# Fewer Dimensions for Higher Thermal Performance: A Review on 2D Nanofluids
## 2 Types of 2D Nanofluids
### Tungsten Disulfide
[93], 2D nanofluids with WS\({}_{2}\) volume fractions of 0.005% vol., 0.01% vol., and 0.02% vol., together with the surfactants sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and CTAB surfactants at 0.05, 0.5%, 1%, and 2% were synthesized and characterized. The authors arrived at the following main conclusions:  
* The maximum enhancement in the agglomerate sizes were of 172%, 245%, and 261% with 0.005% WS\({}_{2}\) and 2% SDS, 0.01% WS\({}_{2}\) and 2% SDS, and 0.02% WS\({}_{2}\) and 0.05% SDS, respectively. Collectively, the zeta potential saw an improvement of 554%, whereas the mean particle size presented an enhancement of up to 411% mainly caused by the adsorption of the surfactant molecules. This would entail a long-term agglomeration risk, resulting in the clustering and sedimentation of nanoparticles. * The maximum TC increases were approximately 2.8%, 1.9%, and 4.5% for combinations of 0.05% SDS and 0.005% WS\({}_{2}\) at temperature values of 25 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, and 70 \({}^{\circ}\)C, respectively. The authors argued that the TC reduced with increasing concentrations of the SDS. Alongside the 0.005% of WS\({}_{2}\) filler concentration, the higher fractions of 0.01% and 0.02% of WS\({}_{2}\) also exhibited higher TC enhancements with lower concentrations of surfactant. * The high-temperature condition revealed a pendular behavior concerning the TC. The reason behind this response may be the eventual detachment of the surfactant molecules and further interaction with the WS\({}_{2}\) nanosheets. * The rheological analysis confirmed the formation of a nanostructured network inside the nanofluids with WS\({}_{2}\) nanosheets and depicted the transition of the nanofluids from viscous to elastic behavior with the inclusion of surfactants. Such transition suggests that a higher pumping power was required to initiate the flow of the working fluid. The maximum decrease in viscosity of 8.2% was found at the minimum concentration of 0.005% vol. of WS\({}_{2}\), confirming the superior fluidity of the developed nanofluids. This reduction was amplified at a concentration of 0.05% vol. of SDS, enlarging the referred value by approximately 10.5%. * The developed nanofluids evolved from non-Newtonian to Newtonian behavior under a 10 s\({}^{-1}\) shear rate. The most prominent conclusion is that surfactants, such as SDS, provide their steric hindrance effect to produce a more uniform dispersion of the WS\({}_{2}\) nanosheets in the ethylene glycol base fluid and contribute to the interfacial tension reduction in the nanofluids, which may also promote a decrease in the viscosity of the system. On the other hand, the WS\({}_{2}\) nanoparticles have already demonstrated [94] great potential as fillers in lubricating and engine oils. WS\({}_{2}\) has shown improved lubricity due to the weak intermolecular interactions among the nanosheets, causing easy shearing with a fullerene-like structure. In lubrication, one of the main factors of the 2D nanofluids that contributes the most towards their lubricating ability is the amount of nanosheets present in the base fluid, as illustrated in Figure 6.  
However, more molecular dynamics are required for the WS\({}_{2}\) nanostructures dispersed in the synthetic oils most commonly used in CSP plants, which are a mixture of biphenyl and diphenyl oxide, to provide useful insights into the heat transfer capability of WS\({}_{2}\) nanofluids. The decoration of WS\({}_{2}\) requires further theoretical and experimental confirmation, given that the decoration of the WS\({}_{2}\) edge may induce considerable beneficial effects on the rheological and heat transfer characteristics of WS\({}_{2}\) nanofluids. Furthermore, there is a lack of published studies considering the interactions between the surfactants, incorporated nanostructures, and liquid medium used in the liquid phase exfoliation of WS\({}_{2}\) nanosheets.