micro-/nano-scale range. Boosting macroscale coupling of asymmetric wettability could revolutionize interfacial separation in emulsions. Scaling Janus membrane thickness from micro/nano to millimeter scales evolves conventional planar structures into spatial membrane architectures, where unique emulsion separation advantages arise from engineered channel widths and asymmetric wettability. Specifically, spatial channel configurations incorporated by controllable wettability membranes at millimeter-scale are used for recycling water and oil from emulsions. Zhi-Kang Xu and coworkers presented a Janus-channel-membrane (JCM), in which a restricted architecture constructed of a group of hydrophobic and hydrophilic membranes is used for concurrent recycling of water and oil from surfactant-stabilized emulsions. When the width of the channel between the pairs of membranes with contrary wettability reduces to a few millimeters, the enrichment and collision behavior of the emulsion within the channel caused by the interference interaction between the pairs of membranes is significantly enhanced, producing a substantial increase in the recovery rate of oil and water. The central component of JCM is a slit formed by hydrophilic and hydrophobic membranes. When the emulsion is delivered to the slit, purified water flows out rapidly from the hydrophilic membrane, whereas the oil is concurrently recycled at the hydrophobic membrane. The hydrophilic membrane performs underwater superoleophobicity, enabling efficient water permeation while rejecting oil. This spatial configuration offers a transformative strategy that addresses key issues in emulsion separation and recovery industry.
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TIE Lu, ZHENG Xiaoping (2025). Asymmetric Wettability Channel of Membranes for Water and Oil Concurrent Recovery from Emulsions. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3365-5
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Frequently Asked Questions
What is the fundamental limitation of conventional Janus membranes that the JCM architecture overcomes?
Conventional Janus membranes have asymmetric wettability that only affects interface separation at micro-/nanoscale thickness, limiting separation to microscopic domains and preventing synchronous dual-phase recovery. The JCM scales the asymmetric wettability to millimeter-scale spatial channels, enabling concurrent recovery of both water and oil from stable emulsions. The millimeter-scale channel width enhances enrichment and collision behavior of emulsion droplets, producing substantial increases in recovery rates.
How does the JCM achieve concurrent recovery of oil and water from surfactant-stabilized emulsions?
The JCM consists of a slit formed by a hydrophilic membrane and a hydrophobic membrane. When emulsion is delivered to the slit, purified water flows rapidly through the hydrophilic membrane, which exhibits underwater superoleophobicity, while oil is concurrently recovered at the hydrophobic membrane. This dual-membrane configuration enables simultaneous separation and recovery of both phases, unlike uniform-wettability membranes that recover only the permeating phase.
What operational parameters enhance the separation efficiency in the JCM?
The width of the channel between the hydrophilic and hydrophobic membranes is reduced to a few millimeters. This millimeter-scale confinement significantly enhances the enrichment and collision behavior of emulsion droplets within the channel due to interference interactions between the membrane pair. The result is a substantial increase in the recovery rate of both oil and water from high-concentration-surfactant-stabilized emulsions.
Can the JCM be scaled for industrial applications?
Yes. Multistage device configurations incorporating JCMs can multi-levelly strengthen separation and recovery efficiency, providing infinite potential in the real industrial field. The spatial configuration of membranes with contrary wettability at the millimeter scale offers a transformative strategy that addresses key issues in emulsion separation and recovery industry, enabling scalable concurrent oil and water recovery from stable emulsions.
What are the advantages of the JCM over conventional separation methods like centrifugation and flocculation?
Centrifugation is density-dependent and flocculation requires chemical additives that cause secondary pollution. The JCM avoids these limitations by using asymmetric wettability to separate and recover both oil and water concurrently from surfactant-stabilized emulsions without chemical additives. It achieves synchronous dual-phase recovery, which conventional uniform-wettability membranes cannot accomplish, and demonstrates distinctive advantages for high-concentration-surfactant-stabilized emulsions.
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