# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Graphene/Graphene Oxide
The specific heat capacity and density of the nanofluids suffered a minor reduction compared with those properties of the base fluid alone. Park and Kim [54] examined the evolution of the TC and viscosity of different types of Gr nanosheets dispersed in water, including those containing GO nanosheets and Gr nanosheets with diverse sizes. The enhancement of the TC in the GO nanofluid was the highest among all tested nanofluids, whereas the maximum increase in the viscosity was found for the Gr nanofluid with the largest nanosheets. Tesfai et al. [55] studied the rheological characteristics of aqueous suspensions in GO nanosheets. The suspensions exhibited a shear thinning behavior under small shear rates, which was followed by a shear-independent behavior. The shear thinning behavior became more intense with an increasing incorporated fraction of the nanosheets. Furthermore, the nanofluids possessed a considerable viscosity due to the high aspect ratio of the GO nanosheets. J jam et al. [56] evaluated the viscosity of GO nanosheets dispersed in water and ethylene glycol. The nanofluids exhibited a shear thinning behavior under low shear rates and a Newtonian behavior under high shear rates. The viscosity of the nanofluid increased by up to \(35\%\) compared with that of the base fluid at \(0.1\%\) wt. and a temperature of \(20\)\({}^{\circ}\)C. The majority of experimental works on Gr and GO nanofluids have stated that the TC is temperature dependent; however, Sun et al. [57] reported inconsistent results, which was expected since a more in-depth understanding is required to establish precise correlations between the thermophysical characteristics of this type of nanofluid and many factors, including the size and incorporated fraction of the nanoparticles, rheological features, stability over time, the aspect ratio of the Gr nanosheets, pH regulation, zeta potential, and operating temperature. Naghash et al. [58] studied the HTC of nanofluids prepared from porous Gr nanosheets. The TC of the \(0.1\%\) wt. nanofluid remained almost unchanged with only a minor increase of \(3.8\%\), while the HTC suffered a considerable enhancement of \(34\%\). Thisconfirmed that aside from TC enhancement, more factors may influence the heat transfer of Gr nanofluids, including the local order of the molecules of the base fluid present in the surroundings of the Gr nanosheets, parallel structure, and \(\pi\)-\(\pi\) stacking. Bahiraei et al. [59] examined the hydrothermal features of an eco-friendly Gr nanofluid flowing in a spiral heat exchanger. The results demonstrated that with an increasing Reynolds number or concentration, the overall HTC and heat transfer rate were increased. Furthermore, the pressure loss was enhanced with an increasing Reynolds number, and the nanofluid exhibited higher pressure losses compared with those using water, especially at high Reynolds numbers. The nanofluids with Gr nanosheets usually have a better convective heat transfer performance than GO nanosheet nanofluids. Indeed, GO nanosheets have sp2-hybridized and sp3-hybridized carbon atoms with epoxide and hydroxyl groups. The presence of the atoms of oxygen and saturated sp3 bonds in the GO nanosheets decreases the TC, promotes phonon scattering, and decreases the heat transfer of nanofluids in reference to the Gr nanosheets. Hu et al. [60] evaluated the pool boiling of Gr nanosheets dispersed in ethylene glycol and water. The authors observed that at lower concentrations, the heat transfer behavior was improved with the increasing concentration of the nanosheets, which was mainly due to the surface wettability enhancement induced by the deposition over time of the nanoparticles. Nonetheless, further enhancements in the concentration decreased the HTC caused by the sedimentation over time in the Gr nanosheets and the resulting blockage of the available nucleation sites. Moreover, at concentrations of Gr inferior to 0.02%, the critical heat flux was increased as the concentration increased. Zhang et al.