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.