A High Thermal Conductivity of MgO-H 2 O Nanoﬂuid Prepared by Two-Step Technique

: In this paper, the main goal is to study the impact of nanopowder volume concentration and ultrasonication treatment time on the stability and thermophysical properties of MgO-DW nanoﬂuid at room temperature. The co-precipitation method was utilized to prepare pure MgO nanoparticles with an average particle size of 33 nm. The prepared MgO nanopowder was characterized by using XRD, SEM, and EDX analyses. Then, MgO-DW nanoﬂuid was obtained with different volume concentrations (i.e., 0.05, 0.1, 0.15, 0.2, and 0.25 vol.%) and different ultrasonication time periods (i.e., 45, 90, 135, and 180 min) by using a novel two-step technique. With volume concentration and ultrasonication time of 0.15 vol.% and 180 min, respectively, good stability was achieved, according to the zeta potential analysis. With increasing volume concentration and ultrasonication time period of the nanoﬂuid samples, the thermal conductivity measurements showed signiﬁcant increases. As a result, the maximum enhancement was found to be 25.08% at a concentration ratio of 0.25 vol.% and agitation time of 180 min. Dynamic viscosity measurements revealed two contrasting trends with volume concentration and ultrasonication time. The lowest value of relative viscosity was gained by 0.05 vol.% MgO-DW nanoﬂuid. The chemical and physical interactions between MgO nanoparticles and DW molecules play an important function in determining the thermal conductivity and dynamic viscosity of MgO-DW nanoﬂuid. These ﬁndings exhibit that MgO-DW nanoﬂuid has the potential to be used as an advanced heat transfer ﬂuid in cooling systems and heat exchangers.


Introduction
High thermal conductivity fluids are referred to as nanofluids and are employed rather than regular fluids to increase heat transfer properties or to prevent overheating. Numerous studies have demonstrated that standard fluids used in heat exchangers, such as water, oil, ethylene glycol, and glycerol, have low thermal conductivity as compared with metallic and ceramic materials [1][2][3]. Therefore, praiseworthy attempts have been made to improve the thermal conductivity of traditional fluids, by distributing ultrafine solid particles. The growing requirement for high efficiency in heat exchangers, storage materials in electronic devices, chemical reactions, boiling processes, and solar energy has prompted researchers to focus on dispersed particles, particularly within a size range of 1-100 nm, within the base fluid [4][5][6][7][8][9].
Maxwell [10,11] was the first to conduct practical research to increase heat conductivity, and he was supported by a large number of researchers. Among the most prominent, Choi, nanofluid of Ag-MgO/water was also conducted by the M. H. Esfa research group [5]. The nanofluid was produced by mixing 25 nm nano Ag (50%), 40 nm nano MgO (50%), and water. Ultrasonic vibration for 3 h and CTAB surfactant were employed to ensure better stability.
Thermal conductivity and dynamic viscosity were measured as a function of nanoparticle volume fraction range between 0% and 2%. The obtained results revealed that both thermal conductivity and dynamic viscosity were increased with increases in the volume fraction of (Ag + MgO) nanoparticles. The effects of particle size, solid concentration, and temperature on the dynamic viscosity of ethylene glycol-based nanofluid containing MgO nanoparticles have been studied by K. Yapici et al. [59]. They conducted experiments at concentrations ranging from 5 to 20 wt.% of MgO nanopowder with particle sizes of 20 and 40 nm. MgO/EG nanofluid was agitated in an ultrasonic bath for 6 h by using a two-step technique. Nonlinear results revealed that the 20 nm and 40 nm MgO nanoparticles exhibited non-Newtonian behavior at weight concentrations higher than 5%. J. Lu et al. [60] used molten salt LiNO 3 -NaNO 3 -KNO 3 with MgO nanoparticles to prepare nanofluid via a two-step technique. The synthesis technique was concluded by heating the mixture of molten salt and MgO nanoparticles to 350 • C and ultrasonication time for 1 h. The effect of nanoparticles on thermal properties was studied with different sizes ranges of 20-100 nm, weight concentrations from 0.5 to 2.0 wt.%, and temperature. The results showed that the enhancement of thermal conductivity of salt with 1 wt.% nano-MgO could be 5.3-11.7%. In an experimental environment, S. R. Nfawa et al. [61] improved the thermal conductivity of a CuO-MgO/H 2 O hybrid nanofluid in an experimental setting. The hybrid nanofluid was synthesized and investigated at various volume fractions (i.e., 0.125-1.25%) of 80% CuO and 20% MgO nanoparticles which were dispersed in water at temperatures ranging from 25 to 50 • C. The zeta potential measurement indicated that the 0.25% (CuO-MgO)/H 2 O hybrid nanofluid had a good stability of −45.1 mV. The study results showed that by increasing the concentration ratio, the CuO-MgO/H 2 O hybrid nanofluid's thermal conductivity was clearly enhanced; however, this enhancement began to decline when the concentration was higher than 1 vol.% as a result of nanoparticle precipitation.
The two-step approach is often supported by the aid of relatively high temperatures, and/or the use of surfactants, and/or a long ultrasonication treatment time, and/or organic base fluids. Therefore, in the present study, we aimed at preparing MgO-DW nanofluid with efficient thermal properties such as better stability, high thermal conductivity, and low viscosity, without using surfactants or stabilizers, and with environmentally friendly nanofluid at room temperature.
Furthermore, the plurality of the concerned investigations has been considered in order to exploit MgO nanopowder as an additive or to synthesize hybrid nanofluid with other nanomaterials, as cited in [62][63][64][65][66]. However, in this work, pure MgO-DW nanofluid was produced with functional properties. This was achieved by using MgO nanopowder with a unique morphology of feathery or fluffy-like nanostructure, while usually, the nano shape used in nanofluid preparation is spherical particles. The abovementioned aspects are all assumed to be novel, making MgO-DW nanofluid an excellent alternative for heat transfer applications.

