The effect of filler geometry on thermo-optical and rheological properties of CuO nanofluid

In the present work, we investigate the effect of nanoparticle crystallite size and shape on the thermal, rheological and optical properties of low-cost copper oxide nanofluid. Nanofluids are prepared by suspending 0.3% volume of nanoparticles in the 70:30 ethylene glycol:distilled water mixture using ultrasonic vibration, and followed by the microwave irradiation. Our results indicate that the zeta potential values of the nanofluids decrease as nanoparticle anisotropy or aspect ratio increases, zeta potential values decreases. Moreover, the increase in the thermal conductivity of the nanofluids is maximum for the nanofluids prepared using nanorods having the highest aspect ratio of 5.84. The rheology data of the CuO nanofluids with different shapes of nanoparticles at a particle volume fraction of 0.3% indicates that the nanofluids predominantly show Newtonian behaviour, exemplified by the shear rate independent viscosity. Moreover, the rheological behaviour is observed to depend on crystallite size and the nanoparticles shape as marked by an increase in the viscosity which follows the order, spherical < cubelike < rectangular < nanobar < nanorods. The comparison of the optical transmittance spectra for the base fluid and CuO suspended nanofluids suggests that the latter have higher adsorption capacity than the base fluid in the range 940 and 1165 nm. The CuO nanofluids with different shapes can be potentially utilised as direct solar absorber and heat transfer fluid and also as a coolant owing to their excellent stability, high thermal conductivity and low viscosity.