**4. Conclusions**

Stable MWCNT-Fe2O3/DIW nanofluids have been prepared and investigated for their electrical conductivity and viscosity at varying temperatures and volume concentrations. Increasing the bi-nanoparticles' concentration enhanced both the viscosity and electrical conductivity of MWCNT-Fe2O3/DIW nanofluids. Additionally, increasing the nanofluid temperature augmented the electrical conductivity, whereas the viscosity was reduced. Relative to the base fluid, maximum enhancements of 1676.4% and 35.7% were achieved for the electrical conductivity and viscosity of MWCNT-Fe2O3/DIW nanofluids, respectively. The obtained results were noticed to be consistent with previous studies in the literature. The hybridization of MWCNT and Fe2O<sup>3</sup> nanoparticles to prepare MWCNT-Fe2O<sup>3</sup> (20:80)/DIW nanofluid was proven to cause a reduction of viscosity, which is advantageous in engineering applications of the fluid. To predict the viscosity and electrical conductivity of the studied MWCNT-Fe2O3/DIW nanofluids, correlations have been developed.

**Author Contributions:** Conceptualization, S.O.G. and M.S.; methodology, S.O.G. and M.S.; investigation, S.O.G.; data analysis, S.O.G. and M.H.A.; writing—original draft preparation, S.O.G. and M.H.A.; writing—review and editing, J.P.M. and S.M.S.M.; supervision, M.S., J.P.M. and S.M.S.M.; funding acquisition, J.P.M. and S.M.S.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was also partially supported by the Fundação para a Ciência e Tecnologia (FCT), Portugal through grant: PTDC/NAN-MAT/29989/2017 (NanCOM).

**Acknowledgments:** The supports of the National Research Foundation (South Africa) under the Renewable and Sus-tainable Energy Doctoral Scholarships and of the FCT, Portugal through project-NanoCOM are hereby acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
