**1. Introduction**

The exceptional thermal and flow properties exhibited by nanofluid compared with those of conventional thermal transporting media have projected this special fluid as a subject of intense global research. Early studies in this context have measured the viscosity and thermal conductivity of nanofluids with diverse base fluids (ethylene glycol (EG), water, propylene glycol, glycerol, etc.) and found that these properties of the nanofluids were enhanced in relation to the base fluids [1–10]. However, underlying mechanisms for such enhancements of these properties of nanofluids, particularly for thermal conductivity,

**Citation:** Giwa, S.O.; Sharifpur, M.; Ahmadi, M.H.; Sohel Murshed, S.M.; Meyer, J.P. Experimental Investigation on Stability, Viscosity, and Electrical Conductivity of Water-Based Hybrid Nanofluid of MWCNT-Fe2O3. *Nanomaterials* **2021**, *11*, 136. https://doi.org/10.3390/ nano11010136

Received: 1 December 2020 Accepted: 1 January 2021 Published: 8 January 2021

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are not yet explored, and there are also controversies and inconsistencies in literature results [10–13]. Outside the viscosity and thermal conductivity, the electrical conductivity (EC), specific heat capacity, dielectric, and density of various nanofluids prepared from different nanoparticles (Cu, MgO, CuO, CNT, SiO2, ZnO, TiO2, Al2O3, Fe2O3, Fe3O4, spinels etc.) and dispersed in several base fluids (water, propylene glycol, ethylene glycol, bioglycol, palm oil, glycerol, ionic fluid, coconut oil, engine oil, etc.) have been subsequently examined at various mass/volume concentrations or fractions for different temperature ranges [14–20].

Hybridization of different types of nanoparticles to prepare hybrid nanofluids was first investigated by Jana and co-workers [21]. The idea behind this was to augment the thermal conductivity of nanofluid over that of conventional fluids. Jana and co-workers [21] synthesized aqueous carbon nanotubes (CNTs), Cu and Au nanofluids, and Cu-CNT and Au-CNT nanofluids and measured their thermal conductivity. They noticed that the hybrid nanofluids yielded a lower thermal conductivity relative to the mono-particle nanofluids. Suresh et al. [22] examined the thermal conductivity and viscosity of water-based Al2O3-Cu (90:10) nanofluids with varying volume concentrations (0.1–2%) at ambient temperature. The enhancement of the thermal conductivity by 1.47–12.11% and viscosity by 8–115%, in comparison with water, was reported. Both properties were noticed to improve as the volume concentration increased. Conflicting with the result of Jana et al. [21], Suresh et al. [22] showed that hybrid nanofluids have higher thermal conductivity than monoparticle nanofluids (Al2O3/water). Similarly, Chen et al. [23], who used water-based Ag-MWCNT nanofluid, reported lower thermal conductivity for water-based MWCNT nanofluid when compared with the hybrid nanofluid. The thermal conductivity of the water-based multiwalled CNT and γ-Al2O<sup>3</sup> (at an equal weight ratio (1:1)) nanofluids for different volume concentrations (0–1%) at the room temperature was examined [24]. A maximum enhancement of 20.68% was achieved with 1 vol%.

Esfe et al. [25] used an equal volume concentration of water-based mono-particle nanofluids (Al2O<sup>3</sup> and MWCNTs) to formulate water-based Al2O3-MWCNT nanofluids in an effort to measure the thermal conductivity at temperatures of 303–323 K. They noticed that the thermal conductivity was augmented when temperature and volume concentration increased in comparison with water. The rheological study of engine oil (EO)-based hybrid nanofluids (Al2O3-MWCNTs (75:25%)) was studied under varying shear rates (1333–13,333 s−<sup>1</sup> ), temperatures (25–50 ◦C), and volume concentrations (0–1%) by Dardan et al. [26]. The hybrid nanofluids were noticed to exhibit Newtonian behaviors. The viscosity improved with volume concentration increase and reduced with temperature rise. The highest enhancement of 46% was observed with 1 vol%. Afrand and co-workers [27] measured the viscosity of EO-based SiO2-MWCNT hybrid nanofluids (at equal volumes of nanoparticles) under varying temperatures (25–60 ◦C) and volume concentrations (0.0625–1%). The hybrid nanofluids showed an enhancement of viscosity as the volume concentrations increased. The hybrid nanofluids were observed to be higher than those of SiO2/EO and MWCNT/EO nanofluids with a maximum enhancement of 37.4% for 1 vol% at 60 ◦C.

In the work of Megatif et al. [28], equal weights of CNT-TiO<sup>2</sup> nanoparticles (0.1, 0.15, and 2.0 wt%) were suspended in water to synthesize the hybrid nanofluids, and the thermal conductivity, density, viscosity, and specific heat capacity were determined at 25–40 ◦C. These properties were found to be improved for the hybrid nanofluids compared to the mono-particle CNT nanofluids. The viscosity, specific heat capacity, and density of the hybrid nanofluids diminished with rising temperature, whereas the thermal conductivity followed the reverse trend. The rheological behavior of MWCNT-SiO<sup>2</sup> (50:50 vol%)/EGwater (50:50 vol%) nanofluids was examined under varying shear rates (0.612–122.3 s−<sup>1</sup> ), volume concentrations (0.0625–2%), and temperatures of 27.5–50 ◦C [29]. The nanofluids examined were found to demonstrate shear-thinning flow as the power-law index was below unity. Recently, Kakavandi and Akbari [30] studied the thermal conductivity of EG-water-based MWCNT-SiC/EG using a similar base fluid and ratio of nanoparticles as

the work of Eshgarf and Afrand [29] with concentrations of 0–0.75 vol% and temperatures of 25–50 ◦C. The thermal conductivity was improved by 33% at 0.75 vol% and 50 ◦C when compared with water-EG.

