Every high-performance composite, from a wind turbine blade to an aircraft fuselage, is only as strong as the invisible boundary where fiber meets resin. For decades, materials scientists have tried to predict that strength by measuring how liquids spread across surfaces, a technique known as contact angle analysis. Now a team of Korean researchers has delivered a finding that could change a routine laboratory practice: the most popular way of measuring contact angles, the quick and simple static droplet method, may be quietly misleading engineers about how well fibers and matrices actually bond.
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
Cite Scienmag News
APA MLA Chicago
Bethany Barker. (October 2, 2026). Dynamic Contact Angle Wins: New Study Rethinks How We Measure Composite Interfaces. Scienmag. https://scienmag.com/dynamic-contact-angle-wins-new-study-rethinks-how-we-measure-composite-interfaces/
Bethany Barker. “Dynamic Contact Angle Wins: New Study Rethinks How We Measure Composite Interfaces.” Scienmag, 2 October 2026, https://scienmag.com/dynamic-contact-angle-wins-new-study-rethinks-how-we-measure-composite-interfaces/. Accessed 2 October 2026.
Bethany Barker. “Dynamic Contact Angle Wins: New Study Rethinks How We Measure Composite Interfaces.” Scienmag. October 2, 2026. https://scienmag.com/dynamic-contact-angle-wins-new-study-rethinks-how-we-measure-composite-interfaces/
Copy citation Download RIS
Tags: advances in industrial and engineering chemistrycomposite interface bondingcompositescontact anglecontact angle measurementdynamic contact angledynamic contact angle analysisepoxyepoxy resin bondingfiber-matrix interfacefiber-resin adhesionglass fiberimplications for wind turbine and aircraft composite designinfluence of contact angle measurement techniquesinterfacial shear strengthlaboratory practices in interface characterizationmaterials science researchmicrodroplet pull-out testOwens-Wendt equationstatic contact anglestatic droplet methodsurface energysurface wettabilitywork of adhesion
