Coconut oil nanofluids with ZnO nanoparticles show dramatic heat-transfer gains in lab tests

The work, published in Discover Nano, compares the polymeric stabilizer polyvinylpyrrolidone (PVP), the anionic surfactant sodium dodecyl sulphate (SDS) and the chelating dye Eriochrome Black T (EBT) at loadings of 0.5, 1.0 and 1.5 percent by weight.

The motivation is environmental as much as thermal. Conventional mineral-oil cutting fluids are widely criticized for toxicity, poor biodegradability and difficult disposal, so modern manufacturing has been pushed toward bio-based alternatives. Coconut oil was chosen as the base fluid because of its high lubricity, oxidative stability and rich content of saturated medium-chain triglycerides. Earlier studies cited by the team indicate that dispersing metal oxide nanoparticles such as zinc oxide, zirconia and alumina in vegetable oils can lower cutting temperature, tool friction and energy use under minimum quantity lubrication conditions, although the new study itself did not include any machining trials.

Zinc oxide was the particle of choice because of its chemical stability, low cost and high intrinsic thermal conductivity. The researchers procured nanoparticles of 99 percent purity and examined them by scanning electron microscopy, which revealed agglomerated clusters with primary particle sizes mainly in the 150 to 320 nanometer range. Drawing on prior literature showing that zinc oxide concentrations below 2 percent deliver substantial thermal conductivity gains while keeping viscosity rise and sedimentation under control, the team fixed the particle loadings at 0.5, 1.0 and 1.5 percent, with surfactant concentrations matched at the same nominal levels.

Preparation followed a two-step protocol without ultrasonication. For each 60-milliliter sample, the surfactant was first dissolved into pre-measured coconut oil by magnetic stirring at 750 revolutions per minute and 45 degrees Celsius for ten minutes. The calculated mass of zinc oxide nanoparticles was then added slowly under continuous stirring, and the mixture was stirred for a further 60 minutes under the same conditions, giving 70 minutes of total mixing. The authors selected these moderate conditions to avoid thermal degradation of the vegetable oil matrix and to prevent sedimentation, taking operating parameters from earlier published work.

Every sample, along with pure coconut oil as a baseline, went through eight characterization tests, all completed within three hours of preparation and repeated three times, with reported standard deviations below 2 percent. Thermal conductivity was measured with a transient hot-wire instrument from Flucon GmbH, viscosity with a Redwood Viscometer No. 1 at 25 degrees Celsius, and density gravimetrically using a calibrated balance. Cloud and pour points were determined with an EIE Instruments apparatus following ASTM D97-based procedures, while flash and fire points were measured with a Pensky-Martens tester in which an ignition source was applied at 2-degree intervals. Chemical characterization was performed by Fourier transform infrared spectroscopy on a Bruker ALPHA II over a range of 4000 to 500 wavenumbers.

The headline result concerns thermal conductivity. Pure coconut oil measured 0.211 watts per meter-kelvin, and every nanofluid exceeded it. The highest value, 1.139 plus or minus 0.023 watts per meter-kelvin, belonged to sample 18MCZE15, the formulation containing 1.5 percent zinc oxide stabilized with 1.5 percent EBT, an enhancement of 439 percent with a 95 percent confidence interval of plus or minus 16 percent. The authors themselves flag this figure as surprisingly high compared with many published studies of low-loading nanofluids, and they offer only speculative explanations, citing possible interfacial effects, partial clustering or percolation, and surfactant-induced surface states. They note that a precise assignment of the dominant mechanism would require targeted XPS and EXAFS measurements, which are planned as future work.

Across concentrations, EBT-stabilized samples averaged 1.130 watts per meter-kelvin, consistently outperforming the other two stabilizers. The team’s proposed explanation is that coordination interactions between EBT groups and zinc ions on the particle surface could increase interfacial vibrations and reduce thermal boundary resistance, but they explicitly state that this hypothesis was not verified experimentally and would need confirmation by XPS, Raman spectroscopy and molecular dynamics simulation. A schematic discussion in the paper also considers Brownian microconvection and weak clustering as possible contributors, while cautioning that excessive aggregation could hinder rather than help heat transport. Because direct phonon transport and interfacial resistance were not measured, the authors treat the interfacial explanation as plausible rather than proven.

