# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Hybrid
[117], 2D nanofluids with synthetic and natural esters were prepared as base fluids with h-BN nanosheets, MoS2 nanosheets, and hybrid h-BN/MoS2 fillers. The nanofluids were synthesized with weight fractions of 0.01, 0.05, 0.10, 0.15, and 0.25% wt. The incorporation of the MoS2 into natural esters showed an ameliorated evolution of the TC similar to that of h-BN. Moreover, the addition of the h-BN/MoS2 nanohybrid to natural esters resulted in considerable enhancements in the TC with maximums of 0.25% wt. of 30% for the FR3 base fluid and 32% for the VG-100 base fluid. The authors argued that the h-BN contributed towards the improvement of the TC and the MOS2 contributed towards the reinforcement of the fluid, which helped to obtain a homogeneous dispersion of the fillers because of its superior lubricant properties. It was proposed that the verified TC increase was caused by the percolation mechanism and by the Brownian motion of the included nanosheets. In cases where the fraction of the 2D nanostructures was augmented in the synthetic or natural esters, the probability of the phonons being scattered into the adjacent nanostructures increased proportionally, leading to increased  
Figure 12: Preparation method of the Gr-gold nanoparticles in ethylene glycol 2D nanofluid. Adapted from [114]. contact conductance. In addition, thermal conduction channels were formed that may enhance the TC because of the percolation mechanism. The heat transfer capability between colliding structures may enhance the TC of the nanofluids. Indeed, a higher temperature value corresponds to a stronger Brownian motion. Consequently, at a temperature of 323 K, the TC of the developed nanofluids was more evident than at other temperature values. The liquid layering at the interface between the 2D nanostructures and the base fluid can also contribute towards TC enhancement. Furthermore, at room temperature it was observed that the impact of the combined usage of h-BN and MoS\({}_{2}\) was similar to that of the single component reinforcements. Nevertheless, the TC became higher than that of the single component nanofluids with increasing temperature. Yarmand et al. [118] prepared a 2D nanofluid with ethylene glycol as the base fluid and a nanocomposite of an empty fruit bunch and GO. The TC determination results showed an enhancement of 6.47% for the 0.06% wt. activated carbon-Gr/ethylene glycol nanofluid. Furthermore, the viscosity of the nanofluids showed a nonlinear increase with the increase in activated carbon-Gr fractions because of the reinforcement of the internal shear stress. The compound nanostructure of activated carbon-Gr enhanced surface interactions in the ethylene glycol. An extremely low concentration of nanoparticles was incorporated to avert sedimentation and an excessive enhancement of the viscosity. The highest increase in the viscosity was by 4.16% at 0.06% wt. The viscosity decreased gradually with increasing temperature, particularly at 45 \({}^{\circ}\)C. The viscosity increase was low for all tested nanofluids, which makes them adequate for heating applications with only small penalties affecting the required initial pumping power. Additionally, the density of the activated carbon-Gr/ethylene glycol nanofluids increased with the increasing fraction of nanoparticles and decreased with increasing temperature. The highest verified density enhancement was by 0.09% at 0.06 wt.% and 20 \({}^{\circ}\)C, and such a negligible enhancement can be caused by the interface effects on the liquid bulk induced by the surface of nanoparticles and by the interactions among the added nanoparticles themselves. When the temperature was increased from 20 \({}^{\circ}\)C to 50 \({}^{\circ}\)C, the specific heat capacity of the nanofluids also increased. The specific heat of the nanofluids at 0.06% wt. was 2.25%, superior to the specific heat of the pure ethylene glycol at 50 \({}^{\circ}\)C. A minor increase was observed in the specific heat of the hybrid nanofluids with the increasing concentration of activated carbon-Gr nanoparticles.