Figure 7. Viscosity enhancement ratio of the nanofluid with suspended graphene nanoparticles at different nanoparticle volume fractions.
The effect of nanoparticle volume fraction on the viscosity enhancement ratio is illustrated in Figure 8 for the nanofluid with suspended graphene oxide nanoparticles.
Surface functionalization can convert oxidized sp 2 graphitic layers into a variety of hydroxy- and carboxylic ionic groups. These groups can carry electrostatic charge and are miscible with water-based fluids. Thus, core-shell structures of graphitic gore and graphene oxide shell are formed. On one hand, surface oxidation helps increasing the stability and decreasing viscosity of nanofluids; but, on the other hand, thermal conductivity of graphene oxides is much smaller than that of graphite and graphene. Therefore, the functionalization process decreases enhancements in thermal conductivity due to formation of surface oxides. In development of nanofluids for heat transfer a fine balance needs to be obtained between increases in thermal conductivity and viscosity. For graphitic nanoparticle suspensions, advanced thermal conductivity is observed when nanoparticle percolation threshold is achieved. The concentration of the percolation threshold will vary with particle morphology, and both platelet diameter and thickness are important for that matter. Besides the important role of surface charges in nanoparticle agglomeration and viscosity of nanofluids, particle shape effect can also play a role in abnormally increased viscosity of graphitic nanofluids. Shear rate dependence of viscosity in suspensions indicates some restriction in fluid movement due to particle alignment and agglomeration. In a steady state, a rod-like particle or elongated agglomerate can have two types of motion due to the Brownian movements: rotational motion around the mid-point, and translational motion in parallel or perpendicular to the long axis. When the average spacing between particles is much larger than the longest dimension of the particle, the rotational and translational motions are not restricted by each other; hence very weak shear thinning behavior is expected. In suspensions of cylinders with higher volume fractions nanoparticles start to interact, so the viscosities at zero shear rate can be much greater than the base fluid viscosity and be very sensitive to the shear. At concentrations of nanomaterials that are significantly above the percolation threshold, an extended microstructure will be created in a nanofluid, obstructing the fluid flow and producing high viscosities. Such enhancements are possible with graphitic nanoparticles that are commercially available at reasonable costs. Surface chemistry and functionalization provides better dispersion stability, lower viscosity, and higher thermal conductivity, enhanced performance with temperature.