A Review of Nanofluid Synthesis

Abstract

Stable colloidal suspensions of nanoparticles in solvents are conventionally termed as “nanofluids”. Several controversial reports in the literature on nanofluids explored the anomalous enhancement of their thermophysical properties (such as thermal conductivity). This has generated considerable interest among several research groups in the last three decades leading to thousands of publications on several aspects of nanofluids, spanning fundamental studies to various applications in engineering and life sciences. The popularity of nanofluids is spawned by their ability to deliver tunable material properties based as well as the ease and flexibility afforded by a multitude of nanosynthesis protocols. The material properties of nanofluids are highly sensitive to small variations in the nanosynthesis protocols. The properties of nanofluids can be tuned by varying their size, shape, morphology (surface functionalization), material composition, structure and mass concentration of nanoparticles as well as a variety of techniques for dispersing the chosen nanoparticles in the liquid solvents (which is often overlooked as an important variable in the nanofluids literature). Hence, due diligence is required in implementing the synthesis protocols for achieving nanofluids with the desired functionality and ensuring the stability. Different techniques for synthesizing nanofluids are summarized and the effect of synthesis conditions on the material properties of nanofluids are reviewed in this study. Several examples for a selected set of candidate nanofluids are discussed as well. This also helps to identify the gaps in the literature as well as recent developments on the topic of large-scale synthesis of nanofluids. This need is particularly acute for providing flexibility in scale-up for large volume manufacturing and batch-fabrication capabilities required in several practical applications (e.g., thermal energy storage and power generation). These applications would require tons of nanofluids to be synthesized on a commercial scale, as opposed to several milligram quantities that are typically synthesized in a research laboratory for a scientific study.

Appendix: Nanofluid Two-Step Synthesis

1.1 Metallic Nanoparticle-Based Nanofluids

Metallic nanoparticle-based nanofluids have drawn much interest due to the high thermal conductivity of metals. It is expected that the metallic suspensions can enhance the thermal transport properties of conventional heat transfer fluids which makes them suitable for heat exchanger applications.

1.1.1 Copper Nanofluids

The use of copper nanoparticle in nanofluids is appealing since copper has relatively high resistance to corrosion. Xuan and Li [96] prepared both water-Cu and oil-Cu nanofluids using the two-step method in which copper nanoparticles of about 100 nm diameter are directly mixed in water and mineral oil. For water-based nanofluid, laurate salt at 9% concentration was used as a dispersant which was observed to hold the water-Cu suspension stable for 30 h in a stationary state with minimal amounts of clustering. For oil-based nanofluids, oleic acid at 22% concentration was dispersed in the nanofluid to stabilize the suspension followed by ultrasonication for 10 h. The oil-Cu nanofluid was proved to be stable for 1 week with no sedimentation. Li [97] prepared aqueous copper nanofluid by mixing copper nanoparticle (~25 nm) in purified water at 0.1% concentration (with and without dispersant under different pH conditions). He observed that the addition of CTAB dispersant enhanced the stability of water-Cu suspensions by reducing the diameter of copper nanoparticles from 5560 to 130 nm. The nanofluid sample without dispersants quickly agglomerated, while the sample with dispersant remained stable with no sedimentation after 1 week. The water-Cu nanofluid samples showed maximum zeta potential value at pH = 9.5 which indicates that the suspension exhibits better dispersion in slightly basic environments. Saterlie [147] prepared water-Cu nanofluid by first synthesizing copper nanoparticles (~100 nm) in-situ using chemical reduction method and then re-dispersing them in water. The nanofluid was stabilized by adding SDBS as dispersing agent and ultrasonication for 50 min. He found that by increasing the Cu particle loading from 0.55 to 1.0%, several agglomerates are formed in the nanofluid with nanoparticle size increasing from 120 to 800 nm.

Surfactant-free copper nanofluids were also explored in various studies. Lu [148] prepared surfactant-free water-Cu and ethanol-Cu nanofluids by mixing copper nanoparticles (~20 nm) with the base fluid at 0.2–2% concentration and ultrasonicating for 10 h. The nanofluids were tested in a flat capillary pumped loop and sedimentation of nanoparticles observed on the heated surface. However, the morphology of the working nanofluid was not examined after the test. Kole [98] dispersed copper nanoparticles (~40 nm) in distilled water at 0.5% concentration with ultrasonication for 10 h. The suspensions remained stable for more than 15 days with no significant sedimentation . Garg [57] prepared EG-Cu nanofluids by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. The particle loading is 2.0% and no sedimentation was observed after a few days.

It is difficult to compare these different studies and draw a general conclusion on the effect of dispersing agent and ultrasonication toward the stability of nanofluid samples, since the nanofluid samples are synthesized under different conditions with different material characteristics. The stability of nanofluids is very sensitive to the variation in the size of the nanoparticles, concentration, pH, ultrasonication time, surfactant, etc. It is also important to notice that most studies have only shown stable nanofluids for the limited period (from few hours to 1 week). That suggests preparing stable samples of copper nanofluids via two-step methods to be used in long-term duty-cycles is still a challenging research topic.

