The new work, published in Frontiers in Thermal Engineering, builds on the level set method, one of the most popular interface-capturing techniques in computational fluid dynamics. In a level set formulation, the interface is not drawn explicitly; instead, it hides inside a smooth function as the set of points where that function equals zero. Typically this function is a signed distance function, meaning its value tells you how far you are from the interface and on which side. Moving the interface is then simply a matter of transporting this function along with the flow. The elegance of the approach explains its enduring popularity, but it comes with a well-known Achilles heel: mass loss.

The trouble arises because the interface must periodically be re-regularized. As the flow deforms the distance function, its defining property—that its gradient has unit magnitude everywhere—degrades, so a reinitialization equation must be solved to restore it. Every round of reinitialization nudges the zero contour slightly, and those tiny nudges accumulate. Over long simulations, droplets can shrink and bubbles can evaporate numerically, not because of any physics but because of the bookkeeping itself. Previous fixes have ranged from coupling level sets with volume-of-fluid methods to seeding the interface with Lagrangian particles, or from adding source terms to solving extra correction equations. Each works to some degree, but all add computational steps, implementation complexity, or both.

The NTNU-led team took a different route, borrowing an idea from a parallel line of research on phase field methods. Recent formulations—the conservative phase field (CPF) method of Chiu and Lin, refined by Jain, and the conservative diffuse interface (CDI) method proposed by Mirjalili and colleagues—advect a hyperbolic tangent profile across the interface instead of a signed distance function. This profile is carried by a compressive-diffusive equation with a regularization term that keeps the interface sharp, and crucially, it can preserve the interface function within strict bounds between zero and one using nothing more exotic than central-difference discretization. No separate reinitialization equation is needed at all.

The researchers realized this conservative advection machinery could be grafted onto their existing sharp interface level set solver designed for co-located grids, a solver they call SI-LSMcol. The result is the new sharp interface conservative level set method on a co-located grid, or SI-CLSMcol. Where the old method solved a pseudo-time reinitialization equation after every advection step, the new method advects the bounded hyperbolic tangent profile in a single step and then recovers the signed distance function through a simple algebraic relation. That substitution removes the most expensive and mass-eroding step in the pipeline while leaving everything else—momentum solving, pressure projection, jump conditions at the interface—untouched.

The co-located grid part of the story matters just as much. Many production codes store velocity and pressure at the same grid points rather than on staggered meshes, which is memory-friendly and simpler to parallelize, but it introduces a risk of pressure–interfacial force imbalance: when surface tension is not discretized in perfect balance with the pressure gradient, unphysical spurious currents ripple through the flow. The team’s approach sidesteps this by treating surface tension not as a volumetric source term but as a jump condition—an interfacial boundary condition—imposed directly at the interface. The new study extends this jump-condition framework, previously demonstrated only in two dimensions, to full three-dimensional Cartesian simulations with high density ratios, showing it remains robust under those demanding conditions.

Validation proceeded through a gauntlet of canonical benchmarks. First came the Young–Laplace law: a static droplet suspended in equilibrium, where the pressure jump across the curved interface must exactly match the surface tension times curvature. Across density ratios of 1, 10, and 100, the new method matched the old one’s accuracy almost exactly, with comparable magnitudes of spurious currents—confirming that swapping the advection equation did not disturb the delicate pressure–surface tension balance. A grid refinement study then revealed the headline result: the conservative method achieved two to three orders of magnitude less mass loss than its predecessor, while converging at a rate between first and second order as the mesh was refined.

The dynamic test cases told a consistent story. For a buoyancy-driven bubble rising in three dimensions, the simulated rise velocities agreed with previously published benchmark results, and the new method cut computational time by a factor of about two while improving mass conservation by a factor of roughly 45. For a falling droplet, validation against level set and coupled level set–volume of fluid results showed excellent agreement in the droplet’s trajectory, with improvements of about 3.25 times in speed and a several-fold gain in mass conservation. The droplet coalescence case—a droplet merging into a quiescent liquid pool, a surface-tension-dominated problem with severe interface deformation—matched experimental data within a few percent, with a 1.66-fold speedup and a 4.28-fold reduction in mass loss.

