The study, published in the journal Advances in Industrial and Engineering Chemistry, was conducted by Jong-Hyun Kim and Dong-Jun Kwon of Gyeongsang National University together with Pyeong-Su Shin of the Korea Institute of Convergence Textile. The researchers set out to answer a deceptively simple question: when you want to know how strongly an epoxy resin will grip a glass fiber, should you trust the angle formed by a droplet sitting on the surface, or the forces recorded as a sample is dipped into a liquid? Their answer, backed by hundreds of measurements, favors the dynamic approach.

The logic behind contact angle analysis is elegant. When a drop of water, diiodomethane, formamide or ethylene glycol is placed on a solid, the shape of that drop reflects the balance of surface tensions at the solid, liquid and air interfaces. By measuring angles with several liquids of known surface tension, researchers can decompose a material’s surface energy into dispersive and polar components using the Owens-Wendt equation, and from there calculate the work of adhesion between two materials. In theory, a higher work of adhesion should translate directly into a stronger interface, quantified as interfacial shear strength, or IFSS, the stress needed to shear the fiber out of its resin jacket.

The team compared two measurement philosophies. The static method is the one most laboratories know well: deposit a droplet, photograph it, and read the angle. For flat epoxy plates the droplet is about a millimeter across; for cylindrical glass fibers, the researchers had to work with droplets of roughly 100 micrometers, either resting on the fiber or embedded along it, and extract the angle from the droplet geometry using the Pythagorean theorem and Young’s equation. The dynamic method, by contrast, never looks at a droplet at all. Instead, a DCAT 11 instrument lowers the sample into a test liquid at a controlled speed of one millimeter per minute to a depth of two millimeters, and records the weight change. From the forces during immersion and withdrawal, the advancing and receding contact angles are calculated through the Wilhelmy principle.

The static method’s weakness revealed itself quickly. On glass fibers, the measured contact angle changed not only with the solvent but also with the size of the droplet, even when the surface tension stayed the same. Large droplets sag under their own gravity, distorting the geometry that the angle calculation depends on. Small droplets evaporate rapidly, so the angle captured in the photograph is often a receding angle rather than the advancing angle that theory requires. The researchers also noted that image quality, resolution, brightness, contrast and viewing angle all introduce errors, and that even a carefully polished surface can carry dust or microcracks that skew the result. In short, the static measurement is hostage to a long list of experimental subtleties.

To settle the question, the researchers built four epoxy systems from two resins with different numbers of epoxide groups, DGEBA and TGDDM, and four hardeners with distinct chemistries: an acid anhydride, an amino hardener, and two variants of diaminodiphenyl sulfone known as 33DDS and 44DDS. They measured surface energies for each cured resin and for the glass fiber, computed the work of adhesion for every fiber-resin pairing by both methods, and then measured the true interfacial shear strength with the microdroplet pull-out test. In that test, tiny resin droplets are cured onto single fibers and pulled off in a universal testing machine fitted with a custom microvice, forty times per condition, at a crosshead speed of 0.5 millimeters per minute. The debonding force, divided by the fiber circumference and embedded length, yields the IFSS.

The pull-out results crowned a clear winner among the resins: the TGDDM/33DDS system, whose high crosslink density and stiffness produced the strongest bond to the glass fiber. But when the researchers checked whether the work of adhesion values predicted that ranking, the two measurement methods diverged sharply. The static method produced work of adhesion values around 100 dyne per centimeter that failed to track the IFSS trend at all. The formulation with the highest measured shear strength showed a comparatively low static work of adhesion, while resins with low amino content scored high adhesion values despite weaker interfaces. The dynamic method returned lower absolute values, near 60 dyne per centimeter, but crucially its ranking of the four resins mirrored the ranking from the mechanical pull-out tests.

