Nanofluids are colloidal suspensions containing solid particles with at least one dimension of <100 nm that can be made of metals, oxides, carbides, nitrides, or carbon dispersed in conventional heat transfer fluids (HTFs). Since the early 1990s, they have received great attention, which has risen exponentially over recent years, due to their potential applications in heat transfer devices and energy systems as a result of their enhanced thermal properties. Moreover, they appear to be promising candidates for conversion and management of energy for responding to global warming and reducing the temperature increase of our planet.

From a practical point of view, the rheological behaviour of nanofluids is an important issue for applications involving fluid flow and has been used to evaluate the required pumping power, pressure drop, convective heat transfer and natural convection. Thus, such flow properties are now the subject of many studies, as shown later in Table 4.1, just beyond thermal conductivity.¹  The dynamic viscosity of HTFs, and their rheological properties at large, can change due to the addition of solid nanoparticles and can result in increased pressure drops,²  which affect the efficiency of energy systems. It is also closely related to the stability, the structure and interaction of solid nanoparticles, their dispersion state and possible presence of aggregates. Consequently, it can help in the understanding of physical and structural properties of nanofluids that can be related to thermal behaviour. Also, in many dimensionless numbers such as the Prandtl, Brinkman, Reynolds, and Rayleigh numbers, viscosity needs to be known. Viscosity also appears in figure of merits calculations that are generally used for the evaluation of nanofluid efficiency compared to base fluids.^3,4