1.1.2 Gold and Silver Nanofluids

Gold and silver nanoparticles have also been used in many studies due to their unique optical, electrical, and thermal properties (i.e., high electrical conductivity, stability, and low sintering temperatures). Such properties make them desirable in wide range of applications including diagnostics, antibacterial agents, heat transfer fluids, and optical fluids [149].

Preparation of gold nanofluids via two-step method is rare as most gold nanofluids were synthesized directly in the target base fluid from chemical reduction approach [150,151,152,153,154]. Silver nanofluids can be prepared by mixing manufactured silver nanoparticles in the base fluid. DisKang [155] prepared water/EG-Ag nanofluids at 0.1–0.4% volume fraction by dispersing silver nanoparticles (8–15 nm) into fluids without additives or stabilizers and ultrasonicating for 3 h. The nanofluid was generally stable for 1 day. Oliveira [156] prepared stable water-Ag nanofluid with 0.15% volume loading and 80 nm diameter nanoparticles using high-pressure homogenizer. The stabilization was achieved by placing the mixture in the high-pressure homogenizer and circulating for 30 min at 400–500 bars. The nanofluids were visually verified to be stable for at least 6 months. Hwang [133] prepared silicone oil-Ag nanofluid by dispersing produced silver nanoparticles in the base fluid with the assistance of various physical treatment techniques. With primary particle size of 35 nm and particle weight loading of 0.5%, he found that without any treatment, Ag nanoparticles were highly agglomerated in the pure fluid with an average nanoparticle size of 335 nm. Such values were reduced drastically to 182 nm, 147 nm, 66 nm, and 45 nm, after using stirrer, an ultrasonic bath, an ultrasonic disruptor, and a high-pressure homogenizer, respectively. Warrier [157] prepared water-Ag nanofluid at the concentration of 1 and 2% with nanoparticle size of 20, 30, 50, and 80 nm, respectively. The suspension was stabilized by both polyvinylpyrrolidone (PVP) at a concentration of 0.3% and ultrasonication process (while no settlement was observed for 2 h) after synthesis. Parametthanuwat [158] prepared water-Ag nanofluid at a concentration of 0.5% by repeated magnetic stirring and ultrasonicating after the addition of oleic acid (OA) (at concentration of 0.5, 1, and 1.5%) and potassium oleate surfactant (OAK⁺). It was found that the OAK⁺ exhibited good adsorption on the silver nanoparticles which helped improve the colloidal stability and non-precipitation period of the silver nanoparticles for up to 48 h.

Gold and silver nanofluids are typically synthesized via one-step method due to their inherent simplicity and competitive costs. It is worth noticing that these nanofluids were shown to be stable over several months—just by physical treatment.

1.1.3 Aluminum Nanofluids

Aluminum nanoparticles are of great interest in a variety of fields due to their high enthalpy of combustion and rapid kinetics. These characteristics make them favorable in fuel engineering field including alloy powder metallurgy parts for automobiles and aircrafts, rocket fuel, igniter, smokes, and tracers [159]. The study of aluminum nanofluid is limited as they are easily oxidized into alumina. Boopathy [160] prepared aluminum nanoparticles (~150 nm) by mechanical milling and dispersing them in distilled water and engine oil with 0.025% volume loading. The nanofluids were stabilized using 1% sodium lauryl (Dodecyl) sulfate as dispersant followed by 10 min of ultrasonication at 20 kHz and for 30 min of magnetic stirring at 1500 rpm. Teipel [161] prepared aluminum nanofluid using paraffin oil and HTPB as the base fluid. The Al particles (~80 nm) were dispersed by stirring for several hours and using ultrasound homogenizer before the suspension was tested for rheological measurements.

Aluminum nanopowders can react with water at high temperature (400–600 °C) to generate hydrogen which enhances fuel combustion [162]. Such feature promotes studies on aluminum nanofluid used in combustion and fuel. Kao [163] prepared aqueous aluminum nanofluid for diesel fuel combustion by producing emulsified nano-aluminum (40–60 nm) liquid using both ultrasonic vibrator and agitator. The work did not discuss the stability of the aluminum suspension. Gan [99] prepared aluminum nanofluid in n-decane and ethanol fuels with 80 nm Al nanoparticles at 10% mass loading by stirring the mixture vigorously and ultrasonication in an ice bath for 5 min. He observed that the suspension of n-decane/nano-Al remains stable for 10 min while ethanol/nano-Al can last for 24 h without obvious sedimentation. Xiu-tian-feng [164] synthesized stable jet fuel-Al nanofluids with 1.0% mass loading by modifying the surface of aluminum nanoparticles with various chemicals. It was found that oleic acid is the most effective surface modification agent which keeps the suspension stable for more than 2 weeks.

The use of aluminum nanoparticles in fuels and combustion requires high enough concentration for achieving considerable contribution to the energy content. Thus, the stabilization of the nano-mixture suspensions for long enough time is crucial for them to be utilized in liquid fuels.