Materials and Method
The co-precipitation method was used to create MgO nanopowder. Analytical grade chemicals such as magnesium sulfate (MgSO 4 ·7H 2 O), sodium bicarbonate (NaHCO 3 ), ammonium hydroxide (NaOH), and sodium dodecyl sulfate (SDS) were utilized without additional purification. NaOH and SDS were both used as a surfactant at room temperature. The chemicals used were all from BDH Chemicals Ltd., Poole, Dorset, UK. First, a 0.8 M solution of NaHCO 3 and NaOH was prepared by dissolving a suitable quantity in deionized water. Similarly, Mg precursor solution was prepared by dissolving MgSO 4 ·7H 2 O in deionized water. Next, 50 mL of 0.5 M MgSO 4 and SDS solution were blended fully by stirring vigorously by utilizing a magnetic stirrer for 20 min at room temperature. Then, using a burette, 50 mL of 0.8 M NaHCO 3 solution was gently poured into it, while it was constantly stirred. Then, 50 mL of 1 M NaOH solution was added in batches into the above resulting solution under stirring. The final combination was left to swirl for 4 h after adding the surfactant and precipitating agent, with no parameters being modeled.
The extremely finely powdered white precipitate MgO was collected at the bottom of the preparation flask once the time was up. The fine powder was carefully separated using a centrifuge. The whole precipitation was washed several times with distilled water. The main purpose of the washing process was to ensure that the precipitate was free from foreign trace elements, especially SDS residue. The resulting MgO powder was dried in an air oven at 60 • C for 2 h. Then, the prepared MgO nanopowder was produced via a calcination process using a furnace at 350 • C for 1 h. All details of the preparation method are explained in Figure 1 (first step). chemicals such as magnesium sulfate (MgSO4·7H2O), sodium bicarbonate (NaHCO3), ammonium hydroxide (NaOH), and sodium dodecyl sulfate (SDS) were utilized without additional purification. NaOH and SDS were both used as a surfactant at room temperature. The chemicals used were all from BDH Chemicals Ltd., Poole, Dorset, UK. First, a 0.8 M solution of NaHCO3 and NaOH was prepared by dissolving a suitable quantity in deionized water. Similarly, Mg precursor solution was prepared by dissolving MgSO4·7H2O in deionized water. Next, 50 mL of 0.5 M MgSO4 and SDS solution were blended fully by stirring vigorously by utilizing a magnetic stirrer for 20 min at room temperature. Then, using a burette, 50 mL of 0.8 M NaHCO3 solution was gently poured into it, while it was constantly stirred. Then, 50 mL of 1 M NaOH solution was added in batches into the above resulting solution under stirring. The final combination was left to swirl for 4 h after adding the surfactant and precipitating agent, with no parameters being modeled.
The extremely finely powdered white precipitate MgO was collected at the bottom of the preparation flask once the time was up. The fine powder was carefully separated using a centrifuge. The whole precipitation was washed several times with distilled water. The main purpose of the washing process was to ensure that the precipitate was free from foreign trace elements, especially SDS residue. The resulting MgO powder was dried in an air oven at 60 °C for 2 h. Then, the prepared MgO nanopowder was produced via a calcination process using a furnace at 350 °C for 1 h. All details of the preparation method are explained in Figure 1

