Dispersion and rheology of nanofluids with various concentrations of organic modified nanoparticles: Modifier and solvent effects

This article describes the relation between nanofluid viscosity and nanoparticle dispersibility. The rheological characteristics of dispersions of metal oxide nanoparticles was studied with covalently bonded organic molecules on their surfaces. Using a supercritical method, surface modification of the metal oxide nanoparticles with organic ligands was accomplished, thereby producing nanofluids of various kinds. By changing both the solvent and surface modifiers, the relation between the dispersive behavior and rheological characteristics of nanofluids was elucidated over widely various concentrations up to 30 vol.%. Then the nanofluid dispersibility was assessed in terms of their transparency, determined using ultraviolet - visible light spectroscopy, to correlate it to the nanofluid rheological behavior. Results reveal that transparent nanofluids behave as Newtonian fluids. Moreover, their relative viscosities converge to a certain viscosity range irrespective of the surface modifier and solvent type, whereas the non-transparent nanofluid viscosities are scattered above the transparent nanofluid viscosity range. After shear thinning, which is observed only for non-transparent nanofluids, the non-transparent nanofluid viscosity decreases and approaches the transparent nanofluid viscosity range. Furthermore, shear thickening occurs only at high nanoparticle concentrations of more than 23 vol.%, fundamentally independent of dispersibility. These findings provide a comprehensive picture of the relation between the nanofluid rheology and dispersive behavior, which is useful for nanofluid design.