Ferrofluids are stable colloidal suspensions of magnetic nanoparticles dispersed in a carrier liquid, and they have emerged as remarkably versatile materials across medicine and engineering. Their dual ability to flow like a liquid and respond to external magnetic fields underlies a wide range of functionalities. In the medical field, they serve as contrast agents to enhance magnetic resonance imaging, and their use in targeted drug delivery systems is being investigated. In engineering, they are used to develop magnetic seals, vibration dampers, and smart lubricants whose viscosity can be tuned on demand. They also hold potential for solar energy systems and can considerably enhance heat transfer efficiency in thermal engineering applications such as cooling systems for electronics, with additional uses in material separation, sensor technology, and energy harvesting devices. When exposed to magnetic fields, ferrofluids can adjust both their viscosity and their thermal conductivity, which makes them useful across a broad span of technological applications.

Characterizing how these fluids flow is a critical aspect of their study, but it is far from straightforward. The rheological response depends on many variables, including particle size distribution, suspended solid concentration, interactions with surfactants, surface free energy, temperature, and impurities in the crystalline structure. The intensity and direction of the applied magnetic field directly influence the response of the fluid, affecting its viscosity, stability, and heat-transfer capacity. Because of this complexity, no universal mathematical relationships that accurately describe the flow behavior of ferrofluids have been proposed, so specific rheological models must be developed to predict their behavior under different operating conditions. Flow properties are typically characterized with rotational rheometers, which correlate rotational speed with shear rate and applied torque with shear stress through geometric constants, allowing stress-versus-shear-rate curves to be constructed and viscosity to be determined at specific shear rates or temperatures.

Specialized commercial equipment can determine the rheological properties of ferrofluids under magnetic fields, such as the Anton Paar magnetorheological characterization accessory compatible with MCR rheometers. However, the researchers note that such systems carry significant limitations: their high cost restricts access, and their poor compatibility with other commercial rheometer references confines measurements to laboratories with particular instrument brands. These constraints motivated the development of an accessory that facilitates ferrofluid characterization through versatile and accessible experimental setups, opening magnetorheological testing to laboratories that use different measurement systems.

The newly developed accessory consists of a central structure coupled to a positioning base and four arms for locating permanent magnets. Each arm includes six guides for magnet capsules, allowing different magnet-to-sample distances and, consequently, different magnetic field intensities. The central structure contains a compartment designed to accommodate a fluid-containing cup with an outer diameter of up to 83 millimeters, leaving space between the cup and the magnetic assembly for a non-magnetic insulating layer or future thermal-control elements. For the Rheotest RN 5.1 configuration, whose cup has an outer diameter of 43 millimeters, this compartment provides an available radial clearance of approximately 20 millimeters, enough to accommodate a non-magnetic insulating layer such as silicone or silicone-foam sheets and potentially allow future integration of a thin heating jacket. Because heating reduces the effective insulation thickness, the team advises that elevated-temperature operation should be performed only with temperature monitoring of the cup, magnets, and printed components, keeping the printed structure below a conservative limit of approximately 45 degrees Celsius and the magnets within their safe operating range.

Importantly, the printed accessory never contacts the tested fluid. The sample remains confined within the rheometer cup, while the accessory acts purely as an external magnetic-field positioning system, so measurement accuracy remains governed by the commercial rheometer’s calibration and specifications. A centralizing element functions as a coaxial alignment platform, ensuring that the rotor’s rotation axis aligns with the symmetry axis of the accessory and that the measuring cup is inserted to an adequate depth within the generated magnetic field. The magnet-holding capsules were designed with dimensions of 46 by 21 by 10 millimeters, matching the neodymium magnets used in the setup and ensuring full coverage of the deformation region defined by the selected rheometric geometry. The capsule design can be adapted to accommodate other commercial magnet sizes, making the system compatible with different rotational rheometers or viscometers. Protective covers mount on the arms to prevent accidental displacement of the magnets caused by strong repulsive or attractive magnetic forces from nearby metallic objects.

A separate gauge assembly was developed to characterize the magnetic field generated by each magnet configuration before testing. This element positions a gaussmeter probe at predefined Cartesian coordinates, replicating the relative magnet positions of the application accessory. Using the side inlets, measurements can be performed in the xy plane at 12 positions corresponding to the top of a single quadrant, and owing to the assembly symmetry, up to 40 measurements can be performed for the entire area containing the ferrofluid. Three variants of top inlets allow measurements in the xz and yz planes at different depths along the z axis, coinciding with the positions of the lateral inlets. The magnitude of the resulting field is then calculated by vector combination of the components measured in the three planes. The gauge is used independently before rheological measurements; during testing, only the magnetic-field application accessory is positioned around the rheometer cup.