1.1.4 Iron Nanofluids

Iron nanomaterials are of great interest as iron is among the most useful magnetic materials as well as the most abundant and widely used elements on earth. Doping iron nanoparticles in fluid manifest both fluid and magnetic properties which open new area of applications in electronic device, spacecraft propulsion, material science, biomedical instruments, and so on [165, 166]. Hong and Yang [167] prepared iron nanofluids with ethylene in which the Fe nanocrystalline powder (~10 nm) was first synthesized by chemical vapor condensation process and then re-dispersed in the base fluid with 0.2, 0.3, and 0.4% volume loading using ultrasonication (20 kHz). They observed an increment in thermal conductivity of the nanofluid with increasing sonication time from 10 to 70 min and ascribed it to the improved stability of suspension with prolonged sonication. Sinha [168] prepared EG-iron nanofluid by synthesizing iron nanopowders from chemical reduction method and mixing them in the base fluid under 50 min ultrasonic irradiation in concentrations of 1.0 vol.%. Agglomeration of nanoparticles was observed since the nano-crystallite sizes of the powders were below 20 nm while the average particle size in the fluid was around 500 nm. Xuan and Li [169] prepared magnetic iron nanofluid by directly dispersing Fe nanoparticles (~26 nm) into deionized water with the volume percentage of range from 1.0 to 5.0%. The suspension was stabilized using 1.0–6.0 vol.% sodium dodecylbenzenesulfonate as activator and the nanosamples showed good stability from few hours to 1 week. Gan [170] studied the combustion of iron nanofluid fuels prepared from dispersing iron nanoparticles (25 nm) in n-decane/ethanol with 5–20 wt.% concentration by hand mixing and ultrasonication . The nanofluid was stabilized with 0.5 wt.% sorbitan oleate as surfactant and mixture remains suspended for few hours.

Although pure iron exhibits better saturation magnetization, they are highly toxic and very sensitive to oxidation without appropriate surface treatment. In contrast, iron oxide nanoparticles are less sensitive to oxidation and, therefore, can give a better and stable performance [171].

1.1.5 Other Metal Nanofluids

Quite few other metal nanoparticles were also explored for synthesizing energy-efficient nano-suspensions. Naphon [172] prepared titanium nanofluid by mixing 21 nm Ti nanoparticles in de-ionized water and alcohol using an ultrasonic homogenizer. The nanofluid was tested in heat pipe without characterization on the suspension stability. Chopkar [173] prepared Al₂Cu and Ag₂Al dispersed nanofluids using the two-step method, in which Al₂Cu (20–30 nm) and Ag₂Al (30–40 nm) nanoparticles were first produced by mechanical alloying using a high energy planetary ball mill followed by dispersing these particles into ethylene glycol and water with volume fractions from 1.0 to 2.0%. The suspension was homogenized by intensive ultrasonic vibration and magnetic stirring with the addition of 1.0 vol.% oleic acid as surfactant. The sample showed good stability with some tendency of agglomeration during the test. However, metal nanoparticles were found more likely to be oxidized at even low temperature [12] which brings instability in the hydrothermal performance. Besides, metal nanofluids suffer from the issue of quick sedimentation and fouling which makes it challenging to use them in practical applications [174, 175].

1.2 Oxide Nanoparticle-Based Nanofluids

Oxide nanomaterials have been intensively used in modern nanotechnology. Their unique physicochemical properties have opened up applications in nanoelectronics, sensors, optics, catalysts, biomedicine, etc. [176,177,178,179,180]. Preparation of nanofluids using oxide nanoparticles via two-step methods is discussed below.

1.2.1 Copper Oxide Nanofluids

Copper oxide nanoparticles are used in diverse applications with a range of useful properties such as high electric/thermal conductivity, electron correlation effect, high atom efficiency, etc. [181, 182]. Choi and Eastman [183] first studied copper oxide nanofluid in water and ethylene glycol, in which they dispersed CuO (~20 nm) nanoparticles produced by gas condensation in the base fluid directly by shaking thoroughly. It was observed that Cu nanoparticles agglomerated into large particles (~100 nm) which could still form the stable suspension in the liquid. Kwak [184] prepared copper oxide nanofluid in ethylene glycol using 10–30 nm Cu nanoparticles at 0.001–1% volume fraction dispersed by ultrasonication. It was found that sonication for 9 h gives the best dispersion and the suspension was stable for 100 days. Namburu [185] prepared copper oxide nanofluid in EG–water mixture (60:40) with a particle size of 29 nm and volume concentration increasing from 1 to 6%. The nanofluid mixture was stirred and agitated thoroughly for 30 min with an ultrasonic agitator for ensuring uniform dispersion. Kulkarni [186] did a similar study by mixing CuO nanoparticles (~29 nm) in deionized water with 5–15% volume fraction. The uniform mixture of nanoparticles in water was attained by thorough stirring and ultrasonicate agitation for half an hour. Li and Peterson [100] prepared water-CuO nanofluid with 29 nm diameter and 2–10% volume fraction. The powder and base fluid were blended by immersing in an ultrasonic bath for 3 h and the suspensions were found to be very stable, with essentially no sedimentation over 7 days. Karthikeyan [101] prepared water/EG-CuO nanofluid using monodispersed CuO particles of 8 nm diameter. The suspension was homogenized by using an ultrasonic horn for 30 min without the addition of surfactant. The study found that the nano-mixture remained stable for more than 3 weeks with particle volume concentration below 1%. Above 1% volume concentration, sedimentation in CuO nanofluid was observed. Rashin [102] prepared copper oxide-coconut oil nanofluid using 20 nm CuO nanoparticles with 0.5–2.5% mass concentration. The nanoparticles were dispersed by only 1 h ultrasonication and the suspension remains stable for 7 days after which the sedimentation starts. Kole [103] prepared stable nanofluid by dispersing 40 nm spherical CuO nanoparticles in gear oil with volume fraction ranging between 0.5 and 2.5%. The mixture was stabilized by mixing with oleic acid, intensive ultrasonication for 4 h and magnetic agitation for 2 h. Although aggregation of CuO nanoparticles was identified with average cluster size ~7 times of the primary diameter , the suspension was stable for more than 30 days without visual sedimentation. Sahooli [120] studied the effect of pH and surfactant concentration on the stability of CuO nanoparticles (4 nm, 1.0 wt.%) in the water-based nanofluid. He proposed that the suspension zeta potential and absorbency were maximized at pH = 8 and 0.095 wt.% PVP, which is the optimum condition for obtaining most stable nanofluid. However, the average particle size measured with PVP surfactant was 63 nm indicating clustering of the nanoparticle.

