Abstract

Biobased lubricants with thermally responsive rheology are increasingly needed for high-efficiency mechanical systems. Graphene nanoplatelet (GnP) nanofluids in vegetable oils are promising candidates, yet their percolation-driven thermo-rheological transitions remain insufficiently understood. In particular, the coupled concentration–temperature landscape controlling network-mediated flow transitions has not previously been established. Here, surfactant-free GnP nanofluids were prepared in high-oleic soybean oil (HOSO) at concentrations of 0.025–2.15% v/v (ϕ), and their rheological response was mapped between 25°C and 80 °C under steady-shear and oscillatory conditions. Below the percolation threshold (ɸ ≤ 0.1% v/v), the nanofluids exhibit near-Newtonian behavior, moderate reversible viscosity enhancement, and classical Arrhenius-type temperature dependence. Above a critical concentration (ɸc ≈ 0.3% v/v), a sharp transition emerges: viscosity amplification up to ∼10⁴ at 80 °C, strong shear-thinning (n ≈ 0.37–0.39), and a concentration-dependent critical temperature window (Tc ≈ 41–50 °C). Within this range, the temperature–viscosity relationship inverts to an anti-Arrhenius regime with large negative activation energies (Ea′ = −15.8 to −36.6 kJ/mol), representing, to our knowledge, the first report of activation-energy sign inversion for viscous flow in a nanofluid system. The magnitude of |Ea′| is comparable to reported π–π interaction energies on graphene surfaces, consistent with thermally activated interparticle association. Oscillatory rheology reveals percolation-type scaling of the storage modulus, indicating fractal network formation. A dimensionless Network Strength Parameter (NSP) is introduced to quantify concentration- and temperature-dependent structural reinforcement. Together, the Tc–ϕ transition map and NSP framework provide new physicochemical insight into thermally activated percolation in nanoparticle suspensions.

Department(s)

Mechanical and Aerospace Engineering

Second Department

Chemical and Biochemical Engineering

Publication Status

Full Text Access

Keywords and Phrases

Biobased lubricants; Graphene nanoplatelet nanofluids; Percolation threshold; Temperature-dependent rheology; Thermo-rheological transitions; Viscoelastic network formation

International Standard Serial Number (ISSN)

1873-4359; 0927-7757

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2026 Elsevier, All rights reserved.

Publication Date

20 Aug 2026

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