## 4 Conclusions  
In this study, the following features were achieved:  
* Clusters of cobalt ferrite (CoFe\({}_{2}\)O\({}_{4}\)) nanoparticles (a novelty in nanofluid design) with average diameters of 97 to 192 nm were synthesized by a solvothermal method, adjust  
Figure 6: Thermal conductivity-enhancement dependence on the thermal conductivity range of CoFe\({}_{2}\)O\({}_{4}\)-based nanofluids. Figure 7: Specific surface area (SSA) as a function of TEM size for the clusters of Fe\({}_{3}\)O\({}_{4}\)/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)[24] and CoFe\({}_{2}\)O\({}_{4}\) [this work]. ing the amount of poly(ethylene glycol) in the reaction medium. Their morphology and structure were confirmed by TEM, XRD, Raman spectroscopy (RS), and vibrating sample magnetometry (VSM). * Nanofluids were designed with different base fluids [water (W), propylene glycol (PG) and PG:W] at 0.1 and 0.5 wt% concentrations of the three synthesized nanoadditives (CoFe\({}_{2}\)O\({}_{4}\) 97 nm, CoFe\({}_{2}\)O\({}_{4}\) 155 nm, and CoFe\({}_{2}\)O\({}_{4}\) 192 nm). * The DLS analyses of the average Z-size over time evidence moderately stable nanofluids with readily recoverable initial dispersion conditions. The smaller size of the nanoadditive and the greater amount of PG favour dispersion stability. * The analysis of the thermal conductivity of the nanofluids reveals temperature-dependences similar to those of the corresponding base fluid. W- and PG:W-based nanofluids obtain larger values as temperature rises, while PG-based nanofluids obtain similar values within the studied temperature range. * The nanofluid thermal-conductivity enhancements are practically temperature-independent. * The lower the thermal conductivity of the base fluid, the greater the enhancement for the same nanoadditive and mass concentration. * The SSA becomes a more preferable parameter than size to describe the thermal conductivity improvements. The thermal conductivity of the CoFe\({}_{2}\)O\({}_{4}\) cluster nanofluids shows decreases as the SSA decreases (the size increases) for all base fluids. A maximum enhancement of 4.2% is reported for the 0.5 wt% CoFe\({}_{2}\)O\({}_{4}\) 97 nm/PG nanofluid. Conceptualization, J.P.V. and L.L. ; methodology, J.P.V. and L.L. ; validation, J.P.V. and L.L. ; formal analysis, J.P.V., A.E. and L.L. ; investigation, J.P.V. and A.E. ; resources, J.P.V. and L.L. ; data curation, J.P.V. and A.E. ; writing--original draft preparation, J.P.V. and A.E. ; writing--review and editing, J.P.V. and L.L. All authors have read and agreed to the published version of the manuscript. Grant PID2020-112846 RB-C21 funded by MCIN/AEI/10.13039/501100011033. Grant PDC2021-121225-C21 funded by MCIN/AEI/10.13039/501100011033 and by \"European Union NextGenerationEU/PRTR\". Not applicable. * _[PERSON] et al. (2008)_ [PERSON]; [PERSON]; [PERSON] Thermophysical and electrokinetic properties of nanofluids--A critical review. _Appl. Therm. Eng._**2008**, _28_, 2109-2125. [CrossRef]
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