Enhanced thermophysical properties of copper nanoparticles dispersed in gear oil

• The prepared Cu-gear oil nanofluids with surfactant are stable for >20 days.
• Thermal conductivity enhances by ∼24% (2 vol% Cu) at room temperature.
• Nanolayer and ballistic transport of phonons causes the observed enhancement.
• Viscosity enhances by ∼71% at room temperature.

Stable and well dispersed Cu-gear oil (IBP Haulic-68) nanofluids having Cu nanoparticles volume concentration between 0.11 and 2% are prepared with oleic acid surfactant. Presence of agglomerated Cu nanoparticles in the prepared nanofluids is confirmed from Dynamic Light Scattering (DLS) data. Thermal conductivity and viscosity measurements are performed both as function of nanoparticle concentration and temperature between 10 and 80 °C. Thermal conductivity enhancement of 24% is observed with 2 vol% of Cu nanoparticle loading at room temperature. The formation of interfacial layer at the nanoparticle–liquid boundary and the ballistic transport of phonons across the percolating aggregates are believed to be responsible for the observed thermal conductivity enhancement. Viscosity of Cu (2 vol%)–gear oil nanofluids shows ∼71% enhancement at 30 °C. Newtonian behavior of gear oil changes to non-Newtonian behavior with the appearance of shear thinning for higher loading of Cu nanoparticles. Modified Krieger–Dougherty equation considering the role of aggregation predicts reasonably well Cu vol% dependence of viscosity of Cu-gear oil nanofluids and its temperature dependence is well accounted by the modified Andrade equation.