Preparation of MgO Nanofluid
A two-step technique was used to make the MgO nanofluid. The first step, i.e., nanopowder preparation, was completed (as described above). The second step involved synthesis of the nanofluid using mechanical procedures. The mechanical procedures were carried out using primary mechanical stirring for 30 min, and then ultrasonic agitation as shown in Figure 1 (second step). Different ultrasonication periods and volume concentrations were used to prepare the nanofluid samples. They were synthesized via dispersing MgO nanoparticles in 100 mL of deionized water as a base fluid without the use of any dispersants or heat. The ultrasonication times were 45, 90, 135, and 180 min. The following relationship [67] was used to compute the volume concentration ratios of 0.05, 0.1, 0.15, 0.2, and 0.25 vol.%: where φ is the volume concentration of MgO nanofluid, V P is the volume of MgO nanopowder, and V T is the total volume of nanofluid sample. Depending on nanopowder density (ρ P ) and base fluid density (ρ bf ), the volume was measured as follows: where m p and m bf are the masses of MgO nanopowder and base fluid (deionized water), respectively; ρ p and ρ bf are the densities of nanopowder and base fluid, respectively. According to the final relation, the weights of MgO nanopowder required to prepare nanofluid samples with different volume ratios are listed in Table 1.

The Characterization Techniques
The structural characteristics of MgO nanopowder were achieved using an XRD-6000 Shimadzu X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of (10−80 • ) in a step of 0.05 • and a scan speed of 5 degree/min. A scanning electron microscope (SEM) analysis was also performed to test the surface morphology of the prepared nanopowder. The SEM device was a Mira 3-scan (Metrohm France, Villebon-sur-Yvette, France) type that ran at a 15 kV accelerating voltage. An EDX approach equipped with a SEM instrument was used to assess the composition and purity of the nanopowder.

The Stability and Particle Size Analysis
Zeta potential was employed as an index of the surface charge of the MgO nanoparticles in the base fluid. The surface charge was considered to be an indicator of the dispersion stability of the MgO nanosuspension. The suspension with high zeta potential was stabilized, while the sample with low value tended to accumulate and suffer from sedimentation, according to ASTM standard (D4187-82) of zeta potential ranges in Table 2   The measurements were carried out using a zeta potential analyzer (Zeta Plus (21521), power 100-240 V 50/60 Hz, USA). To ensure data reliability, each experiment was repeated three times before the mean results were estimated. After four days of preparation, the zeta potential measurements were obtained. A particle size analyzer (Zetasizer, Brookhaven Instruments, New York, NY, USA) can also detect particle size by using a dynamic light scattering (DLS) process. The container is illuminated by a laser, and the particles scatter the light which is measured using a detector. A fundamental property of Brownian motion is that small particles move much faster than large particles. There is an association between the size of a particle and its velocity due to Brownian motion. Based on this physical feature, a particle size analyzer measures the Brownian motion of the particles in a specimen and connects this to a size based on established theories [70]. Therefore, a particle size analyzer was used to determine the average particle size of the prepared MgO nanopowder at room temperature. The scattering angle and polydispersity index were 15 • and 0.061, respectively.

Measurements of Thermophysical Properties
With varying volume concentrations of 0.05, 0.1, 0.15, 0.2, and 0.25 vol. percent, the thermal conductivity and dynamic viscosity of MgO-DW nanofluid were measured. They were also measured with various ultrasonication treatment times of 45, 90, 135, and 180 min. The thermal conductivity was tested using a Sweden company device (Hot Disk TPS 500 S, KIJTALEY, Johanneberg Science Park, Göteborg, Sweden), which is a type of thermal constants analyzer that uses the standard isotropic method. The dynamic viscosity measurements were carried out using a viscometer, Model HTD13145 Six-Speed Rotational Viscometer (Qingdao Haitongda Special Instrument Co., Ltd., Qingdao, China). The thermal conductivity and dynamic viscosity measurements were both achieved at room temperature. The enhancement ratio of thermal conductivity values of MgO-DW nanofluid can be estimated using the following relationship [71]: where K nf and K bf are the thermal conductivity of the nanofluid and the base fluid, respectively. Figure 2 shows the XRD spectrum used to analyze the crystalline structure of the obtained MgO nanopowder. The pattern shows an ordered polycrystalline structure with five intense peaks. The low intensity of the diffraction background was clearly indicative of the pure phase of the intended material. Therefore, no distinguishing peaks of any impurities were discovered. The surfactant SDS [CH 3 (CH 2 ) 11 OSO 3 Na] acts as a catalyst, preventing any fast reactions in the solution. In addition, it acts as a dispersion factor of the formed particles [72].

