# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Molybdenum Disulfide  
MoS2 nanofluids have already proven to be an attractive option for application in, for instance, volumetric parabolic trough collectors. These nanofluids can improve by 5% or more the thermal efficiency of such collectors and, at the same time, reduce the initial required pumping power by around one-fifth. Furthermore, the use of MoS2 nanofluids diminishes the thermal stress on volumetric receivers and the overall required investment. Molybdenum disulfide is a 2D transition metal dichalcogenide with MX\({}_{2}\) as chemical formula, where X denotes a chalcogen, such as sulfur, selenium, and tellurium, and M denotes the transition metal. MoS\({}_{2}\) possesses improved thermal and chemical stabilities. It can be applied as a catalyst and lubricant because of its superior characteristics, including inertness, anisotropy, and photoconrosion resistance. MoS\({}_{2}\) nanostructures have good thermophysical properties and great anti-friction properties, making it a promising material as reinforcement in the cooling process of nanofluids. Moreover, parameters including the type and concentration of nanoparticles, the nature of the base fluid, the synthesis procedure, and the stability/surfactant addition balance affect the thermophysical characteristics of the nanofluids, including the viscosity and thermal transport. Su et al. [95] investigated the thermal stability of MoS\({}_{2}\) in water and oil nanofluids at weight fractions from 0.01% wt. to 0.5% wt. The authors found that with the enhancement in the amount of MoS\({}_{2}\), the TC of the nanofluids was increased. The investigation team also reported that the TC improvement was higher in water-based nanofluids than in oil-based nanofluids. This fact was explained as the consequence of the improved dispersion and stability over time of the water nanofluids. Nagarajan et al. [96] prepared MoS\({}_{2}\) nanosheets to be suspended in SAE 20W50 diesel engine oil to produce an effective lubricant. MoS\({}_{2}\) nanoparticles were synthesized using a microwave hydrothermal reactor. The schematic diagram of the synthesis procedure is presented in Figure 7.  
The tribological and oxidation analysis showed that the lubricant with a weight fraction of 0.01% wt. of MoS\({}_{2}\) nanoparticles provided better results than those of other concentrations and, hence, the nanofluid with this fraction was further evaluated for its TC. The authors reported an improvement in the TC of the 0.01% wt. nanofluid of approximately 10% compared with that of the base engine oil alone. Due to the lower amount of the MoS\({}_{2}\) nanoparticles, the enhancement in the TC was due to the molecular  
Figure 7: Schematic diagram of microwave-synthesized molybdenum disulfide nanoparticles. Adapted from [96]. oil and the nanoparticles. The researchers argued that the TC behavior indicated that this enhancement was due to the Brownian motion of the nanosheets. The TC and heat transfer capability of a nanofluid depend on the Brownian motion of the incorporated nanoparticles. Indeed, the Brownian motion-driven convection and effective thermal conduction by the percolating paths mechanism of the nanoparticles are the most likely factors that promote the improved heat conduction in nanofluids. The percolation mechanism involves the formation of thermal conductive paths based on the enhanced thermal conductance that improves the overall TC. A schematic diagram of the percolation mechanism is presented in Figure 8. The Brownian motion allows the direct solid-solid transport of heat from one particle to the adjacent one, resulting in an increase in the thermal conductivity. The Brownian motion and resulting micro mixing of the nanoparticles and clusters, as well as aggregation kinetics of nanoparticles and clusters, are the main issues in the evaluation of the thermal conductivity of nanofluids. Any effort to understand the behavior of nanofluids is not complete without considering at the same time all these effects.