In general, it was found that the copper oxide nanofluids can be quite stable for moderate time period without the presence of the surfactant, if the nanoparticle concentration is low. The time scale for non-sedimentation could vary from few days to months depending on the ultrasonication and stirring condition. Aggregation of ultrafine nanoparticle is inevitable in CuO nanofluid but the nanoscale cluster can still be stable in the mixture suspension.

1.2.2 Alumina (Al₂O₃) Nanofluids

Alumina nanoparticles are among the most widely used due to their abundance and low cost of mass production. The use of alumina nanoparticles in different base fluids has drawn considerable interest in applications including electronic cooling, deep drilling, thermal energy storage, etc. [187]. Beck [188] dispersed 20 nm diameter alumina nanoparticles in ethylene glycol with mass fraction ranging from 3.26 to 12.2%. The nanofluid was stabilized by ultrasonic mixing for several minutes and the suspension remained uniform during the experiments. Timofeeva [189] prepared nanofluids of alumina particles in water and ethylene glycol with three different particle size (11, 20, and 40 nm) and two different volume concentration (0.01% and 0.1%). The mixture was sonicated continuously for 5–20 h in an ultrasonic bath and highly agglomerated nanoparticles were observed in the experiments. It was found that particle with smaller diameter tends to form larger agglomerates and the agglomeration size increases with time as the sample ages. However, nanosamples were still found to be stable in both water and ethylene glycol. Esmaeilzadeh [190] prepared water-alumina nanofluid using 15 nm Al₂O₃ nanoparticle with 0.5 and 1% volume fractions. The mixture was stabilized through a 4 h process of ultrasonication and electromagnetic stirring. No sedimentation was observed throughout the testing period. Sarathi [191] dispersed 50 nm Al₂O₃ nanoparticles in distilled water by magnetic stirring for 3 h and ultrasonication for few hours. Sedimentation of particles was still observed after the sonication and stirrer was used during the experiment to minimize the sedimentation.

Use of surfactant and pH control can significantly enhance the stability of alumina nanofluid. Sharma [192] prepared stable water-Al₂O₃ nanofluid by mixing SDBS with one-tenth the mass of the nanoparticle (~47 nm) in the suspension. The nanofluid was observed to be stable for over a week if the volume concentration is less than 3%. With higher concentration, some sedimentation was observed. Teng [193] prepared water-Al₂O₃ fluid using 0.3 wt.% chitosan as the dispersing agent. The mixture with 0.5, 1.0, and 3.0 wt.% Al₂O₃ nanoparticles showed good suspension for 1 month during which the sample was placed statically. Jung [121] prepared water-based alumina nanofluid using a horn-type ultrasonic disrupter for 2 h. The nano-suspensions with 0–0.1 vol% Al₂O₃ nanoparticles (45 nm) were observed to be stable for more than 1 month with/without the addition of polyvinyl alcohol surfactant. Khairul made a more comprehensive study on the effects of surfactant toward the stability of Al₂O₃ nanofluid. He used different weight fractions from 0.05 to 0.2% of the dispersant SDBS to stabilize the water-Al₂O₃ (10 nm) nanofluid with nanoparticle weight ratio in the range of 0.05–0.15%. It was found that aggregation of nanoparticle still presents in the fluid, but 0.1% SDBS gives lowest mean aggregation size and maximum zeta potential which is an indication of good stability. Ho [194] prepared 0.1–4 vol.% water-Al₂O₃ nanofluid with the particle size of 33 nm. The nano-suspension was stable for at least 2 weeks after magnetic stirring for 2 h. and adjusting pH value to 3. Jacob [195] used similar method prepare stable suspensions of Al₂O₃ nanoparticles (~50 nm) in de-ionized water with 0.25, 0.5, and 1% volume. The mixture was stabilized by adjusting the pH value away from the isoelectric point and sonication for 5–6 h.

In general, it was found that the alumina nanofluids only exhibit short-term stability with mechanical stabilization methods. The stabilization period was enhanced to months if appropriate amount of dispersant was used.