Esfe et al. [31] experimentally determined the thermal conductivity of DWCNT-ZnO/EG (10:90) with concentration and temperature ranges of 0.045–1.9 vol% and 30–50 ◦C, respectively. At 50 ◦C and 1.9 vol%, the thermal conductivity was augmented by 24.9%. They revealed that the addition of 10% DWCNT nanoparticles to 90% ZnO nanoparticles to formulate the hybrid nanofluids caused the thermal conductivity of EG-based ZnO nanofluids to be enhanced. Additionally, their cost analysis showed that it was more economical to use hybrid nanofluids than mono-particle nanofluids. Moldoveanu et al. [32] studied the thermal conductivity of aqueous mono-particle (TiO<sup>2</sup> and Al2O<sup>3</sup> for 1–3 vol%) and hybrid (Al2O<sup>3</sup> (0.05 vol%)-TiO<sup>2</sup> (0.05–2.5 vol%) nanofluids at temperatures of 20 to 50 ◦C. The measured property was enhanced by 10.7–14.1%, 8.5–17.7%, and 15.3–19.2% for aqueous TiO2, Al2O3, and Al2O3-TiO<sup>2</sup> nanofluids, respectively. An investigation into the impact of variation in the volume concentration (0–2.3%) and temperature (25–50 ◦C) on the thermal conductivity of EG-based hybrid nanofluids of functionalized MWCNT-Fe3O<sup>4</sup> (at equal volumes) was carried out by Harandi et al. [33]. The thermal conductivity of the hybrid nanofluid was augmented by 30% at 50 ◦C and 2.3 vol%. The rheological behavior of an identical hybrid nanofluid (at equal amounts) and temperature range like that of Harandi et al. [33] was performed using shear rates of 12.24–73.44 s−<sup>1</sup> and volume concentration of 0.1–1.8% [34]. The result showed that the nanofluids exhibited Newtonian behavior for volume concentrations of 0.1–0.8% and non-Newtonian flow for the nanofluids beyond 0.8 vol%.

Shi et al. [35] investigated the thermophysical properties (viscosity, thermal conductivity, and specific thermal capacity) of Fe3O<sup>4</sup> and Fe3O4-MWCNT nanofluids with 0.25 vol%. They reported higher thermal conductivity and viscosity and lower specific heat capacity for Fe3O4-MWCNT nanofluid compared to Fe3O4. The introduction of MWCNT particles to formulate the hybrid nanofluid was observed to enhance the thermal conductivity and viscosity but attenuated the specific heat capacity as the Fe3O<sup>4</sup> nanoparticles possess a better specific heat capacity than the MWCNT particles. Recently, a study on the thermal properties (viscosity, density, surface tension, specific heat capacity, and thermal conductivity) of three-nanocomponent water-based hybrid nanofluids was conducted by Mousavi et al. [36]. Hybrid nanofluids of CuO-MgO-TiO2/deionized water (DIW) were formulated in five different mixture ratios with volume concentrations of 0.1–0.5 vol%, and the thermal properties were measured at temperatures of 15–60 ◦C. The results showed that the hybrid nanofluid with a mixture ratio of 60:30:10 (CuO-MgO-TiO2) was the best as it had the lowest viscosity (36.4% for 0.5 vol% at 60 ◦C), highest thermal conductivity (78.6% for 0.1 vol% at 15 ◦C), and lowest surface tension when compared with DIW. Goodarzi et al. [37] investigated the behavior and viscosity of EO-based ZnO-MWCNT (25:75) nanofluids under changing shear rates (666.5–13,300 s−<sup>1</sup> ), temperatures (5–55 ◦C), and volume concentrations (0.05–0.8%). They reported a Newtonian flow for all the samples and at the studied temperatures. A temperature rise was observed to reduce the viscosity, whereas increasing the concentration improved the viscosity of the hybrid nanofluids, with an enhancement of 5–20%.

The above literature survey supports the fact that the hybridization of nanoparticles engaged in formulating hybrid nanofluids yielded an improvement in their thermal properties. However, there are limited studies in the open literature regarding the stability and the thermal properties of hybrid ferrofluids. In addition, there is a scarcity of documentation on the thermophysical properties of MWCNT nanoparticle-based ferrofluids (F3O4, Fe2O3, Co3O4, etc.) at different mixing ratios of bi-nanoparticles. This present study involved an experimental measurement of the electrical conductivity and viscosity of MWCNT-Fe2O3/DIW nanofluids with a bi-nanoparticle mixing ratio of 80:20 (weight % basis) for volume concentrations and temperatures ranging from 0.05% to 1.5% and 15 ◦C to 55 ◦C, respectively. Furthermore, research progress in this context revealed that there is

a notable dearth of knowledge on the electrical conductivity of hybrid nanofluids in the public domain.
