# Preparation Techniques of TiO\({}_{2}\) Nanofluids and Challenges: A Review
## 2 Preparation Methods
### Combination Usage  
Combination usage involves the utilization of a combination of different processes such as ultrasonication, magnetic stirring, and adjusting pH value of the base fluid to accomplish nanofluids with better dispersion efficiency and stability. [PERSON] et al. [79] used a combination of magnetic stirring and ultrasonication techniques for the preparation of TiO\({}_{2}\) nanofluid with different concentrations by using diathermic oil (0.86 g/cm\({}^{3}\), -10-320 \({}^{\circ}\)C) as the base fluid. Figure 14 shows the transmission electron microscope (TEM) image of the dried nanoparticles. Figure 14: TEM image of the dried TiO\({}_{2}\) nanoparticle [79]. Figure 13: Zeta potential of the TiO\({}_{2}\) particle (0.024 %vol.) in distilled water [78]. The process involved the dispersion of anatase type TiO\({}_{2}\) nanoparticles into the base fluid, adding oleic acid as a surfactant to increase the stability of the solution, stirred magnetically at 500 rpm and ultrasonicated for 1 h to homogenize the mixture at room temperature. The diameter of the particles just after the experiment was 22 nm and 19.3 nm with 0.4 vol. % and 0.2 vol. %, respectively, but increased to 48.5 nm when measured after 10 days. [PERSON] et al. [80] compared the stability of the TiO\({}_{2}\)-H\({}_{2}\)O binary nanofluids prepared by using titanium dioxide nanotubes (TiNTs) and titanium dioxide nanosheets (TiNSs) and found that the stability of nanofluid improved by using a combination of TiNTs and TiNSs powder rather than the usage of a single powder of TiNTs or TiNSs with the same concentration. They got the desired binary nanofluids by adding the mixed powder of TiNTs and TiNSs into the base fluid and stirred up to 30 min using the magnetic stirrer to ensure better suspension of powder into the base fluid. To get a more stable solution, ultrasonic agitation was performed with the help of ultrasonic cleaner for 1 h.  
[PERSON] et al. [81] dispersed TiO\({}_{2}\)-CNTs nanocomposite powder into a mixture of water and propylene (50:50), stirred and agitated thoroughly for 30 min to synthesize the desired TiO\({}_{2}\)-CNTs nanofluid. The thermal conductivity of all samples was measured by using the apparatus (H471) and the study concluded that there was an intensive increase in thermal conductivity with a temperature below 28 \({}^{\circ}\)C, after that, it increased linearly with temperature. [PERSON] et al. [82] investigated the effect of temperature, size of nanoparticles, the addition of surfactants and concentration on the dynamic viscosity of water-based TiO\({}_{2}\). The nanofluids of TiO\({}_{2}\) were formulated by using a two-step method and stabilized by adding polycarbonate and tri-oxadecane acid, adjusting pH value and dispersing the nanoparticles into the base fluid while sonicated. [PERSON] and [PERSON] [83] revealed that nanofluid prepared with a low concentration of TiO\({}_{2}\) nanoparticles and sodium dodecyl sulfate (SDS) surfactant (0.1 wt. %) showed greater thermal conductivity along with the highest stability. Nanofluid preparation involved the mixing of base fluid and surfactant with the help of a magnetic stirrer and it was ultrasonicated while adding the nanoparticles for proper dispersion. [PERSON] et al. [84] used magnetic stirring and ultrasonication to prepare a homogeneously dispersed solution of TiO\({}_{2}\). The experimental results showed an increase in viscosity and thermal conductivity of Titania by increasing the volume concentration of nanoparticles. [PERSON] et al. [85] improved the thermal conductivity of TiO\({}_{2}\) nanofluid with the addition of small amounts of nanoparticles of modified silver (Ag). Firstly, the composite of TiO\({}_{2}\) was prepared by using the combination of small (70%) and large nanoparticle (30%) with the help of a mechanical stirrer. For the purpose of doing a comparison, they prepared the three samples of nanofluids with 1, 2 and 3 wt. % concentration of nanocomposite particles mixed with flattened Ag nanoparticles and dispersed into water.