What makes the advance practical rather than merely academic is its plug-and-play character. Because the jump-condition source terms and the overall solution algorithm remain unchanged, the conservative advection equation can be dropped into an existing sharp interface solver with minimal re-engineering. The authors recommend choosing the regularization parameters so the interface function stays strictly bounded between zero and one, keeping the transition layer as thin as stability allows—a choice that sharpens the estimates of interface normals and curvature on which the whole sharp interface framework depends. The interface advection itself is discretized with a fifth-order WENO scheme in space and a third-order Runge–Kutta scheme in time, while the flow equations use a semi-explicit pressure-projection algorithm with a third-order TVD scheme for momentum advection.

For engineers simulating boiling, atomization, droplet dynamics, or any application where two immiscible fluids interact over long times, the implications are straightforward: simulations that keep their mass, run faster, and behave identically to trusted methods when checked against known physics. The work was supported by the Research Council of Norway and computed on the IDUN cluster at NTNU. As multiphase flow simulation pushes into ever more complex regimes—phase change, turbulent sprays, dense suspensions—methods that quietly eliminate accumulated numerical error while saving computation are exactly the kind of unglamorous but essential progress on which the field depends. This one, its authors show, can be adopted almost as easily as it was conceived.

Subject of Research: A mass-conservative sharp interface level set method for two-phase flow simulation on co-located grids

Article Title: A fast and improved mass-conservative approach for a sharp interface-level set method on co-located grid

Article References: Chirammel, S. S., Mukherjee, A., Brandt, L., & Solsvik, J. (2026). A fast and improved mass-conservative approach for a sharp interface-level set method on co-located grid. Frontiers in Thermal Engineering, 6, Article 1916556. https://doi.org/10.3389/fther.2026.1916556

Image Credits: AI Generated

DOI: 10.3389/fther.2026.1916556

Keywords: level set method, multiphase flow, mass conservation, co-located grid, sharp interface, computational fluid dynamics, surface tension, phase field method, two-phase flow, numerical simulation, jump conditions, reinitialization

The research, published in National Science Review, was led by Prof. Wei Xu of the Institute of Urban Environment and Prof. Ru-Jin Huang of the Institute of Earth Environment, both of the Chinese Academy of Sciences, together with Prof. Jurgita Ovadnevaite of the University of Galway. The team reports the first size-resolved observational constraint on the number size distribution of marine primary organic aerosols, achieved through long-term aerosol mass spectrometry, hygroscopicity measurements and machine-learning-based source apportionment. The measurements were carried out at the Mace Head Atmospheric Research Station on the west coast of Ireland, a location chosen for its frequent exposure to clean marine air masses arriving from the open North Atlantic, far from continental pollution sources that could confound the signal.

The reason this measurement matters so much lies in a fundamental distinction between mass and number. Marine primary organic aerosols have been characterized in terms of their mass for roughly two decades, and models have generally inferred their number concentrations indirectly from those mass estimates. But cloud droplet number concentrations, and therefore the brightness or albedo of marine clouds, depend on how many particles are available in the atmosphere rather than on how much material they collectively contain. A population of many small particles can seed far more cloud droplets than an equal mass concentrated in a few large ones. Without direct number measurements, the models were essentially guessing at the particle population that governs cloud formation over much of the planet.

Isolating the organic fraction from the rest of the sea spray required an elegant physical trick. At 90 percent relative humidity, sea salt and sulphate particles absorb water readily and grow substantially, whereas marine primary organic aerosol particles are considerably less hygroscopic and swell far less. The researchers exploited this contrast using a humidified tandem differential mobility analyser, an instrument that classifies particles according to how much they grow in humid air. Combined with concurrent measurements of particle size, number and chemical composition, this approach allowed the team to resolve the mPOA number size distribution down to an unprecedented 35 nanometres, a size range directly relevant to cloud droplet activation.

