A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol, and oil.

Nanofluids have many potentially heat transfer applications, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines, engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, in grinding, machining and in boiler flue gas temperature reduction. They exhibit enhanced thermal conductivity and convective heat transfer coefficient compared to the base fluid. Knowledge of the rheological behaviour of nanofluids is critical in deciding their suitability for convective heat transfer applications. Nanofluids also have special acoustical properties and in ultrasonic fields display shear-wave reconversion of an incident compressional wave; the effect becomes more pronounced as concentration increases.

In computational fluid dynamics (CFD), nanofluids can be assumed to be single phase fluids; however, almost all academic papers use a two-phase assumption. Classical theory of single phase fluids can be applied, where physical properties of nanofluid is taken as a function of properties of both constituents and their concentrations. An alternative approach simulates nanofluids using a two-component model.

The spreading of a nanofluid droplet is enhanced by the solid-like ordering structure of nanoparticles assembled near the contact line by diffusion, which gives rise to a structural disjoining pressure in the vicinity of the contact line. However, such enhancement is not observed for small droplets with diameter of nanometer scale, because the wetting time scale is much smaller than the diffusion time scale.

Thermal conductivity, viscosity, density, specific heat, and surface tension are significant thermophysical properties of nanofluids. Parameters such as nanoparticle type, size, shape, volume concentration, fluid temperature, and nanofluid preparation method affect thermophysical properties.

Nanofluids are produced by several techniques:

Base liquids include water, ethylene glycol, and oils have been used. Although stabilization can be a challenge, on-going research indicates that it is possible. Nano-materials used so far in nanofluid synthesis include metallic particles, oxide particles, carbon nanotubes, graphene nano-flakes and ceramic particles.

A biologically-based, environmentally friendly approach for the covalent functionalization of multi-walled carbon nanotubes (MWCNTs) using clove buds was developed. No toxic/hazardous acids are typically used in common carbon nanomaterial functionalization procedures, as employed in this synthesis. The MWCNTs are functionalized in one pot using a free radical grafting reaction. The clove-functionalized MWCNTs are then dispersed in distilled water (DI water), producing a highly stable MWCNT aqueous suspension (MWCNTs Nanofluid).