The study, led by Wenjun Zhang of the Frontiers Science Center for Deep Ocean Multispheres and Earth System at Ocean University of China, together with colleagues at the University of Shanghai for Science and Technology and Shandong University, introduces what the authors call a heterogeneous wetting membrane, or heterowettability membrane. Unlike conventional membranes, which present a single uniform wettability to the fluid they contact, this membrane features spatially patterned regions of hydrophilic and hydrophobic surface chemistry arranged side by side. The design is paired with a bidirectional gradient contraction channel module, and it is this combination that allows one device to do the work of two.

The central insight of the work lies in how the two components divide their labor. The gradient contraction channel is not merely a piece of plumbing. As fluid flows through a channel whose cross-section narrows progressively, the velocity field becomes non-uniform, generating a spatially varying shear stress across the flow. That non-uniform shear field acts directly on the emulsion droplets, deforming them and gradually destabilizing the surfactant shells that keep them intact. In effect, the channel performs a gentle, continuous demulsification before the fluid ever reaches the membrane surface, priming the droplets to coalesce and release their contents at the membrane interface.

Once the partially demulsified dispersion arrives at the membrane, the patterned wettability takes over. Droplets of the dispersed phase encounter surface regions whose chemistry matches their own continuous phase affinity, and the resulting surface energy gradients drive them to migrate in opposite directions across the membrane. Water droplets in an oil continuous phase are drawn toward hydrophilic zones, while oil droplets in water are pulled toward hydrophobic zones. Coupled with the shear-induced demulsification and directional phase transport, this wettability-guided routing allows oil-in-water and water-in-oil emulsions to be processed simultaneously and in parallel, each yielding its own purified phase.

A critical challenge in any such dual-function design is preventing the two phases from crossing into each other’s collection pathways, a failure mode known as continuous phase crossover. The researchers addressed this with what they describe as a pressure field–Laplace anchoring mechanism. By carefully controlling the transmembrane pressure and exploiting the Laplace pressure barriers that arise at the boundary between wetting and non-wetting regions of the membrane, the device suppresses unwanted crossover of the continuous phases. In essence, the capillary pressure required to force a non-wetting liquid through a pore region acts as a self-regulating valve, keeping each phase confined to its designated pathway without the need for external actuation or switching.

The team’s experimental characterization, documented across six main figures and an extensive supplementary package of more than sixty figures and sixteen tables, probed the interfacial restructuring and retained fluid states on the membrane during operation. Contact angle measurements captured in real-time video show directed water transport along the heterogeneous wetting interface, driven by surface energy gradients. Dynamic observations of the oil-water interface on both hydrophilic and hydrophobic membrane regions under varying transmembrane pressure reveal how the fluid states are maintained and how the phase-selective droplet transfer is controlled by the interplay of membrane wettability and channel geometry.

Perhaps the most subtle finding of the study concerns the dual role of shear stress. Shear is essential for demulsification, breaking droplets apart so their phases can be separated. But excessive shear at the membrane surface risks re-emulsification, tearing newly coalesced phases back into fine droplets and undoing the separation. The researchers mapped this trade-off in detail, identifying the operating window in which the gradient contraction channel delivers enough shear to destabilize droplets while the membrane interface experiences conditions that favor coalescence and phase transport rather than droplet re-formation. This balance, they show, is what sustains the device’s remarkable efficiency over extended operation.

The practical implications are considerable. Anti-fouling performance and long-term stability, the two metrics that most often doom membrane technologies in real wastewater service, were both notably enhanced in the new system. Because the opposing emulsion streams are demulsified before contact with the membrane and routed along chemically matched pathways, foulants are less likely to accumulate and clog the pores. The authors also report a techno-economic assessment supported by collaborators at Ocean University of China, suggesting the team has begun to evaluate not just laboratory performance but the commercial viability of the approach. For industries ranging from petroleum refining to food processing and textile manufacturing, where both emulsion types arise in the same facility and are currently handled by separate, sequential treatment trains, a single device that purifies both streams continuously could simplify plant design and reduce operating costs.