The Structural Studies
Hence, the synthesized material is of high purity, free of any secondary products, and nanoscale.  [73,74]. The specified X-ray peaks matched well with the crystallographic structure according to the JCPDS standard file no. 45-0946 of polycrystalline MgO. The crystallite size (D) was calculated based on the full width at half maximum (FWHM) of the intensive diffraction peak (i.e., the plane 200) by using the Debye-Scherrer's formula [75,76]: where A is an empirical shape factor equal to 0.94, λ is the wavelength of the X-ray source, and θ is the angular position of the diffraction peak.
where A is an empirical shape factor equal to 0.94, λ is the wavelength of the X-ray source, and θ is the angular position of the diffraction peak. The crystallite size was calculated to be 22.78 nm. The sharp diffraction peaks indicated a high degree of crystallization. The nanostructure of the prepared MgO particles was also revealed by the average particle size of less than 100 nm. All the diffraction peaks of MgO nanopowder revealed an obvious shift towards lower 2θ positions, as seen in Table 3. This result may be attributed to the nano scale of the prepared particles, whereas the high density of crystalline boundaries in the nano structure may cause microstrain.  The lattice constant and the unit cell volume were estimated by using the Formulas (5) and (6)  The crystallite size was calculated to be 22.78 nm. The sharp diffraction peaks indicated a high degree of crystallization. The nanostructure of the prepared MgO particles was also revealed by the average particle size of less than 100 nm. All the diffraction peaks of MgO nanopowder revealed an obvious shift towards lower 2θ positions, as seen in Table 3. This result may be attributed to the nano scale of the prepared particles, whereas the high density of crystalline boundaries in the nano structure may cause microstrain. The lattice constant and the unit cell volume were estimated by using the Formulas (5) and (6) where d hkl is the interplanetary crystalline distance; h, k, and l are the Miller indices; and "a" is the lattice constant. From the results in Table 4, it can be noticed there is relative stretching in the d hkl spacing and lattice constant values as compared with the standard values, since the variation in both is approximately 0.0011 Å of the interplanetary distance and 0.0023 Å of lattice constant. Consequently, the volume of the unit cell showed relative enlargement, which might be due to the nanoscale of MgO particles. The elemental structure, purity, and surface morphology of the synthesized MgO nanostructure were tested using SEM and EDX techniques. The energy dispersive X-ray spectrum was achieved at the range of 0-10 keV, as shown in Figure 3a. One can clearly observe three evident bands at nearly 0.25 keV, 0.55 keV, and 1.3 keV which correspond to the elements of C, O, and Mg, respectively. The spectrum demonstrated a distinct absorption band of magnesium element with weight and atomic ratios of 50.08% and 47.55%, respectively. As stated in the inset table inside the Figure, the oxygen element ratios are 46.95% and 50.76%. Therefore, the EDX spectroscopy results confirmed the formation of MgO, as was previously demonstrated by XRD analysis. In addition, according to XRD results the prepared material has a pure phase, but the EDX results show a small band of carbon element.

Stability Measurement of MgO-DW Nanofluid
The stability of the nano suspension contents in the base medium must be carefully monitored since it has a significant influence on the thermal characteristics. Nanoparticle aggregation may cause precipitation and blockage, hence, decreasing the thermal conductivity of the nanofluid. The zeta potential refers to the quantity of electrostatic repulsion potential presented between the charged nanoparticles and the base fluid. The zeta poten- The presence of carbon peak in the prepared sample was attributed to the carbon holder utilized in the imaging and/or the sodium carbonate used in the preparation process. A similar result was reported in the literature [77]. The morphology of MgO nanostructure was anatomized at various magnifications, as displayed in Figure 3b-d. Figure 3b exhibits agglomerated structures with high density. It shows what looks like snowballs or bright white cotton balls that are diffused on the surface of the agglomerated structures. Interestingly, the SEM images (Figure 3c,d) exhibit feathery or fluffy-like nanostructures that assemble together to form dense clouds.
The fluffy-like nanostructures have varied orientations, resulting in the intermingled morphology within the clouds. The average particle size of MgO nanostructures was measured to be 33 nm.

