## 4 Research Perspectives  
According to the above discussion, nanofluids can be regarded as functionalized colloids with special requirements of low-particle loading to prevent from agglomeration, sedimentation and  
Figure 7: Temperature and streamline profiles of a coolant at 6 L/min in a car radiator: (**a**) base fluid; (**b**) \(Al_{2}O_{3}\) nanofluid (\(\varphi=0.3\) vol.%); (**c**) \(CuO\) nanofluid (\(\varphi=0.3\) vol.%). Results obtained by [PERSON] and [PERSON] [21] for a 50:50 EG-water mixture and reproduced with permission from Elsevier. clogging of the transport lines, favourable rheological behaviour to limit the pumping power required to transport them, and of course, high thermal performances for thermal energy storage and/or heat transfer. However, the characterization of the thermophysical properties and performances of nanofluid coolants, especially in the sub-zero temperature regime, still remain very limited, and only very few works have been devoted to this research topic for refrigeration applications. On the contrary, more research has been published for high temperatures, especially to increase the heat transfer rate in car radiators. More research has still to be done on the nanofluid synthesis, stability and compatibility with other materials of the thermal systems. For commercial or industrial applications, the preparation and stability of ethylene- or propylene-glycol-based nanofluids should be more carefully considered. To go towards their potential commercialization, their preparation should be as simple as possible by limiting the control of pH, the use of surfactants and the sonification step. A similar study to that of [PERSON] et al. [76] done for \(Al_{2}O_{3}\) water-based nanofluids should be performed for these other nano-coolants to see, among other things, if surfactants are necessary to guarantee their long-term stability. Further experimental research investigations on the effective thermal conductivity, heat capacity, density, and dynamic viscosity are also needed to better understand the heat transfer and transport characteristics of nanofluids over a wider range of low temperatures [\(-\)50; 0 \({}^{\circ}\)C]. Appropriate measurement techniques, especially the thermal hot wire technique for the thermal conductivity and the stress-controlled rheometer for the dynamic viscosity, should be preferred over the KD2 pro device and pressure drop/flow rate measurements to get more reliable experimental data. They could serve hereafter as reference data to develop accurate and more specific correlations for these properties, which could be implemented in future numerical models. Particular attention should also be paid to the hysteresis phenomenon when temperature varies. This has been observed experimentally by [PERSON] [77] through simultaneous measurements of dynamic viscosity and thermal conductivity for \(Al_{2}O_{3}\) water-based nanofluids, even at low concentrations. It may have a large impact on the pumping capacity of a given system, while providing, at the same time, useful information regarding the dispersion regime and stability of the suspensions. To validate numerical models, more measurements are also required in canonical thancomplex geometries, including pipes or channels with or without turbulence promoters. Local temperature, velocity, and concentration measurements are specifically required for detailed comparisons. Each system should be characterized also over a whole life cycle by quantifying the degradation of the nanofluid performances over hundreds of heating/cooling cycles. It would give better insight into the long-term stability of nano-coolants in real systems. The performances of nanofluid flows are difficult to assess, and comparisons between two nanofluids are rarely discussed regarding objective criteria, as their characteristics depend on so many parameters like the nanoparticle size, type and concentration, the geometry of the considered system, and the operating parameters (flow rate, temperature, pressure).