Viscosity told a more modest story. All nine samples fell between 0.0696 and 0.08567 pascal-seconds at ambient temperature, only 13 to 40 percent above the base fluid’s 0.06131 pascal-seconds. The lowest viscosity belonged to sample 15MCZS15, an SDS-stabilized formulation, which the authors suggest makes it suitable for pumping applications. The narrow viscosity spread across a threefold variation in particle loading was interpreted as evidence that surfactant stabilization limited agglomeration. To rationalize the differences between SDS and EBT samples, the team calculated Debye lengths under conservative ionization assumptions and found electrostatic screening to be very weak in the low-permittivity coconut oil medium, concluding that double-layer repulsion is unlikely to dominate and that molecular size and adsorption geometry, with EBT forming thicker chelating layers or bridged aggregates, better explain the viscosity pattern.

Thermal safety improved markedly across the board. Flash and fire points rose in all nine samples, with the best values of 243 and 291 degrees Celsius again recorded for the EBT-stabilized 18MCZE15, against 153 and 196 degrees Celsius for neat coconut oil, an increase the authors summarize as roughly 59 percent for both measures. Density increased only slightly, from 0.910 grams per milliliter for the base oil to between 0.912 and 0.947 grams per milliliter for the nanofluids. Infrared spectroscopy confirmed the triglyceride structure of the coconut oil matrix and detected surfactant-specific bands for each stabilizer, along with a broad zinc oxide absorption near 450 wavenumbers, which the authors read as successful incorporation without chemical decomposition. Field-emission electron microscopy after dispersion showed micron-scale clusters in all systems, and energy-dispersive X-ray analysis confirmed the expected elements, including zinc and sulfur for SDS samples and zinc and nitrogen for the PVP and EBT formulations.

Stability was assessed photographically: samples stored in transparent bottles were photographed at zero, one and seven days under identical conditions, and no visible settling, color change or phase separation appeared in any formulation within the week. The authors are careful to stress that this is only a qualitative, preliminary indication, and that quantitative methods such as zeta potential measurement, dynamic light scattering, turbidity testing or accelerated sedimentation are needed before any claim of long-term stability. They also caution that the study did not include atomization, droplet-size distribution, tribological or cutting trials, so suitability for minimum quantity lubrication machining remains unproven; a simple estimate of Reynolds number around 57 for flow through a 0.5-millimeter orifice suggests viscous-dominated behavior and potentially reduced atomization performance relative to conventional MQL fluids. Within those limits, the team positions the EBT-stabilized formulation, with its record thermal conductivity and improved flash and fire points, as a promising candidate for biodegradable nano cutting fluids, while SDS offers the lowest viscosity and PVP the most consistent performance relative to the base oil.

Subject of Research: Thermophysical and thermal safety characterization of surfactant-stabilized ZnO-coconut oil nanofluids for sustainable cutting fluid applications

Article Title: “Comparative performance evaluation of ZnO-coconut oil nanofluids stabilized with PVP, SDS and EBT surfactants for sustainable applications”

Article References: Savalia, D., Anadani, P., & Das Lala, S. (2026). “Comparative performance evaluation of ZnO-coconut oil nanofluids stabilized with PVP, SDS and EBT surfactants for sustainable applications”. Discover Nano, 21(1), Article 566. https://doi.org/10.1186/s11671-026-05087-3

Image Credits: AI Generated

DOI: 10.1186/s11671-026-05087-3

Keywords: ZnO nanoparticles, coconut oil, nanofluids, surfactants, thermal conductivity, PVP, SDS, Eriochrome Black T, cutting fluids, colloidal stability, flash point, biodegradable lubricants