The explanation lies in how much contact each method actually creates. During dynamic immersion, the liquid wets a large and uniform area of the sample, so the recorded forces integrate genuine chemical interactions across the whole solid-liquid interface. A static droplet, by contrast, touches only a small patch, and its shape is at the mercy of gravity, evaporation and gradual impregnation of the liquid into the substrate over time. The researchers also observed measurement noise in the dynamic force curves for single fibers, a consequence of the tiny sensing area, yet the weight-based data still proved more quantitative and reproducible than droplet photography. For micro-scale fibers in particular, the sensitivity of the balance becomes the decisive factor in measurement quality.

The implications reach well beyond epoxy and glass. Contact angle measurements are a standard tool for evaluating sizing agents on carbon fibers, plasma treatments on basalt, and surface modifications across the fiber-reinforced composites industry. If static droplet measurements can invert the apparent ranking of materials, then design decisions based on them, from sizing formulations to resin selection, may rest on shaky ground. The Korean team’s conclusion is pragmatic: the static method remains attractive for its simplicity, but for reliable analysis of interfacial properties, the dynamic contact angle method is the more appropriate choice, because it is the one whose surface energy predictions actually agree with what happens when fiber meets resin under load.

There is a broader lesson in the result about the gap between thermodynamic prediction and mechanical reality. The work of adhesion is a theoretical quantity, calculated from idealized surfaces and equilibrium angles, while interfacial shear strength reflects crosslink density, matrix modulus, curing conditions and the messy mechanics of debonding. The dynamic method, by sampling a larger interface and delivering advancing angles uncontaminated by evaporation or gravity, comes closer to bridging that gap. As composites push into ever more demanding applications, the study suggests that the humble dipping experiment, harder to perform but richer in information, deserves a more central place in the materials laboratory than the familiar photograph of a droplet.

Subject of Research: Comparison of static and dynamic contact angle methods for predicting fiber-matrix interfacial properties in epoxy-glass fiber composites

Article Title: Evaluation of static and dynamic contact angle methods for their correlation with interfacial properties in composites

Article References: Evaluation of static and dynamic contact angle methods for their correlation with interfacial properties in composites. (n.d.). https://doi.org/10.1007/s44405-025-00016-x

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00016-x

Keywords: contact angle, composites, glass fiber, epoxy, surface energy, interfacial shear strength, work of adhesion, microdroplet pull-out test, dynamic contact angle, static contact angle, fiber-matrix interface, Owens-Wendt equation

Rubrene has long been a benchmark material in the organic electronics community. In single-crystal form it can carry charges at mobilities of up to 20 square centimeters per volt per second, rivalling some amorphous inorganic semiconductors. The catch is that those spectacular numbers come from painstakingly grown individual crystals, not from the thin films that real devices need. When rubrene is evaporated directly onto silicon dioxide, the standard insulator of transistor technology, thermal annealing rarely produces uniform, large-area crystalline domains. The result is a patchwork of grains with wildly varying quality, and devices whose performance cannot be trusted from batch to batch. Reproducibility, not raw mobility, has become the bottleneck for commercialization.

The Korean team’s strategy was to insert an ultrathin polystyrene interlayer between the silicon dioxide and the rubrene, and then to engineer that interlayer’s chemistry with remarkable precision. They prepared three versions of the polymer surface. The first was plain, as-cast polystyrene. The second, which they call PS-x, was irradiated for twenty minutes with 365-nanometer ultraviolet light in ambient air. The third, dubbed PS-net, received the same UV dose and was then rinsed briefly in isopropyl alcohol. Twenty nanometers of rubrene were subsequently evaporated onto each surface under high vacuum, and the films were annealed in a nitrogen glovebox.

The ultraviolet treatment sounds crude, but it triggers a rich photochemistry. UV photons cleave carbon-hydrogen bonds in the polystyrene backbone, generating carbon radicals that react with oxygen in the air to form peroxy species. Some of these radicals link neighboring polymer chains together into a crosslinked network, while others drive the opposite process, beta-scission and ring-opening reactions that chop the polymer into small, polar, low-molecular-weight fragments. Fourier-transform infrared spectroscopy confirmed the chemical transformation: the characteristic benzene-ring and carbon-hydrogen absorption bands weakened after irradiation, while new carbonyl signals appeared between 1700 and 1850 per centimeter, marking the increased polarity of the modified surface.