The work also builds on, and departs from, a rich lineage of bioinspired separation research. Prior advances include superwetting nanofiber membranes, CO2-responsive switchable membranes, Janus membranes with asymmetric affinities, liquid-infused aerogel systems capable of on-demand emulsification and demulsification, and a Janus channel of membranes reported in Science in 2024 that achieved concurrent oil and water recovery from emulsions. What distinguishes the new study is its explicit targeting of opposing emulsions within a single synchronous process, achieved not by switching a membrane’s properties over time but by distributing different wetting behaviors across space and letting the channel geometry do the rest. The result, the authors write, establishes a new paradigm for synchronous separation and parallel processing of opposite emulsions, paving the way for efficient integrated oil-water separation technology.

Challenges remain before the system can be scaled from laboratory modules to industrial service. Membrane fabrication must be reproducible at large areas, the patterned wettability must withstand years of exposure to surfactants, solvents and temperature swings, and the pressure operating window must tolerate the variability of real waste streams. Yet the fundamental demonstration is striking: a membrane that treats wettability not as a fixed property to be chosen but as a design variable to be patterned, working in concert with fluid mechanics rather than against it. If the reported efficiencies and stability hold up at scale, the humble membrane may finally be ready to handle the full, messy spectrum of emulsified oily waste in one continuous pass.

Subject of Research: A patterned heterowettability membrane with a gradient contraction channel for synchronous, parallel separation of opposing oil-in-water and water-in-oil emulsions.

Article Title: Synchronous and parallel separation of opposing emulsions enabled by a heterowettability membrane

Article References: Zhang, W., Zhang, D., Guan, Y., Li, Y., Geng, S., Li, B., Chen, D., Wang, J., Bao, M., & Wang, Z. (2026). Synchronous and parallel separation of opposing emulsions enabled by a heterowettability membrane. Nature Water. https://doi.org/10.1038/s44221-026-00703-z

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00703-z

Keywords: heterowettability membrane, emulsion separation, oil-water separation, demulsification, shear-induced demulsification, water-in-oil emulsion, oil-in-water emulsion, membrane technology, anti-fouling, Laplace pressure, directional liquid transport, water purification

This pioneering work, led by Lu, Huang, Zhang, and their colleagues, shifts the focus from merely tuning physical pore sizes to engineering the electrochemical landscape across membrane surfaces. Utilizing advanced multimodal atomic force microscopy (AFM) techniques, the researchers visualized nanoscale charge heterogeneities, extracting detailed three-dimensional maps of surface potential, mechanical phase, and functional group distribution on polyamide membranes. The metrological innovations underpinning this methodology enable unprecedented quantitative insight into spatial variations of surface charge—a critical parameter previously shadowed by the dominant emphasis on pore size.

Their findings reveal a counterintuitive phenomenon: membranes with ostensibly “optimized” narrow PSDs may underperform in ion selectivity if the surface charge distribution remains heterogeneous. Conversely, membranes exhibiting enhanced charge homogeneity demonstrate sharper discrimination capabilities between ions, especially in complex separations like the lithium–magnesium mixtures crucial for next-generation resource recovery. This discovery overturns established thought, underscoring that homogenous electrostatic environments on membrane surfaces significantly modulate ion transport and rejection beyond what pore size alone can achieve.

Critically, the research team employed a polyethyleneimine (PEI)-based multivariate functionalization strategy to program stepwise enhancements in the spatial homogeneity of electropositive amine groups on polyamide membranes. This chemical engineering approach results in a progressively uniform surface charge distribution that directly correlates with improved ion selectivity. The membranes designed this way distinctly outperform those optimized solely via PSD manipulation, especially in challenging separations involving ions of similar size but differing charge characteristics.

The implications of this discovery extend wide across membrane science and engineering. By decoupling ion selectivity from the traditional constraint of pore size precision, the study opens new pathways for designing membranes with superior performance through facile surface charge modulation. This could expedite the development of next-generation nanofiltration devices that are simpler to manufacture, more robust, and highly selective, meeting urgent global needs in clean water production, waste brine valorization, and critical metal recovery.

