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

The motivation behind the work stems from a growing environmental reckoning with conventional surfactants. These workhorse molecules, which make up 10 to 20 percent of most cosmetic emulsions, are designed to reduce tension at the boundary between water and oil, allowing otherwise incompatible phases to blend into smooth, shelf-stable products. Yet many synthetic surfactants persist in the environment for years. Perfluorinated compounds in particular are prized by formulators for their robustness but are notoriously resistant to degradation, while linear alkylbenzene sulfonates have been linked to biochemical and pathological harm in aquatic ecosystems. Regulators and consumers alike have been pushing the cosmetics industry to find biodegradable alternatives that do not sacrifice performance.

Biosurfactants, amphiphilic molecules produced by microorganisms that combine a water-loving polar head with an oil-loving hydrophobic tail, have long been touted as such an alternative. Lactic acid bacteria are especially attractive producers because they carry the Generally Recognized as Safe designation, meaning their metabolites face fewer regulatory hurdles in food and personal-care applications. The obstacle has been practical: production yields are often low and fermentation substrates expensive, pushing up costs and limiting commercial adoption. Researchers have therefore been hunting for high-performing producer strains in unconventional places, from cassava-processing effluents to traditional fermented foods.

Pendidam, the spontaneously fermented skimmed cow’s milk consumed around Ngaoundere in Cameroon’s Adamaoua region, had already caught the attention of the research group. Earlier work by Mbawala and colleagues had shown that the drink teems with Lactobacillus strains capable of producing surface-active compounds. In the new study, the team collected six Pendidam samples from vendors at four sites across the city, including the Petit marché, Dang, Wakwa and the Grand marché, and transported them to the laboratory under refrigeration within two hours of purchase.

From those samples the researchers isolated twelve presumptive Lactobacillus strains, all of which proved to be Gram-positive, non-motile, catalase- and oxidase-negative rods with heterofermentative metabolism, growing across a temperature range of 15 to 45 degrees Celsius and tolerating salt concentrations up to 6.5 percent. The authors are careful to note that these phenotypic traits, while consistent with members of the former Lactobacillus group and suggestive of the reclassified genus Lactiplantibacillus, are not sufficiently discriminating for definitive taxonomic assignment following the genus’s recent fragmentation into more than twenty genera; molecular identification by 16S rRNA gene sequencing remains a stated priority for future work.

Screening for biosurfactant production relied on two complementary assays. In the hydrocarbon overlay test, bacterial colonies grown on agar were covered with palm oil; the appearance of clear halos containing microemulsions signaled secretion of surface-active compounds. The oil spreading test provided quantitative confirmation: drops of cell-free culture supernatant were deposited onto a thin oil layer floating on water, and the diameter of the resulting clear zone, which correlates with biosurfactant concentration, was measured. Five isolates, subsequently designated S1 through S5, produced clear zones ranging from 6 to 28 millimeters, comfortably exceeding the 3-millimeter threshold above the negative control that the field considers evidence of genuine biosurfactant production.

The crude biosurfactants were then extracted by ethanol precipitation from scaled-up cultures and put through their paces. Emulsification indices, a measure of the emulsified layer formed when biosurfactant solution is mixed with palm oil, ranged from 67.33 to 72 percent, values slightly below those reported for some purified lactobacilli biosurfactants in the literature but respectable for unpurified crude extracts. More striking were the surface tension measurements. At a concentration of 4 grams per 100 milliliters, solutions of the biosurfactants from isolates S1 and S5 lowered the surface tension of water from 70.90 millinewtons per meter to levels comparable with sodium dodecyl sulfate, the benchmark anionic surfactant used as the positive control.

The decisive test came when the biosurfactants were formulated into water-in-oil beauty milks alongside xanthan gum, sesame oil, glycerol and shea butter, with the commercial sorbitan monostearate SPAN 60 and a marketed body milk serving as positive controls. Laser diffraction analysis using a Malvern Mastersizer 2000 revealed that emulsions stabilized with the bacterial surfactants formed droplets between 4.94 and 7.61 micrometers in diameter, dramatically finer than the 16.34-micrometer droplets of the negative control. The standout was isolate S5, whose biosurfactant yielded the smallest droplets of all, significantly finer even than those produced by the synthetic SPAN 60. According to the authors, this reflects rapid adsorption of the amphiphilic molecules at newly created oil-water interfaces during high-shear homogenization, where they form protective films that generate steric and, depending on molecular composition, electrostatic repulsion between droplets, suppressing coalescence and Ostwald ripening.

Viscosity told a complementary story. Formulations containing the bacterial biosurfactants ranged from 967 to 1715 millipascal-seconds, compared with roughly 1998 millipascal-seconds for both controls, and the differences tracked closely with droplet size. Emulsions with the finest droplets, such as those made with the S5 biosurfactant, maintained relatively high viscosity, consistent with emulsion theory: smaller droplets present a larger total interfacial area, increasing interactions between dispersed and continuous phases and raising resistance to flow. Higher viscosity in turn slows droplet mobility, reducing collision frequency and the risk of creaming or phase separation during storage. The combination of fine droplet size and appropriate rheology, the authors argue, positions the S5 biosurfactant as the most promising candidate of the five tested for cosmetic formulation.

The study’s novelty lies less in the individual measurements than in its integrated approach and its source material. Biosurfactants from lactic acid bacteria have mostly been studied for antimicrobial, antiadhesive and probiotic properties; direct deployment as emulsifiers in cosmetic products has rarely been evaluated, and never before with strains from Pendidam. By connecting microbiological screening with rigorous physicochemical characterization of finished emulsions, the work makes the case that traditional fermented foods are an underexplored reservoir of industrially useful microbes, and one that comes with built-in cultural and economic advantages for the communities that produce them. The path to commercialization still requires purification and structural characterization of the biosurfactants, definitive molecular identification of the producing strains, long-term stability testing and safety evaluation, but the baseline data now exist. For an industry under pressure to replace persistent synthetic surfactants with biodegradable molecules, the answer may have been fermenting quietly in a Cameroonian milk calabash all along.

Subject of Research: Biosurfactants from Lactobacilli isolated from Pendidam fermented milk for stabilizing water-in-oil cosmetic emulsions

Article Title: Improvement of water in oil emulsion physicochemical stability using biosurfactants from indigenous Lactobacilli isolated from Pendidam

Article References: Improvement of water in oil emulsion physicochemical stability using biosurfactants from indigenous Lactobacilli isolated from Pendidam. (n.d.). https://doi.org/10.1007/s44508-026-00021-y

Image Credits: AI Generated

DOI: 10.1007/s44508-026-00021-y

Keywords: biosurfactants, Lactobacilli, Pendidam, emulsifiers, cosmetics, lactic acid bacteria, water-in-oil emulsion, droplet size, viscosity, surface tension, fermented milk, sustainable ingredients