1.2.3 Titanium Dioxide Nanofluids

Titanium dioxide is being widely used in various consumer goods and products including cosmetics, paints, dyes, plastics, drugs, etc. Nanoscale TiO₂ has high diffraction index and strong light scattering capability which makes them highly used in radiation protection productions, photocatalyst and photovoltaic [196,197,198,199,200]. The use of TiO₂ nanoparticle in nanofluid is promising due to its excellent chemical/physical stability and low cost from commercial manufacturers.

Ding and Wen [201] dispersed 30–40 nm TiO₂ nanoparticles in distilled water with 0.024% volume concentration. The stabilization of nanoparticles in water was realized by (1) cleaning of the bottles in ultrasonic bath; (2) adjusting the pH of the base liquid to pH = 3; (3) ultrasonification of the bottles containing dispersion for 15 min; and (4) shear mixing of the dispersion under the homogenizer for 30–180 min. The dispersion was found to be very stable for at least a couple of weeks without visually observable sedimentation. Murshed [202] prepared titanium dioxide nanofluid by dispersing TiO₂ nanoparticles in rod-shapes of ∅10 nm × 40 nm (diameter by length) and in spherical shapes of ∅15 nm in deionized water. The nanoparticles were dispersed uniformly using ultrasonic dismembrator for 8–10 h with/without 0.01–0.02 vol.% oleic acid and CTAB surfactants. It was found that nanoparticles agglomerated into large clusters without surfactant, and adding surfactant brought better stability which is indicated by the increment in nanofluid thermal conductivity. Turgut [203] prepared water-based TiO₂ nanofluid with particle size of 21 nm and particle volume concentration from 0.2 to 3%. The mixture was homogenized using ultrasonic vibration which breaks down the agglomerations. Yue-fan [204] prepared titanium dioxide colloidal suspension by dispersing TiO₂ nanoparticles (<20 nm) in transformer mineral oil with 0.003–0.05 g/L concentration. The particles were dispersed by just ultrasonic route and the suspension was stable for 24 h. He and Jin [205] prepared aqueous TiO₂ nanofluid by dispersing 20 nm diameter dry titanium dioxide nanoparticles in distilled water with 1.0, 2.5, and 4.9% mass concentration. The mixture was stabilized by first applying ultrasonication for 30 min, then processed in a medium-mill and finally adjusting the pH value to 11. The particle size distribution after each process stage was ~500 nm, ~120 nm, and ~95 nm while the nanofluids were found to be very stable for months. Charkraborty [206] prepared 0.1 wt.% water-based TiO₂ nanofluid using dry particles with size in the range of 30–50 nm. The nanofluid was homogenized by high shear mixer which breaks down the agglomerate and addition of 0.01 wt.% surfactant which ensures longer stability. Fedele [207] studied the characterization of water-based nanofluids containing TiO₂ (~72 nm) nanoparticles in mass concentrations ranging between 1 and 35%. The nanofluids were stabilized using 1–5 wt.% acetic acid as dispersant with 1 h sonication. The mean diameter of the static suspension decreases to around 51 nm after 35 days, indicating a partial precipitation. Such value returned to 76 nm after re-sonication for 1 h, suggesting the absence of further aggregation in the suspension. Said [208] dispersed 21 nm TiO₂ spherical particles in distilled water with 0.1% and 0.3% volume concentration. The homogenous dispersed solution was obtained after adding PEG 400 dispersant with two times the mass of the particles and passing through 30 cycles in a high-pressure homogenizer. No visual sign of aggregation and sedimentation was observed for a period of a month. Muthusamy [209] prepared stable titanium dioxide nanofluid by dispersing 50 nm diameter TiO₂ particles in ethylene glycol with 0.5, 1.0, and 1.5% volume concentration. The suspension was stabilized by merely mechanical stirring process and proved to be stable for more than 3 weeks with ~220 nm local clusters.

The titanium dioxide nanofluid exhibits relatively good stability (from few weeks to months). Different stabilization conditions were required for achieving optimum dispersion depending on the base fluid and particle characteristics.

