# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Hybrid
With increasing temperature, the surface energy of the nanoparticles decreased, hence diminishing their agglomeration and the viscosity of the nanofluid, leading to a greater Brownian motion of the nanoparticles. Thus, the TC changed from 9.21% at 25 \({}^{\circ}\)C to 83.44% at 40 \({}^{\circ}\)C for the 0.2% vol. nanofluid. Furthermore, it was also observed that an increase in the concentration of the rGO-Fe\({}_{3}\)O\({}_{4}\) nanoparticles led to a greater TC. The nanoparticles as solids suspended in fluids are continuously in motion and can be considered as heat carriages transporting the energy in the form of heat and as natural involuntary stirrers that induce the convection process to enhance the TC of the nanofluids by increasing the motion of the particles and, consequently, the collisions between them. On the other hand, the specific heat of the 0.2% vol. nanofluids decreased by 0.25% compared with that of the water itself. It has already been argued that the existing interactions at the interface between the nanoparticles and the base liquid are paramount for the reduction in the specific heat of the nanofluids. An increase in the number of nanoparticles alters these collisional effects and, in turn, the heat transfer by conduction. Furthermore, the rheological analysis of the nanofluids revealed a shear thinning non-Newtonian behavior. In addition, the HTC increased from 3444.1 W/m\({}^{2}\cdot\)K at 0.01% vol. to 4289.5 W/m\({}^{2}\cdot\)K at 0.02% vol. at the outlet and at a Reynolds number of 7510 \(\pm\) 5. The HTC also increased from 1043.3 to 4289.5 W/m\({}^{2}\cdot\)K with the enhancement of the Reynolds number from 940 to 7510 \(\pm\) 5 at a volume fraction of 0.02% vol. Finally, it was revealed that the nanofluids had a negligible impact on the pressure drop and friction factor, which suppresses any need for additional pumping power requirements. Mbambo et al. [114] synthesized nanofluids with gold nanoparticle-decorated Gr nanosheets dispersed in ethylene glycol. The Gr/gold nanohybrids were prepared using a pulsed Nd:YAG laser to ablate a bulk graphite substrate followed by the gold in the ethylene glycol. The synthesis method is depicted in the schematic diagram of Figure 12.  
Figure 11: Schematic diagram of the rGO-Fe\({}_{2}\)O\({}_{3}\) nanocomposite. Adapted from [113]. Furthermore, TC measurements in the temperature range of 25-45 \({}^{\circ}\)C revealed that the Gr-gold/ethylene glycol 2D nanofluid possessed an enhanced TC of 0.41 W\(\cdot\)m\({}^{-1}\).K\({}^{-1}\) compared with that of the Gr/ethylene glycol nanofluid at 0.35 W\(\cdot\)m\({}^{-1}\).K\({}^{-1}\), gold/ethylene glycol nanofluid at 0.39 W\(\cdot\)m\({}^{-1}\).K\({}^{-1}\), and ethylene glycol alone at 0.33 W\(\cdot\)m\({}^{-1}\).K\({}^{-1}\). The TC of the ethylene glycol and Gr/ethylene glycol nanofluids showed enhancements of 1.8% and 3%, respectively, with rising temperature values between 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C. The ethylene glycol base fluid possesses poor heat transfer features [115], and the increase found in the Gr/ethylene glycol TC with temperature may be caused by a decrease in the interfacial thermal resistance between the ethylene glycol and the nanoparticles at higher temperatures, and to the high TC of Gr. Additionally, alterations in the TC as a function of the temperature are expected to be caused by the Brownian motion of the nanoparticles [116]. The Gr-gold/ethylene glycol nanofluid exhibited a TC increase of up to 26% in the temperature values between 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C compared with the base fluid itself. This increase is higher than those numbers previously reported for Gr composite-based ethylene glycol nanofluids. The authors explained the TC of the developed nanofluid based on the synergistic effect between the gold nanoparticles and the Gr, which both possess intrinsic high thermal conductivities. The obtained results show this hybrid nanofluid to be suitable for improved heat transfer purposes. Furthermore, in the work carried out by Taha-Tijerina et al.