What emerged from the data was a distribution with two distinct modes rather than one. The measured size distribution is bimodal, with a smaller mode centred near 60 nanometres and a broader mode spanning roughly 100 to 200 nanometres. By contrast, the parameterization embedded in several major climate models produces a unimodal distribution centred at 120 nanometres, with no representation of the smaller mode at all. That omission is not a minor detail. Particles near 60 nanometres are precisely the sizes that can activate into cloud droplets at the moderate supersaturations typical of marine boundary layer clouds, so an entire population of potential cloud condensation nuclei was effectively invisible to the models.

The quantitative consequences are substantial. For particles larger than 35 nanometres, the model parameterization underestimates the total mPOA number concentration by approximately a factor of three. In the 30 to 70 nanometre range, the underestimate grows to between fivefold and sevenfold. Translated into cloud condensation nuclei, the mPOA contribution is underestimated by about 16 percent at a supersaturation of 0.25 percent and by as much as 64 percent at 0.5 percent supersaturation, assuming a 30 percent reduction in surface tension caused by the organic material. Surface tension matters here because organic films on particles can lower the energy barrier for droplet formation, allowing smaller particles to activate than would otherwise be possible. The affected models include GEOS-Chem, UKESM and CMAQ, which infer mPOA number from mass, as well as E3SM and CESM2, which represent mPOA without any size-resolved description.

To gauge what this means for the climate system, the researchers combined satellite observations of cloud properties with simulations from the GEOS-Chem model. As an order-of-magnitude estimate, the additional particles correspond to a first aerosol indirect effect of approximately -0.03 to -0.04 watts per square metre when averaged over the global ocean. Over biologically productive waters, where mPOA emissions are highest because phytoplankton activity is most intense, the effect is substantially larger, reaching -0.1 to -0.2 watts per square metre once the surface tension reduction is taken into account. These are cooling effects, since more cloud droplets produce brighter, more reflective clouds that bounce more sunlight back to space. While the global average figure may appear modest, it is concentrated over some of the most climatically sensitive ocean regions on Earth.

The authors are careful to frame these radiative estimates as sensitivity calculations rather than definitive global assessments. The figures quantify the magnitude of the mPOA influence that would obtain if the size distribution derived at Mace Head were representative of the global ocean, which is an assumption the data cannot yet confirm. Because the observations come from a single site, regional differences in phytoplankton community composition, sea state and atmospheric ageing may modify the size distribution elsewhere. Even so, the environmental conditions sampled at Mace Head were broad: chlorophyll-a concentrations ranged from approximately 0.1 to 10 milligrams per cubic metre and wind speeds from 5 to 20 metres per second, encompassing a substantial portion of the parameter space that characterizes the global ocean, although not all of it.

The implications extend well beyond correcting a single parameterization. Aerosol-cloud interactions remain among the largest sources of uncertainty in climate projections, and the total aerosol forcing that has masked part of greenhouse warming depends critically on getting natural aerosol backgrounds right. An underestimate of a natural aerosol source does not simply introduce a local error; it propagates into estimates of anthropogenic forcing, because the radiative effect of human-emitted particles is calculated relative to a preindustrial baseline in which marine organics played a role. If models have been undercounting the natural cloud-seeding particles over the oceans, the historical trajectory of aerosol cooling, and by extension the sensitivity of the climate system to greenhouse gases, may need refinement in the marine sector.

