*2.3. Hybrid Nanofluid Preparation and Stability*

In the formulation of the hybrid nanofluids (Fe2O<sup>3</sup> (80%) and MWCNTs (20%)), a two-step method was used. To ensure proper stability, the pH and electrical conductivity of the formulated MWCNT-Fe2O3/DIW nanofluids were monitored while SDS amounts and sonication parameters (amplitude, frequency, and sonication time) were optimized at a 0.1% volume concentration and room temperature (20 ◦C). SDS to bi-nanoparticle weight ratios of 0.4–1.0 were examined. After obtaining the optimum values of amplitude, frequency, sonicating time, and the ratio of SDS to hybrid nanoparticle weight (dispersion fraction), hybrid nanofluids of various volume concentrations (0.1–1.5%) were formulated according to Equation (1).

$$\varphi = \left(\frac{X\_{Fe\_2O\_3}\left(\frac{M}{\rho}\right)\_{Fe\_2O\_3} + X\_{MWCNT}\left(\frac{M}{\rho}\right)\_{MWCNT}}{X\_{Fe\_2O\_3}\left(\frac{M}{\rho}\right)\_{Fe\_2O\_3} + X\_{MWCNT}\left(\frac{M}{\rho}\right)\_{MWCNT} + \left(\frac{M}{\rho}\right)\_{DIN}}\right) \tag{1}$$

where *X* = ratio of each nanoparticle type; *M* = weight of material; *ρ* = density of material.

The morphology and dispersion of the bi-particles in the hybrid nanofluids were monitored using TEM. Viscosity, UV–visible spectrophotometry, and visual inspection methods

were employed to monitor the nanofluids' stability. The nanofluid can be considered stable if one of the properties has not changed during the period of stability check. However, for our case, both the UV–visible spectrophotometry and viscosity techniques were carried out for 43 h while the visual inspection was done at weekly intervals for a month.