1.2.4 Zinc Oxide Nanofluids

Zinc oxide is also among some of the widest used nanomaterials with its good electrical, electrochemical, and structural properties [210, 211]. ZnO nanoparticles exhibit in various form (particle, rod, thin film) and can be used in electroluminescent devices, chemical sensors, solar cells, etc. [212]. Zhang [213] prepared water-based zinc oxide nanofluid with 20 g/L concentration by dissolving ZnO nanoparticles (60–200 nm) in distilled water, sonicating for 30 min and milling for another 3 h. The average particle size increased slightly from 198.4 to 225.9 nm after being stored for 120 days, indicating good stability of the nano-suspension over time. Yu [214] dispersed dry ZnO nanoparticles (10–20 nm) in ethylene glycol with volume concentration ranging from 0.2 to 5%. The nanofluid was stirred and sonicated continuously for 15 min to 12 h. It was found that the average particle size decreases rapidly in the first 3 h and remained 210 nm afterward. The measured average particle size in the formulated nanofluids is much larger than the size of primary particles indicating ultrasonification was not able to break the agglomerates completely. Sagadevan [215] prepared ZnO nanoparticles (15–20 nm) first by solvothermal reaction and dispersed them in polyvinyl alcohol with magnetic stirring process and ultrasonic vibrator for 5 h. The dispersed mixture was clear and stable for up to 2 weeks. Esfe and Saedodin [216] prepared EG-ZnO nanofluid using 18 nm ZnO nanoparticles with volume concentration ranging from 0.25 to 5.0%. The suspensions were subjected to ultrasonic vibrator for 3–5 h in order to get a uniform dispersion and a stable suspension. Subramaniyan [104] prepared water-ZnO nanofluid by dispersing 0.1%–0.4 wt.% ZnO nanoparticles in water using ultrasonication for 20 min. It was found that ZnO nanofluids with 0.3 wt.% showed highest stability with the maximum zeta potential values. Visual sedimentation showed that 0.3 wt.% nanofluid is stable for 20–24 h without any trace of sedimentation but all the other fluids settle within 6–12 h. Raykar and Singh [105] synthesized water-soluble ZnO nanoparticle (non-spherical, 100–150 nm) via chemical precipitation method and dispersed them in deionized water. The mixture was sonicated for 1 h with the addition of acetylacetone as dispersant. The nanofluid was found to be stable over 9 months to 1 year. Suganthi and Rajan [106] prepared stable ZnO–water nanofluids with particle volume concentrations in the range of 0.25–2 vol.%. They dispersed ZnO nanoparticles (35–45 nm) into water with sodium hexametaphosphate (SHMP) surfactant under high shear homogenization for 20 min, followed by ultrasonication for 180 min. The high colloidal stability was verified by high absolute value of zeta potential as well as visual observation. Saliani [107] dispersed ZnO nanoparticles (4.45 nm) in glycerol with the aid of a magnetic stirrer. Ammonium citrate with the same mass of nanoparticles were used as a dispersant to enhance the stability of the nanofluids, and the suspensions were stable for at least several months with no sedimentation observed during the period.

It is noticed that the stability of zinc oxide nanofluids could be significantly enhanced by adding proper surfactant which could be potentially helpful for using them in the long-term application.

1.2.5 Iron Oxide Nanofluids

Iron oxide nanoparticles have attracted considerable interest due to their superparamagnetic properties and their potential biomedical applications arising from its biocompatibility and non-toxicity [217]. Asadzadeh [108] dispersed 0.05 vol% and 0.1 vol.% Fe₃O₄ nanoparticles (<50 nm) in ethylene glycol using vigorous mechanical agitation and ultrasonication for 1 h. The suspension was stable for 12 h without observable sedimentation. Sheikhabahai [109] prepared Fe₃O₄ nanofluid using EG–water mixture (50 vol.%-50 vol.%) with 0.02–0.1% particle volume loading. The 50 nm diameter Fe₃O₄ nanoparticles were added into the base fluid gradually under ultrasonic mixing for an hour. The nanofluid was stabilized with another hour of sonication and no sedimentation was observed for 8 h. Sundar [110] prepared water-based nanofluid using 36 nm Fe₃O₄ nanoparticles at 0.02, 0.1, 0.3, and 0.6% volume concentrations. The particles were uniformly dispersed in the base fluid at pH value of 3 with 2 h sonification. The uniform dispersion of the nanoparticles is established by visual observation for nanoparticle sedimentation and measuring the densities of nanofluid at different locations in the container. Župan and Renjo [111] prepared water-based ferrofluids by dispersing 50 nm diameter Fe₃O₄ nanoparticles in deionized water using ultrasonic bath for 90 min. The sonified colloid was stable for 1 h without dispersant or activating agent. However, visible sedimentation was observed in the bottom of the suspension after 24 h. Phuoc and Massoudi [218] dispersed Fe₂O₃ nanoparticles (20–40 nm) in deionized water with 0.2 wt.% Polyvinylpyrrolidone (PVP) or Poly(ethylene oxide) (PEO) as surfactant. The suspension was homogenized by magnetic mixing and ultrasonication for 30 min. It was observed that these nanofluids could remain stable for 2 weeks if the particle concentration is less than 2% and less than 1 week if the concentration is higher. Goshayeshi [219] prepared γ- and α-Fe₂O₃/Kerosene nanofluids with 2.0% volume concentration. The nanoparticles were added into the base fluids with oleic acid surfactant and stirred constantly, followed by 5 h sonication. The Fe₂O₃ nanoparticles could readily disperse in organic solvent and the suspension was stable for 10 days. Salari [220] prepared aqueous iron oxide nanofluids by dispersing 0.1–0.3 wt.% Fe₃O₄ nanoparticles (~20 nm) into the water using motorized magnetic stirrer with speed of 250 rpm for 30 min. The suspension was stabilized by adding 0.1 vol.% nonylphenol ethoxylate surfactant into DIW, ultrasonication for 30 min and adjusting pH values. The most stable nanofluid was obtained when pH = 8.43 and the suspension was stable for 25 days.

It can be seen from various studies that the iron oxide nanofluids exhibit relatively shorter stabilization period (less than a month) even with pH control and surfactant.