The findings also carry practical weight for emerging proposals to brighten marine clouds deliberately as a means of cooling the planet. Marine cloud brightening schemes depend on an accurate characterization of the natural marine aerosol baseline, since the effectiveness of adding particles to low-lying ocean clouds hinges on how many particles are already there and how close the cloud is to saturation with condensation nuclei. A baseline underestimated by a factor of three fundamentally changes those calculations. With the first direct size-resolved measurements now in hand, climate modelers have a concrete observational target to correct their parameterizations against, and atmospheric scientists have a clear agenda: extend such measurements to other ocean basins, different plankton communities and varying sea states, to determine how universal the bimodal signature observed over the North Atlantic truly is. The ocean’s influence on clouds, it turns out, is richer and more finely structured than the models assumed.

Subject of Research: Size-resolved measurement of marine primary organic aerosols and their role as cloud condensation nuclei in climate models

Article Title: Marine organic particles that seed ocean clouds may be three times more abundant than climate models assume

Article References: Marine organic particles that seed ocean clouds may be three times more abundant than climate models assume. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: marine primary organic aerosol, sea spray aerosol, cloud condensation nuclei, aerosol-cloud interactions, climate models, Mace Head, North Atlantic, bimodal size distribution, aerosol indirect effect, marine cloud brightening, phytoplankton, surface tension

The team, led by Bekir Sami Yilbas, began by creating model hydrophobic surfaces on glass slides using functionalized silica nanoparticles roughly 30 nanometers in size, deposited by dip coating from a solution of silane precursors. Scanning electron microscopy revealed a landscape of nano-sized hills and valleys where trapped air acts as a cushion, shortening the three-phase contact line where liquid, solid and gas meet. The resulting surfaces boasted an average contact angle of 142 degrees with a contact angle hysteresis of only about 2 degrees, meaning droplets roll rather than slide under remarkably small driving forces. A spreading coefficient of minus 106.5 millijoules per square meter confirmed that water refuses to wet the coating, the fundamental prerequisite for self-cleaning behavior.

Dust for the experiments was no laboratory substitute but real environmental dust collected from photovoltaic panels in Dammam, Saudi Arabia, laid down in layers about 150 micrometers thick, matching regional accumulation levels. Energy-dispersive spectroscopy showed the dust contained silicon, calcium, sodium, sulfur, magnesium, potassium, iron and chlorine, with salts and clay-aggregated hematite and gypsum among the constituents. Crucially, when droplet water meets this dust, soluble salt components dissolve: inductively coupled plasma mass spectrometry measured sodium concentrations of about 45,300 parts per billion, with potassium and chlorine at similar levels. This dissolution nudged the surface tension of the fluid from 72 to roughly 72.6 millinewtons per meter and shaved the contact angle from 142 to 141.7 degrees, a small shift with measurable consequences for droplet adhesion.

The central mechanism of dust removal is cloaking, the process by which droplet fluid spreads over and infuses individual dust particles, drawing them into the rolling liquid under capillary action. Using high-speed cameras operating at 300 to 500 frames per second, the researchers measured cloaking velocities averaging around 0.3 millimeters per second, decaying almost exponentially with time. The numbers work out strikingly in favor of cleaning: complete cloaking of particles between 1.2 and 10 micrometers takes about 0.05 seconds, during which a 40-microliter droplet travels only about 34 micrometers, far less than its roughly 2-millimeter contact length on the surface. In other words, the droplet lingers over each patch of dust long enough to swallow it before moving on. An Ohnesorge number of about 0.06 confirmed that viscous shear losses during cloaking are negligible compared with the capillary infusion driving the process.

To peer inside the moving droplet, the team built a three-dimensional computational model coupling the Navier-Stokes equations with heat transfer, evaporation and Marangoni stresses, using a level-set scheme to track the water-air interface. Simulations employed up to 543,827 tetrahedral elements after mesh convergence tests showed velocity and pressure differences below one percent against finer grids, with time steps as small as 10 nanoseconds. The model incorporated a dynamic contact angle, a Furmidge-type retention force capturing contact-angle hysteresis, and temperature-dependent fluid properties. Evaporation was included through a diffusion-convection formulation, though over the short rolling durations simulated the droplet lost negligible volume, so evaporation played only a minor role in the overall dynamics.

