*3.1. Preparation of Hybrid Nanofluids*

The preparation of stable hybrid nanofluids is a very vital step prior to their characterization and thermophysical property measurement, hence the need to optimize several parameters toward achieving this. The electrical conductivity has been reported as a viable property that can be employed to achieve the critical micelle concentration (CMC) of the surfactant utilized for nanofluid preparation [41,42]. The point of inflection of this property is known to be the CMC. In this study, the optimum ratio of SDS to hybrid nanoparticle weight (dispersion fraction), sonicator amplitude, and sonicating time were determined through the monitoring of the pH and electrical conductivity of MWCNT-Fe2O3/DIW nanofluids (0.1 vol%) at room temperature, as presented in Figures 1 and 2. A turning point (inflection) was observed for the pH and electrical conductivity at an optimum dispersion fraction of 0.5 (Figure 1). Again, the point of inflection was noticed at 120 min for the electrical conductivity in determining the optimum sonication time (Figure 2). The optimum sonication time also indicated a turning point for pH, leading to the optimum (lowest) pH for the MWCNT-Fe2O3/DIW nanofluid. In this present study, the hybrid nanofluids (at all volume concentrations) were prepared by sonicating at an amplitude of 70%, frequency of 70%, and sonication time of 120 min using a dispersion fraction of 0.5. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 7 of 20

**Figure 1.** Optimum dispersion fraction of sodium dodecyl sulphate (SDS) in hybrid nanofluids via electrical conductivity and pH monitoring. **Figure 1.** Optimum dispersion fraction of sodium dodecyl sulphate (SDS) in hybrid nanofluids via electrical conductivity and pH monitoring.