Stability Measurement of MgO-DW Nanofluid
The stability of the nano suspension contents in the base medium must be carefully monitored since it has a significant influence on the thermal characteristics. Nanoparticle aggregation may cause precipitation and blockage, hence, decreasing the thermal conductivity of the nanofluid. The zeta potential refers to the quantity of electrostatic repulsion potential presented between the charged nanoparticles and the base fluid. The zeta potential was investigated at various volume concentrations and ultrasonication times of MgO-DW nanofluid, as listed in the Table 5A,B. It can be seen that all the values of zeta potential have a negative sign, due to the adsorption of the OH − ions (negative ions) on the surface of MgO nanoparticles. The states of zeta potential values were determined according to the ASTM standard in Table 2   Good stability was obtained when the electrostatic repulsion forces overcame the attraction forces such as Van der Waals forces between the MgO nanoparticles. For this reason, this phenomenon could prevent accumulation, therefore, no sedimentation of dispersion nanoparticles occurred within the base fluid. A low zeta potential value was found at 0.25 vol.% MgO-DW nanofluid, which indicated the presence of some clusters or aggregates. However, this sample was moderately stable. All the samples in part B were prepared with a concentration ratio of 0.15 vol.% nanoparticles. It can be seen that ultrasonication time is directly proportional to zeta potential values. Therefore, as ultrasonication time increased from 45 to 180 min, the ZP values of the synthesized MgO nanofluids also increased from −38.9 to −62.1 mV. The portion of nanoparticles that survive in a well dispersed state in nanofluid increases as the ultrasonication treatment period increases, raising the zeta potential values. The last result confirmed the possibility of improving the dispersion stability of the nanofluids via an increasing ultrasonication agitation time. The current findings agree with those reported in [61].

Thermal Conductivity
The thermal conductivity of MgO-DW nanofluid is one of the most important operators that has a direct effect on heat transfer rate. Variations in thermal conductivity and relative thermal conductivity with respect to the nanoparticle concentration ratio ranging from 0.05 to 0.25 vol.% was studied at room temperature 27 • C ± 2, as shown in Figure 4a,b. It is apparent from Figure 4 that the thermal conductivity and relative thermal conductivity increased with the solid content of dispersed MgO nanostructures.  The greatest thermal conductivity enhancement (by using Relation (3)) was found to be 25.08% at the highest concentration of 0.25 vol.%. In addition, the lowest improvement was 5.68% at a volume concentration of 0.05 vol.%, as observed in Table 6. Generally, increasing the volume concentrations of MgO nanoparticles in the fluid minimizes the distance between moving particles, resulting in more molecular collisions. An increase in the accelerated nanoparticles in the base fluid enhances the thermal conductivity of the nanofluid.
Furthermore, the development of clusters and aggregates can take the shape of chainlike structures which provide a transformation path of the thermal energy among the nanoparticles. This may explain the high improvement ratio of thermal conductivity in the case of 0.25 vol.% MgO-DW nanofluid, despite the low zeta potential value of this sample. The results agree very well with the researchers' observations in [79][80][81]. Table 6. The improvement ratios of the thermal conductivity with solid volume concentrations of MgO/DW nanofluid at room temperature.