The simulations exposed an oscillatory interior world. As the droplet rolls, heat from the inclined surface diffuses upward through the contact area, lowering surface tension locally and generating Marangoni currents that swirl fluid through the droplet interior. Bulk-averaged temperature, velocity and pressure all oscillate in time, with the temperature oscillations tied to slight variations in the contact area caused by droplet wobbling, and the faster velocity oscillations reflecting the fluid’s inertial response. Raising the surface temperature from 25 to 60 degrees Celsius lengthens the three-phase contact line, because warmer liquid spreads more readily, which in turn increases the adhesion force and amplifies the amplitude of wobbling. The location of maximum velocity and pressure inside the droplet shifts continuously with time and temperature, reshaping the wetted footprint as the droplet descends.

Wobbling emerged as a star of the show. The droplet’s center of mass shifts as it rolls, stretching the wetted area into an almost elliptical shape and producing a wavy droplet path across dusty surfaces. High-speed recordings and a mode-classification algorithm applied to the video data revealed that the wobbling resembles the l equals 2 oscillation mode, with frequencies between roughly 40 and 32 hertz for droplets from 10 to 50 microliters, matching theoretical predictions from the Rayleigh-like frequency formula. Predicted normalized apex heights of about 1.42 agreed closely with the experimental value of 1.37. Far from being a nuisance, this wobbling temporarily enlarges the wetted region on each oscillation cycle, actively widening the swath of dust removed from the surface.

Dust, however, exacts a price. Experiments on dusted surfaces showed translational velocities consistently below those on clean surfaces, because cloaking spreads fluid over the particles, enlarging the wetted diameter and adhesion force, increasing interfacial friction, dissolving salts that alter surface tension, and loading the droplet with captured mass. By comparing kinetic energies on dusty and clean surfaces, the team quantified the energy dissipated to dust at about 20 percent. They distilled the relationship into a force ratio scaling with distance along the incline, with a prefactor between 0.7 and 2 and an exponent between 0.19 and 0.23 depending on the tilt angle from 10 to 30 degrees. Meanwhile, the ratio of rotational to translational velocity settled near 0.95, slightly below the value of 1 expected for ideal marble-like rolling, a deficit attributed to dynamic pressure in the surrounding air. Only when the rotational Weber number exceeded about 12 did noticeable slip appear, and even then the slip contribution remained a modest 5 percent of translational velocity.

The cleaning efficiency data carry a practical punch. A 40-microliter droplet removed about 2.1 microliters of dust, while a 20-microliter droplet removed 1.34 microliters, so larger droplets clean more area in absolute terms, yet the dust-to-droplet volume ratio actually falls from about 0.067 to 0.046 as droplets grow from 20 to 60 microliters, because dust removal scales more slowly than droplet volume. The ratio of dust removed by a 10-microliter droplet to that removed by a 40-microliter droplet was about 0.74, underlining how strongly droplet size governs the cleaned footprint. The researchers note that their integrated framework of experiments, three-dimensional simulation and analytical modeling provides a superior physical foundation for self-cleaning in energy harvesting and industrial applications, and they plan to extend the work to multiple-droplet systems, where collective rolling dynamics could transform how solar farms and other dust-exposed infrastructure keep themselves clean.