1.2.6 Silicon Dioxide Nanofluids

Silicon dioxide nanoparticles are of great interest in a variety of biomedical applications due to their stability, low toxicity and capability for functionalization with different molecules and polymers [221]. Fazeli [222] prepared water-SiO₂ nanofluids by dispersing 18 nm silica nanoparticles in distilled water with 3.5, 4, 4.5, and 5% volume concentration. The suspension was stabilized using ultrasonic bath for 90 min without any surfactant and the nanofluids were stable for a period of 72 h without any visible settlement. Jin [223] prepared 0.005–0.1% mass fraction silica nanofluid in mineral oil using 10–20 nm size SiO₂ nanoparticle. The particles were dispersed uniformly in the base fluid using magnetic stirring for 15 min and ultrasonication for 2 h. The 0.005% and 0.01% silica nanofluids were found to be stable for around 1 month, for the 0.02% silica nanofluid the stability of the suspension was reduced to 2 days, and the 0.1% silica nanofluid was stable for less than 24 h. Rafati [224] prepared silica nanofluid using a mixture of deionized water (75 vol.%) and ethylene glycol (25 vol.%) as the base fluid. SiO₂ nanoparticles with 14 nm average size were dispersed in the base fluid with 0.5, 1.0, and 1.5% volume concentration using ultrasonication. The nanofluids showed great stability even after 1 week. Noghrehabadi [225] dispersed 12 nm SiO₂ nanoparticles in water with 1% mass concentration using vertical mixer and ultrasonication for 60 min. The nanofluid was homogenized in ultrasonic bath every day to break down the agglomeration and minimize the sedimentation. Sharif [226] prepared polyalkylene glycol-SiO₂ nanolubricant by dispersing 30 nm SiO₂ nanoparticles in the base fluid using a magnetic stirrer for 1 h, and then surged using ultrasonic bath vibrator for 2 h. Minimum sedimentation was observed 1 month after the preparation of nanofluid with 0.2–1.5% volume concentration . Liu and Liao [227] prepared silica nanofluid in both water and alcohol. SiO₂ nanoparticles with 35 nm average diameter were dispersed in the base fluid with 0.2–2% mass concentration. The nanofluids were mixed with 0.5 vol.% SDBS surfactant and surged in super-sonic water bath for 12 h. The experimental results showed that the stability and uniformity of nanofluids were good at least in 1 month. Zhang [122] studied the influence of ultrasonication, dispersants, and pH on the stability of water-silica nanofluids. The nanofluids were prepared by dispersing 1.0 wt.% SiO₂ nanoparticles (~50 nm) in water with mechanical force agitation, ultrasonication, and addition of SDBS. It was found that the silica clusters were effective dispersed with average size of 63 nm in suspension under the sonication power of 500 W and sonication time of 120 min. The maximum absolute zeta potential was attained with SDBS concentration of 1.0% and pH value of 9.5. The good stability was also verified with long-term test in which the particle size remained unchanged after 7 days. Yang and Liu [114] prepared stable water-based nanofluid by dispersing surface-functionalized SiO₂ nanoparticles (30 nm) in water with 10% mass fraction. The nanofluid was kept stable for 12 months. Bagwe [228] prepared silica nanoparticles with different functional groups (including carboxylate, amine, amine/phosphonate, polyethylene glycol, octadecyl, and carboxylate/octadecyl groups) in water and studied their aggregation behavior in water. It was found that the nanoparticles prepared with appropriate amount of amine/phosphonate functional group were stable for more than 8 months in aqueous solution.

The silica-based nanofluids have relatively low stability using only physical stabilization method. The addition of surfactant has some marginal effect on the improvement of the stability, but surface modification could effectively make silica nanoparticles sustain much longer in the suspension environment.

1.3 Organic Nanofluids

Organic nanoparticles exhibit superior electric and thermal properties owing to their unique metal lattice or graphite structures, and have attracted attention for a wide range of applications in different fields. Dispersing carbon-based nanoparticles in liquid has not been as easy as other nanoparticle, as carbon-based particles have a strong tendency to agglomerate due to the strong intermolecular force. It has shown that pristine CNTs will precipitate rapidly in most of fluids even with prolonged sonication [229]. Consequently, surfactant and surface modifications have been used in almost all carbon-based nanofluid preparations.

Wen and Ding [230] prepared the stable aqueous suspension of MWCNT with 0–0.84% volume concentration. The prepared sample was stabilized following a sequence of steps involving: (1) ultrasonicating CNT sample in water bath for 36 h; (2) dispersing CNT in distill water with 20% by weight of sodium dodecyl benzene sulfonate (SDBS) with respect of CNTs; (3) ultrasonicating mixture in water bath for 24 h; (4) treating suspension with high-speed magnetic stirrer for 1 h. The aqueous CNT nanofluid was found to be very stable from months without sedimentation. Wusiman et al. [231] prepared MWCNTS in aqueous suspension with surfactant SDBS and sodium dodecyl sulfate (SDS). They changed the MWCNT concentration from 0.1 to 1 wt.%, with CNT/surfactant ratio varied from 1/1 to 4/1. The suspension was only subjected to ultrasonic mixing for 20 min and the samples with 3/1 CNT-surfactant ratio was found to be most stable for more than 1 month without sedimentation. SDBS was found slightly superior than SDS in their study with better thermal performance. Rashmi et al. [232] prepared aqueous dispersion of CNTs in the presence of gum arabic (GA), with concentrations of CNT and GA varying in the range of 0.01–0.1 wt.% and 0.25–5 wt.% respectively. The mixture was homogenized at 28,000 rpm for 10 min and further sonicated in water bath for 1–24 h. It was found that the optimum concentration of GA varies from 1.0 to 2.5 wt.% with increasing CNT concentration, and the nanofluid was found to be stable for more than 40 days. Quite few other surfactants have also been tested to show effective enhancement on the suspension stability of CNT nanofluid including polyvinylpyrrolidone (PVP) [233], hexadecyltrimethyl ammonium bromide (CTAB) [234], chitosan [235], and gemini surfactant [236]. The optimum concentration for each surfactant is dependent on its own property and the interaction with the carbon molecules.