The device was fabricated by fused filament fabrication, an accessible and cost-effective 3D printing technique that enables rapid iteration of parts with complex geometries. Polylactic acid was chosen for its dimensional stability, biodegradability, low thermal shrinkage, and adequate mechanical strength for structural laboratory components. The components were printed on an FLSUN Super Racer 3D-FDM printer equipped with a 0.2-millimeter nozzle at a printing speed of 100 millimeters per second and a layer height of 0.2 millimeters. A total of 1,375 grams of PLA was used to form the magnetic assembly, with a total printing time of 64 hours. The bill of materials shows that the printed polymer parts account for only a small share of the cost; the largest single expenses were an USD 89 gaussmeter and four magnets at USD 10 each. Although developed around the Rheotest RN 5.1, which measures viscosities from 5 to 105 millipascal-seconds at shear rates up to 2,000 per second, the accessory is also compatible with other column-type rotational rheometers, such as those manufactured by T.A. Instruments, PCE Instruments, and Thermo Fisher, provided a custom centralizer is designed to match the cup diameter.

To validate the assembly’s functionality, the team performed rheological tests using magnetite ferrofluids in an ethylene glycol base at six volumetric concentrations: 0.01, 0.05, 0.1, 0.3, 0.5, and 1 percent. The particles, sized between 60 and 100 nanometers, are larger than those commonly reported for classical ferrofluids but remain within the nanoscale and are considerably smaller than the micrometer-sized particles typically used in conventional magnetorheological fluids. The validation was designed as a functional evaluation of the accessory rather than a full metrological comparison with a commercial magnetorheological system. Each test applied a preconditioning phase at a constant shear rate of 20 per second for 10 seconds, followed by a ramp-up from 0 to 200 per second in 60 seconds and a ramp-down from 200 to 0 per second in 60 seconds, with every test replicated three times. The applied field was not assumed to be spatially uniform; instead, its distribution was characterized with the gauge element, with measured components combined vectorially to estimate the resulting field magnitude. Across the tested magnet arrangements, the strongest characterized configuration reached 269.6 gauss, corresponding to arrangement 2 at the π/2 arm setting, while the weakest measured 24.6 gauss.

The average shear stress versus shear rate curves showed a Bingham plastic response, marked by an approximately linear relationship between shear stress and shear rate after flow onset and a non-zero stress intercept associated with the apparent yield stress. At the lowest concentration, 0.01 percent, the curves were not clearly differentiated by the applied magnetic field, indicating minor field-induced changes under highly diluted conditions. As concentration increased, the curves shifted toward higher stress values mainly through an increase in the apparent yield stress, while their slopes remained relatively similar. Plastic viscosity stayed close to 17 millipascal-seconds across field intensities and remained within a narrow range of roughly 16 to 18 millipascal-seconds for most field conditions, indicating that concentration did not substantially modify the viscous contribution. In contrast, the apparent yield stress showed a stronger dependence on both solid concentration and magnetic field intensity, with its regression slope increasing markedly at higher fields, especially around 105 and 167 gauss. The team concludes that the dominant field-induced effect in the tested range was particle structuring that increased the stress required to initiate flow rather than a significant increase in plastic viscosity.

The authors acknowledge several limitations. Temporal variations of the magnetic field during each run were not directly monitored, although tests were performed at room temperature without active heating using short measurement sequences, so significant thermally induced field variations were not expected. Continuous field monitoring and quantitative temporal stability assessment remain to be addressed in future work, and temperature profiles of the fluid, magnets, and printed components were not measured, so high-temperature operation was not experimentally validated. The durability of the PLA components may be affected by elevated temperature, humidity, ultraviolet radiation, chemical cleaning agents, or accidental damage, and the arm slots, magnet capsules, covers, and centralizing elements should be visually inspected before use and replaced if deformation, cracking, looseness, or loss of magnet positioning accuracy is observed. Within these operating limits, the researchers present the low-cost modular assembly as a practical platform for evaluating magnetic suspensions with different formulations, concentrations, and field configurations, bringing magnetorheological characterization to laboratories that previously lacked the budget or instrument brand for specialized commercial systems.

Subject of Research: Low-cost modular system for magneto-rheological testing in rheometers using 3D printing

Article Title: Low-cost modular system for magneto-rheological testing in rheometers using 3D printing

Article References: Álvarez-Gil, L., Alcalde-Castro, J. J., Saldarriaga-Agudelo, W., & Rojas-Reyes, N. (2026). Low-cost modular system for magneto-rheological testing in rheometers using 3D printing. HardwareX, Article e00853. https://doi.org/10.1016/j.ohx.2026.e00853

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00853

Keywords: Low-cost, modular, system, magneto-rheological, testing, rheometers, printing, scientific research