Subject of Research: Three-dimensional rolling dynamics of water droplets on inclined hydrophobic surfaces and their role in environmental dust removal

Article Title: 3D rolling motion of droplet on inclined hydrophobic surface at different temperatures: Environmental dust removal

Article References: Yilbas, B. S., Hegazy, M. A., Hassan, G., Abubakar, A. A., Al-Qahtani, H., Al-Sharafi, A., & Nouioua, M. (2026). 3D rolling motion of droplet on inclined hydrophobic surface at different temperatures: Environmental dust removal. Results in Engineering, 32, Article 113103. https://doi.org/10.1016/j.rineng.2026.113103

Image Credits: AI Generated

DOI: Not provided

Keywords: hydrophobic surfaces, droplet rolling, self-cleaning, dust removal, Marangoni flow, contact angle hysteresis, cloaking, photovoltaic panels, wobbling oscillations, 3D simulation, surface tension, solar energy

The most striking claim in the study is also the easiest to dismiss at first glance: the droplets never actually touch the crease that stops them. Instead, they respond from a distance. When a soft elastic surface is compressed past a critical strain, it buckles into a crease—a sharp, downward fold in which the material folds against itself. Around this fold, the surface deforms and curves in a characteristic way. For a liquid droplet resting on that surface, the local curvature changes the energetic landscape of wetting. The crease effectively raises an energy barrier, an elastic-cum-capillary hill that the droplet must climb to continue its journey. Small droplets, which carry less gravitational and inertial momentum relative to their surface energy, slow down as they approach this barrier and stop short of it. Larger droplets simply plow over the fold and continue on their way. The result is a sharply defined size threshold, a gate that admits droplets above a cutoff and blocks everything below it.

That threshold is not fixed. Because the crease exists only while the surface is compressed, the amount of compression becomes a single mechanical dial that reprograms the gate on the fly. The researchers found that the effect is dramatically nonlinear: squeezing the surface just 15 percent harder quadruples the critical droplet size that can pass. In practical terms, an operator can tune the same physical fold to block droplets of one size in the morning and admit them in the afternoon, without ever patterning the surface or swapping out components. ‘What we did not expect is that drops of different sizes are not sensing the same thing,’ Pandey says. According to the team, large droplets respond primarily to how steep the fold is—the overall geometric sharpness of the deformation—while small droplets respond to how rapidly that steepness changes, a subtler gradient effect tied to the spatial variation of the surface curvature around the crease. This size-dependent asymmetry is what gives the crease its ability to discriminate between droplets with such precision.

The physics behind the phenomenon sits at the intersection of elasticity and capillarity, a field researchers call elastocapillarity. Soft materials deform easily under the pull of surface tension, and liquids in turn feel the geometry of the elastic substrate they rest on. A crease concentrates both effects into a tiny region: the fold itself is a self-contact singularity in the elastic body, and the surrounding deformation field extends outward over a distance set by the elastic modulus and the droplet’s own length scales. A droplet approaching the crease deforms the soft surface beneath it, and that deformation feeds back into the droplet’s motion. Near a crease, this coupled elastocapillary interaction becomes repulsive for droplets below the critical size—the paper’s authors describe the mechanism as elastocapillary repulsion—so the droplet decelerates as if it were rolling up an invisible incline. Nothing in the traditional wetting literature required the barrier to be felt remotely, which is why the distance-sensing behavior caught the team’s attention.

Once a single gate is understood, the natural question is what a network of gates can do. The researchers went well beyond the one-fold demonstration. By arranging creases in deliberate geometries on the compressed surface, they guided droplets along prescribed paths, sorted them by size, and even sorted them by surface tension, since the barrier that repels a droplet depends on its capillary characteristics as well as its dimensions. The creases can store what the team describes as a kind of droplet memory, in which the configuration of the surface keeps track of past inputs—a sequence of droplets leaves a mechanical record in how the gates have been used or the state of the flow. They also showed that a continuous stream of droplets arriving at a crease can be reshaped into fewer, larger pulses, a form of flow coalescence that could be valuable wherever fluid needs to be delivered in discrete, metered doses rather than a dribble of small drops.