The decisive evidence for crosslinking came from a clever solubility test. The researchers patterned UV light onto selected regions of a polystyrene film and then rinsed the whole sample in isopropyl alcohol, a solvent that normally cannot touch pristine polystyrene. Scanning electron microscopy showed that only the irradiated regions survived the rinse. Crosslinking had rendered the exposed polymer insoluble, while the untouched areas dissolved away. The subsequent alcohol wash on the PS-net samples serves a second, subtler purpose: it strips away the soluble chain-scission fragments, leaving behind a cleaner, purely crosslinked network at the surface.

When rubrene was deposited and annealed on these different surfaces, the contrast was dramatic. On bare silicon dioxide and on untreated polystyrene, no crystalline domains appeared at all, even after annealing. On PS-x, spherulitic crystals formed only partially and unevenly. But on PS-net, the film blossomed into highly uniform spherulitic domains that swept across the entire surface. Polarized optical microscopy, which exploits the birefringence of rubrene crystals, revealed a classic nucleation-and-growth mechanism: new nuclei appeared during annealing, expanded radially, and after roughly sixty seconds the whole film had crystallized. Prolonged annealing beyond that point caused no further ripening, indicating the crystals had settled into a deeply stable energy state, with the final grain size fixed by the early nucleation density.

The speed is the headline. Previous polymer-assisted approaches to rubrene crystallization were far slower. One study by Bronshtein, Rybtchinski and colleagues needed more than 300 seconds of rapid thermal processing under high vacuum with argon flow to grow single crystals from a rubrene-polystyrene blend. Another group required roughly ten minutes of solvent vapor annealing in chloroform. The new interlayer method achieves uniform, large-area crystal growth in just sixty to ninety seconds, a tenfold or better acceleration that matters enormously for manufacturing throughput and, crucially, avoids the solvent-blending route whose crystal growth is notoriously sensitive to solvent choice and ambient conditions.

Temperature proved to be a razor-thin dial. Below 130 degrees Celsius, nothing crystallized; the thermal energy was simply insufficient to mobilize the rubrene molecules. At 140 degrees, small domains emerged that previous work assigns to the triclinic crystal phase. Between 150 and 160 degrees, triclinic regions gradually gave way to orthorhombic domains, the crystal structure favored for charge transport. Only in the narrow window of 170 to 180 degrees did large orthorhombic spherulites blanket the entire film, with triclinic domains vanishing entirely. That window, consistent with earlier findings by Holmes and Fielitz, is punishingly tight, and it underscores how little slack process engineers have when optimizing organic transistor fabrication.

What makes the crosslinked interlayer so special? Surface measurements offered clues but no complete answer. Atomic force microscopy showed all three polymer films were remarkably smooth, with roughness below one nanometer, though PS-net was slightly rougher at 0.804 nanometers than its counterparts at roughly 0.49 nanometers, perhaps aiding nucleation. Contact-angle measurements traced a rising surface energy, from 27.5 millijoules per square meter for pristine polystyrene to 38.5 for PS-x and 44.1 for PS-net, the latter closely matching rubrene’s own surface energy. Yet the energy-matching story collapsed as a full explanation: silicon dioxide and PMMA, with even higher surface energies, supported no polycrystalline growth, while HMDS-treated silicon dioxide, whose surface energy resembles PS-net, also failed. Something beyond wetting physics was at work.