Pre-treating CNTs in acid endows them with carboxylic acid and hydroxyl groups, which could effectively prevent the CNTs from aggregating over time [237]. Osorio et al. prepared functionalized CNTs by soaking CNTs in three different acid environments for 2 h: (1) H₂SO₄/HNO₃/HCl; (2) H₂SO₄/HNO₃; and (3) HNO₃. The treated CNTs were dispersed in water and the subsequent sedimentation over 20 days showed the good stability of the sample soaked in multi-acid environment. Zhang et al. [238] prepared water-soluble CNTs by the introduction of potassium carboxylate (−COOK) using potassium persulfate (KPS) as oxidant. The KPS-treated SWNTs was dispersed in deionized water with ultrasonic water bath and found to be stable over 1 month. Narisi et al. [239] prepared stable CNT nanofluid in water using a combination approach of surface functionalization, surfactant, and ultrasonication. The CNTs were first treated with KPS oxidant, and then dispersed in water with 0.25 wt.% SDS undergoing 45 mins’ ultrasonication using both probe and bath ultrasonicator. The average particle diameter was examined using dynamic light scattering which remained constant (~200 nm) 2 months after preparation.

Although we have been discussing CNT-based nanofluid here, the concept and preparation method is quite similar for graphene/graphene oxide/fullerene-based nanofluid . Owing to their common surface properties, the dispersing method is widely acceptable between different types of carbon nanoparticles.

1.4 Special Nanofluid (High/Low Temperature)

For very high-temperature and low-temperature nanofluid, the preparation of nanofluid involves special steps as the mixing process cannot be performed in room environment. These nanofluids are usually based on materials which are not in liquid form at room temperature and atmospheric pressure (i.e., molten salt, liquified gas, low temperature refrigerant).

1.4.1 High-Temperature Nanofluid

For materials which are in the solid state at room environment, the preparation of nanofluid is usually performed by first dissolving the target material in a room temperature liquid (usually water), then dispersing nanoparticle in the solution, and finally evaporating water out and heating the composite to high temperature where it transformed into liquid. A typical example is the molten salt-based nanofluid which melts at temperature more than 200 °C. Shin and Banerjee [240] prepared silica nanofluids in alkali chloride eutectic. They first dissolve all chloride salt in distilled water, then dispersed 1.0 wt.% SiO₂ nanoparticle via ultrasonication bath for 100 min, and evaporated water in the vial on a hot plate at 200 °C until dried completely. The nanofluid showed good stability as particle size remained constant after repeated DSC cycling tests. Jo and Banerjee [241] prepared graphite nanofluid in molten carbonate salt using gum arabic as the surfactant. In his study, the surfactant and graphite nanoparticle were first dispersed in distilled water with 2 h sonication, then require amount of K₂CO₃ and Li₂CO₃ were dissolved in the suspension liquid with additional 3 h sonication . The final mixture was transferred to a petri dish and heated on a hot plate at 100 °C until fully dehydration. The nanomaterials showed good dispersibility with consistent thermophysical property measurements from repeated tests. Most other molten salt nanofluid preparations [242,243,244,245,246] have followed the same procedure used by Shin and Jo.

1.4.2 Low-Temperature Nanofluid

Most of the widely used refrigerants are in vapor state under room environment (i.e., R134a, R410a). Hence, preparing well-dispersed nanofluid using these materials is usually accomplished by first dispersing nanoparticle in a secondary fluid, and then putting the dispersed fluid into the refrigeration system before the refrigerant fills the test loop. Bi et al. [247] mixed TiO₂ nanoparticle into R134a by first dispersing the nanoparticle into mineral oil via conventional approaches, then put the mixture into the compressor to let the refrigerant mixing with the nanoparticle. Jwo et al. [248] followed a similar approach to mix Al₂O₃ nanoparticles in R134a using POE oil. Subramani and Prakash [249] prepared SUNISO 3GS oil-based nanolubricant with 0.06 wt.% Al₂O₃ nanoparticle which is stable for 3 days without coagulation or deposition. They then filled the nanolubricant in the compressor where it mixes with R134a.

In certain cases, nanoparticles can be added directly into the low temperature liquid as well. Anderson [250] dispersed MWCNT into liquid oxygen (LOX) by tipping the nanoparticles into the LOX gently and slowly. It is mentioned that great cares were taken at this point to avoid micro-scale boiling. The dispersion was then performed by ultrasonicating the mixture with a pre-cooled probe sonicator for 20 s. However, the study on cryogenic nanofluid is rather limited, which may be due to the inherent technical difficulty and narrow application field.