The most ambitious demonstration pushes the analogy between droplet gates and electronic circuitry to its logical conclusion: computation. The team routed two separate streams of droplets into a single crease junction to build a half adder, the elementary arithmetic unit that combines two binary inputs into a sum and a carry. In an electronic processor, a half adder is built from transistors; here, it was built from nothing more than the mechanical state of a compressed elastic surface and the trajectories of liquid droplets. The logic is embodied in geometry: whether a droplet from one stream arrives at the junction and crosses, stops, or merges with a droplet from the other stream depends on the gate state, and the outputs can be read from the flow emerging on the far side. Because the creases appear and vanish with compression, the entire circuit is rewritable. ‘We can switch a gate off, let everything through, and switch it back on,’ Pandey explains. ‘Nothing is permanently patterned into the surface, and the control comes down to a single mechanical variable.’ That last point matters enormously for anyone who has tried to reconfigure a microfluidic chip: conventional devices encode their channels and valves in lithographically etched structures, so changing the routing means fabricating a new chip. A crease circuit changes its mind instantly.

The ability to manipulate droplets without batteries, motors, valves, or embedded circuitry is what gives the technique its practical appeal. Portable diagnostic devices—lateral-flow tests and the growing family of microfluidic assays designed to analyze tiny samples of blood, saliva, or other fluids—depend on moving, metering, and merging microliter volumes reliably and cheaply. A surface that sorts droplets by size or composition, merges them on demand, and holds a record of what has passed through could perform some of that fluid management passively, driven purely by the mechanics of the substrate. The same qualities suit the approach to water harvesting: systems that collect drinking water from fog or humid air rely on capturing, coalescing, and transporting droplets, and a creased surface that passively gates and reshapes droplet flow could improve how efficiently collected water is channeled and consolidated.

The research also carries implications for anyone working with soft materials, where creases are usually regarded as a nuisance. Compressible gels, elastomers, and biological tissues develop creases under load, and engineers typically design against them, treating the folds as failure modes or sources of unwanted friction and adhesion. This study reframes the fold as a functional element—a component rather than a defect. In that sense the work belongs to a broader movement in soft-matter engineering that treats mechanical instabilities as programmable resources: buckling plates that morph into target shapes, wrinkles that encode strain history, and now creases that compute with droplets. The underlying physics, elastocapillary repulsion mediated by substrate deformation, is likely to operate in any system where a liquid moves across a surface soft enough to respond to it, from lab-on-chip devices built from hydrogels to condensation on flexible coatings.

The study, titled ‘Creases gate and steer droplets via elastocapillary repulsion,’ was published in the Proceedings of the National Academy of Sciences on September 22, 2026. Alongside Pandey, the co-authors are Zixuan Wu, a postdoctoral researcher in Pandey’s group at Syracuse University; Gavin Linton, an undergraduate in mechanical and aerospace engineering; and Stefan Karpitschka, a professor of physics at the University of Konstanz in Germany. The collaboration brought together expertise in soft-matter mechanics at Syracuse and in capillary dynamics at Konstanz, a pairing well suited to a phenomenon that lives precisely at the boundary between the two disciplines.

For now, the demonstration remains at the level of laboratory surfaces and dyed droplets lining up along a fold, smaller drops halted before the crease while larger ones sail across. But the conceptual payoff is immediate. It establishes that a single mechanical variable—compression—can program a hierarchy of fluidic behaviors: gating by size, sorting by surface tension, path guidance, pulse formation, memory, and even arithmetic. In a field where adding functionality usually means adding components, a technology that achieves all of this by simply squeezing a sheet of rubber, and reverses it by letting go, offers an unusually elegant answer to the question of how small, cheap, and simple fluid control can become. The traffic lights of the microfluidic world, it turns out, may need nothing more than a wrinkle in the road.

Subject of Research: Elastocapillary gating of droplets by compression-induced creases in soft elastic surfaces

Article Title: New research shows how engineers turn tiny creases into droplet traffic controllers

Article References: New research shows how engineers turn tiny creases into droplet traffic controllers. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: droplets, creases, soft matter, elastocapillarity, microfluidics, surface tension, elastic surfaces, droplet sorting, logic operations, water harvesting, diagnostics, Syracuse University

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

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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