The missing ingredient, the authors argue, is the thermal dynamics of the interlayer itself. Differential scanning calorimetry showed a clear glass transition for pristine polystyrene but none for the UV-modified films, a signature of restricted segmental motion caused by crosslinking. In the 170-to-180-degree window, the crosslinked network apparently provides just enough surface mobility for rubrene molecules to diffuse and reorder into orthorhombic crystals. Too rigid a surface, like HMDS-treated silicon dioxide, blocks diffusion; too mobile a surface, like linear polystyrene, lets the polymer chains churn and disrupt ordering. The residual chain-scission fragments in unrinsed PS-x likely interfere with long-range molecular ordering, echoing earlier pentacene studies in which shortened polystyrene chains suppressed surface diffusion. The researchers caution that this mechanistic picture remains partly speculative, but the practical recipe is clear: remove the unstable fragments, tune the crosslinking, and the polymer underlayer becomes a template that guides small molecules into the crystal phase devices need. For organic field-effect transistors, where structural uniformity and batch-to-batch reproducibility are prerequisites for any commercial future, a sixty-second annealing step on a UV-crosslinked plastic film may prove to be exactly the kind of unglamorous enabling technology the field has been waiting for.

Subject of Research: Interfacial engineering of polymer interlayers to control rubrene thin-film crystallization for organic electronics

Article Title: Interfacial engineering of rubrene thin films with crosslinked polymer layers

Article References: Im, J., & Choi, H. H. (2025). Interfacial engineering of rubrene thin films with crosslinked polymer layers. Advances in Industrial and Engineering Chemistry, 1(1), Article 20. https://doi.org/10.1007/s44405-025-00023-y

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00023-y

Keywords: rubrene, organic semiconductors, polystyrene, crosslinking, spherulites, thin films, crystallization, surface energy, UV irradiation, organic field-effect transistors, polymer interlayers, thermal annealing

The challenge the team set out to solve is fundamental rather than merely technical. Two-dimensional intermetallic compounds, in which two or more metal species arrange themselves into ordered crystallographic sublattices, and high-entropy metallic alloys, in which four or more elements share a single disordered lattice, are both thermodynamically precarious when squeezed into a few atomic layers. Their intrinsic instability means that conventional vapor-phase or solution-based growth tends to produce three-dimensional particles rather than flat crystals, and kinetic factors during growth are equally difficult to control. Without a way to stabilize flat nuclei and steer their lateral expansion, the rich emergent physics predicted for these systems remained largely out of experimental reach.

The researchers call their solution an adsorption–conversion strategy, and its mechanics are worth unpacking. In a typical growth run, a metal halide precursor such as copper chloride is deposited onto a metallic matrix, for example copper germanide, in the presence of a potassium chloride salt flux. As the precursor decomposes on the hot surface, the liberated metal atoms do not simply sit there: they embed themselves into the metallic matrix below, converting the local composition into a new intermetallic phase. Meanwhile, the residual chlorine atoms adsorb onto the exposed crystal faces. Theoretical calculations show that this double action is what tips the energetic balance in favor of two dimensions.

The numbers from the team’s density functional theory calculations illustrate the effect vividly. For the copper–germanium system, embedding additional copper into a copper germanide matrix to form the richer Cu3Ge phase lowers the surface energy of the (001) facet from 0.069 electronvolts to 0.044 electronvolts, a critical first step that triggers the conversion of bulky three-dimensional Cu2Ge particles into flat Cu3Ge nanostructures. Chlorine adsorption then drives the surface energy down further, to 0.01 electronvolts. Because different crystal facets respond differently to chlorine, the energy gap between competing faces widens, and in the language of classical crystal morphology dating back to Wulff’s 1901 analysis, the crystal is pushed to grow laterally as thin plates rather than isotropically as cubes. The potassium chloride flux plays a supporting role, enhancing the wettability and fluidity of the molten system so that the flat nuclei can spread into large, uniform nanoflakes.

Control experiments confirm that both ingredients are essential. When only potassium chloride is present, the growth yields nothing but three-dimensional particles. Replace it with copper chloride, and two-dimensional nucleation begins. The team extended the same logic to iron, demonstrating through ab initio molecular dynamics simulations of the iron chloride and iron germanide interface that the identical mechanism of metal incorporation and chlorine-mediated surface stabilization operates there too, underscoring the generality of the approach rather than a lucky quirk of one chemistry.