Concentration Ratios
Thermal Conductivity Enhancement The thermal conductivity of MgO is approximately 48.4 (W/m· • K) [57] which is significantly greater than water's value of 0.598 (W/M· • K) [78]. As a result, raising the concentration ratio of MgO nanopowder would undoubtedly improve and increase the thermal conductivity of the prepared nanofluid. Notably, the utilized MgO nanoparticles have a high specific surface area that comes from a nanoscale of 33 nm. Hence, there is a significant rate of heat transfer between dispersed nanoparticles and deionized water molecules, which increases as their volume concentrations increase.
In other words, an improvement in relative thermal conductivity means increased interactions between nanoparticles. The interactions are ascribed to the evolution of intense networks of dispersed nanoparticles inside the base fluid and the stir action caused by the Brownian motion of these nanoparticles. Furthermore, the Brownian motion becomes faster in the case of a nanostructure.
The greatest thermal conductivity enhancement (by using Relation (3)) was found to be 25.08% at the highest concentration of 0.25 vol.%. In addition, the lowest improvement was 5.68% at a volume concentration of 0.05 vol.%, as observed in Table 6. Generally, increasing the volume concentrations of MgO nanoparticles in the fluid minimizes the distance between moving particles, resulting in more molecular collisions. An increase in the accelerated nanoparticles in the base fluid enhances the thermal conductivity of the nanofluid. Furthermore, the development of clusters and aggregates can take the shape of chainlike structures which provide a transformation path of the thermal energy among the nanoparticles. This may explain the high improvement ratio of thermal conductivity in the case of 0.25 vol.% MgO-DW nanofluid, despite the low zeta potential value of this sample. The results agree very well with the researchers' observations in [79][80][81].
The major aim of ultrasonication treatment is to ensure proper dispersion and break up of any residual agglomerations in the dispersed phase (MgO nanopowder) within the base fluid. Figure 5 reveals the changes in thermal conductivity with respect to the ultrasonication treatment time of MgO-DW nanofluid. The evident trend in Figure 5 is that the values of thermal conductivity increase with ultrasonication time.
Appl. Sci. 2022, 12, x FOR PEER REVIEW 12 of 19 The major aim of ultrasonication treatment is to ensure proper dispersion and break up of any residual agglomerations in the dispersed phase (MgO nanopowder) within the base fluid. Figure 5 reveals the changes in thermal conductivity with respect to the ultrasonication treatment time of MgO-DW nanofluid. The evident trend in Figure 5 is that the values of thermal conductivity increase with ultrasonication time.
The increase in thermal conductivity with an increase in ultrasonication time may be attributed to enhancing dispersion stability, which was confirmed via zeta potential analysis (see Table 5). Therefore, the majority of the nanoparticles in the MgO nanosuspension will participate in the heat transfer process, increasing with ultrasonication agitation time, thereby, increasing its thermal conductivity. The differences in thermal conductivity were constrictive between 135 and 180 min of ultrasonication, and hence, ultrasonication influence was marginalized after 135 min. Therefore, the enhancement rate in thermal conductivity was insignificant over these periods, as shown in Table 7. The authors in [82][83][84][85][86] found a similar behavior when they investigated the thermal conductivity of MWCNT, Fe3O4/CNT, TiO2, CNT, and Al2O3 nanofluids, respectively, at various ultrasonication times. Therefore, at the ultrasonication time of 180 min, the 0.25 vol.% MgO-DW nanofluid showed better thermal transport characteristics due to the higher volume concentration of MgO nanoparticles in the suspended state.   The increase in thermal conductivity with an increase in ultrasonication time may be attributed to enhancing dispersion stability, which was confirmed via zeta potential analysis (see Table 5). Therefore, the majority of the nanoparticles in the MgO nanosuspension will participate in the heat transfer process, increasing with ultrasonication agitation time, thereby, increasing its thermal conductivity. The differences in thermal conductivity were constrictive between 135 and 180 min of ultrasonication, and hence, ultrasonication influence was marginalized after 135 min. Therefore, the enhancement rate in thermal conductivity was insignificant over these periods, as shown in Table 7. The authors in [82][83][84][85][86] found a similar behavior when they investigated the thermal conductivity of MWCNT, Fe 3 O 4 /CNT, TiO 2 , CNT, and Al 2 O 3 nanofluids, respectively, at various ultrasonication times. Therefore, at the ultrasonication time of 180 min, the 0.25 vol.% MgO-DW nanofluid showed better thermal transport characteristics due to the higher volume concentration of MgO nanoparticles in the suspended state.