One of the most remarkable aspects of this research is the integration of multimodal AFM for charge mapping, a cutting-edge metrological advance. By correlating surface potential images with phase shifts and functional group chemical signatures, the team created a holistic representation of the nanoelectrochemical landscape. This comprehensive mapping capability allows the differentiation of subtle charge patch distributions that conventional characterization methods would overlook, explaining why previously puzzling disparities in membrane performance often arose despite similar pore size specifications.

Furthermore, the study’s revelation that charge homogeneity outweighs pore size in selective ion transport challenges membrane developers to rethink fundamental design principles. The heterogeneous distribution of charges creates localized “hot spots” or “cold spots” that disrupt uniform ion partitioning, leading to less predictable and often degraded selectivity. Homogenizing this charge landscape mitigates such variability, enabling more consistent and controllable ion rejection behavior.

Experimentally, the researchers demonstrated these concepts by fabricating a series of polyamide membranes with varying degrees of charged surface uniformity but comparable pore size distributions. They rigorously quantified ion selectivity metrics and correlated them with nanoscale charge uniformity indices derived from AFM data. The data conclusively showed that membrane selectivity systematically improves with increasing charge homogeneity, even when PSD remains essentially constant. Such a clear decoupling of these two factors was previously unattainable in the field.

Beyond lithium–magnesium separations, the fundamental concept of charge homogeneity governing ion selectivity may have transformative applications across diverse membrane-based processes. These include desalination, wastewater treatment, resource recovery from brines, and selective electrolyte separation for energy applications. The ability to tune ion rejection profiles through electrostatic nanoengineering promises to catalyze innovation in sustainable membrane technologies worldwide.

Another significant advantage of focusing on charge homogeneity is the simplified manufacturing complexity it affords. Traditional approaches demand ultrafine control of membrane pore sizes—often at the nanometer scale—requiring sophisticated fabrication techniques that are difficult to scale. In contrast, chemical modulation of surface charge distribution via polymers like polyethyleneimine can be achieved through accessible, scalable processes such as layer-by-layer assembly or surface grafting. This could democratize access to high-performance nanofiltration membranes beyond specialized laboratories.

The study also sheds light on the fundamental ion transport mechanisms within charged membranes. Uniform surface charge distributions enhance electrostatic exclusion effects and uniform potential barriers, which work synergistically to differentiate ions not just by size exclusion but by ionic charge density and valence. This nuanced electrochemical interplay explains why traditional size-based models failed to fully capture the observed selectivity patterns.

Importantly, this research aligns with ongoing trends toward multifunctional membrane surfaces that combine tailored pore architectures with chemically active moieties to achieve superior selectivity, permeability, and anti-fouling properties. Understanding the predominant role of charge homogeneity enables more rational design principles, where surface chemistry and nanoelectrostatics are engineered in unison rather than in isolation.

Moreover, the authors’ multidisciplinary approach, combining materials chemistry, nanoscale metrology, and membrane engineering, exemplifies the integrative research needed to tackle complex separation challenges. Their methodology could inspire future studies to explore charge homogeneity effects in a wider range of membrane materials, including emerging 2D materials, ionomers, and biomimetic structures.

While this discovery propels membrane science forward, it also raises intriguing questions about how charge homogeneity evolves during membrane aging, fouling, or chemical degradation. Future work could focus on maintaining or dynamically tuning nanocharge uniformity under real-world operating conditions, further enhancing membrane lifespan and reliability.

In summary, this visionary study ushers in a paradigm shift by spotlighting nanoscale surface charge homogeneity as the dominant factor over pore size distribution in dictating ion selectivity of polyamide membranes. By advancing novel characterization tools and surface engineering strategies, the research provides both fundamental insights and practical routes to fabricate next-generation ion-selective membranes. The direct impact on critical applications—ranging from clean water to resource recovery—affirms its significance and potential to inspire transformative advancements in sustainable membrane technologies.

Subject of Research: Ion selectivity in polyamide nanofiltration membranes and the role of nanoscale surface charge homogeneity.

Article Title: Impact of charge homogeneity on ion selectivity in polyamide membranes.

Article References:
Lu, D., Huang, M., Zhang, C. et al. Impact of charge homogeneity on ion selectivity in polyamide membranes. Nat Water (2025). https://doi.org/10.1038/s44221-025-00498-5

Image Credits: AI Generated