The breadth of the resulting materials library is arguably the paper’s most striking feature. Using the adsorption–conversion route, the researchers synthesized ultrathin single crystals based on iron, cobalt, nickel, copper and palladium, spanning ordered intermetallics such as Fe3Sn2, Pd2Bi3, Ni3Sn2 and Co6Ga, multicomponent compounds such as CoFe2Ge2 and (Fe, Co, Cu)3Sn2, and genuine high-entropy alloys including (Cr, Fe, Co, Cu)3(Ge, Sn, Sb)2. Atomic-resolution scanning transmission electron microscopy confirmed that the flakes are high-quality single crystals with well-ordered atomic arrangements, and energy-dispersive mapping showed homogeneous elemental distributions without discernible aggregation even in the most compositionally complex samples. Most of the twenty-five characterized nanoflake compositions could be thinned below five nanometers, with the thinnest, Pd2Bi3, reaching just 1.9 nanometers.

Synthesis did not proceed blindly; the team distilled clear design principles that should guide future attempts. The valence electron concentration of the constituent elements governs which crystal lattice is stable, with higher values favoring close-packed hexagonal and face-centered structures and lower values permitting body-centered arrangements, while substrate symmetry can occasionally impose hexagonal stacking even on bcc-forming chemistries through quasi-van-der-Waals epitaxy. Oxygen affinity proved to be the other decisive variable. An Ellingham-type analysis of oxidation free energies explains why early transition metals such as vanadium, chromium and manganese are difficult to incorporate, since their high oxygen affinity leads to oxide contamination, whereas late transition metals like copper, palladium and nickel form readily. Even the difficult cases are not hopeless: when a hygroscopic precursor such as vanadium dichloride is rigorously dried in an oxygen-free environment, two-dimensional vanadium antimonide can indeed be grown.

The payoff for all this synthetic effort shows up in the electrical measurements. The ultrathin alloys exhibit conductivities reaching 3 × 10^7 siemens per meter, a figure that places them among the most conductive thin-film materials ever reported, and an ultralow temperature coefficient of resistance of just 2 parts per million per kelvin. That second number is the one likely to excite engineers: a resistance that barely changes between room temperature and cryogenic conditions is exactly what precision electronics, sensors and metrology instruments demand, and high-entropy alloys have long been theorized to achieve such behavior by balancing multiple electron-scattering channels. Magnetotransport measurements on CoFe2Ge2 flakes of roughly 9 and 22 nanometers revealed room-temperature ferromagnetism in both thickness regimes, opening a path toward combining high conductivity, temperature-stable resistance and magnetism in a single ultrathin platform.

What emerges from this work is less a single new material than a versatile materials platform. By establishing a reproducible, mechanistically understood route to two-dimensional single crystals of intermetallics and high-entropy alloys across much of the transition-metal series, the researchers have handed condensed-matter physicists a fresh playground in which ordered and disordered metallic lattices can be stacked, patterned and tuned at the nanometer scale. The same platform may serve technologists pursuing interconnects, magnetic devices and precision resistors. Given that the field of two-dimensional metals only reached the ångström thickness limit in 2025, the rapid arrival of a general synthesis for ultrathin intermetallics and high-entropy alloys suggests that the metallic frontier of two-dimensional materials science is now opening in earnest.

Subject of Research: Synthesis of two-dimensional intermetallic compounds and high-entropy metallic alloys via an adsorption–conversion growth strategy

Article Title: Adsorption–conversion synthesis of ultrathin intermetallics and high-entropy alloys

Article References: Zhang, P., Zhao, F., Wang, X., Wang, H., Si, K., Gong, Y., Li, B., Wei, J., Jia, Y., Gao, B., Liu, Z., Zou, X., & Duan, X. (2026). Adsorption–conversion synthesis of ultrathin intermetallics and high-entropy alloys. Nature Materials. https://doi.org/10.1038/s41563-026-02752-x

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02752-x

Keywords: two-dimensional materials, intermetallic compounds, high-entropy alloys, adsorption–conversion synthesis, chloride-mediated growth, surface energy, single crystals, electrical conductivity, temperature coefficient of resistance, ferromagnetism, nanoflakes, materials synthesis