Dynamic Viscosity
The concentration ratio of nanoparticles and ultrasonication time are major factors that influence the viscosity of nanofluids. Figure 6a,b shows the changes in viscosity and relative viscosity with volume concentration ratios of MgO-DW nanofluid, at room temperature. Appl. Sci. 2022, 12, x FOR PEER REVIEW 13 of 19 relative viscosity with volume concentration ratios of MgO-DW nanofluid, at room temperature. Figure 6a shows that the viscosity of MgO-DW nanofluid is directly proportional to the MgO nanoparticles loading. The viscosity of nanofluid samples was gradually increased with increasing concentrations of MgO nanopowder. The intersection on the viscosity axis corresponded to the viscosity value of base fluid as 0.001 (Pa·sec). The increase in nanofluid viscosity is narrow over the range of nanoparticle concentration of ≤0.05 vol.%, while the increasing is fast over a range of more than 0.05 to 0.25 vol.% of the MgO nanoparticle concentration. Furthermore, the resistance to flow increased to 0.0052 (Pa·Sec) at a higher solid content of 0.25 vol.% MgO-DW nanofluid. The dynamic viscosity behavior of MgO-DW nanofluid is harmonious with the predictions in [87,88]. The viscosity of nanofluids is compared to that of base fluid producing relative viscosity, as seen in Figure 6b. It shows how much the nanofluid viscosity differs from the viscosity of the base fluid. From Figure 6, it can be observed that the relative viscosity increases with increasing volume concentrations of MgO-DW nanofluid.    [87,88]. The viscosity of nanofluids is compared to that of base fluid producing relative viscosity, as seen in Figure 6b. It shows how much the nanofluid viscosity differs from the viscosity of the base fluid. From Figure 6, it can be observed that the relative viscosity increases with increasing volume concentrations of MgO-DW nanofluid.
The lowest value of relative viscosity, i.e., 1.2, was obtained by 0.05 vol.% MgO-DW nanofluid, with respect to a base fluid viscosity, while the higher value was obtained by nano solid content of 0.25 vol.%. When the MgO nanoparticle content increases within the base fluid, the Brownian motion is restricted as a result of intense collisions between nanoparticles. Subsequently, the relative viscosity of MgO-DW nanofluid increases with nanopowder volume concentration.
The effects of ultrasonication time on the viscosity of the MgO-DW nanofluid were investigated, as shown in Figure 7. The results show that the viscosity experiences a sharp decrease by increasing the ultrasonication treatment time. On the one hand, the minimal value of dynamic viscosity was measured at 180 min of ultrasonication treatment, indicating a decreasing tendency. On the other hand, the maximum value of viscosity was obtained by the nanofluid sample which was synthesized by stirring only without ultrasonication treatment of 0.0122 Pa·sec. This trend is identical to the results reported by A. Asadi et al. [89] and I. M. Mahbubul et al. [90]. Because the nanoparticles are uniformly dispersed in the base fluid, there are few or no clusters. As a result, the majority of nanoparticles, if not all, are involved in the flow, lowering the resistance to the viscometer's spindle. Additionally, the motion of the various layers of the nanofluid is made easier, hence, the dynamic viscosity decreases.
It is worth noting that increasing the ultrasonication time improves the dispersion stability and the thermal conductivity, while decreasing the dynamic viscosity of the prepared nanofluid. Ultrasonication time and dynamic viscosity are highly important in engineering applications where both of them have direct influences on the heat transfer performance, pumping power, and pressure drop.

Conclusions
A novel two-step technique was employed for preparing efficient MgO-DW nanofluid, at room temperature, without using surfactants and/or organic base fluids. On the one hand, the minimal value of dynamic viscosity was measured at 180 min of ultrasonication treatment, indicating a decreasing tendency. On the other hand, the maximum value of viscosity was obtained by the nanofluid sample which was synthesized by stirring only without ultrasonication treatment of 0.0122 Pa·sec. This trend is identical to the results reported by A. Asadi et al. [89] and I. M. Mahbubul et al. [90]. Because the nanoparticles are uniformly dispersed in the base fluid, there are few or no clusters. As a result, the majority of nanoparticles, if not all, are involved in the flow, lowering the resistance to the viscometer's spindle. Additionally, the motion of the various layers of the nanofluid is made easier, hence, the dynamic viscosity decreases.
It is worth noting that increasing the ultrasonication time improves the dispersion stability and the thermal conductivity, while decreasing the dynamic viscosity of the prepared nanofluid. Ultrasonication time and dynamic viscosity are highly important in engineering applications where both of them have direct influences on the heat transfer performance, pumping power, and pressure drop.

Conclusions
A novel two-step technique was employed for preparing efficient MgO-DW nanofluid, at room temperature, without using surfactants and/or organic base fluids. Based on the experimental valuation, the following conclusions can be drawn: - The co-precipitation method was followed to make crystalline and pure MgO nanopowder with an average particle size of 33 nm.

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