# Preparation Techniques of TiO\({}_{2}\) Nanofluids and Challenges: A Review
## 2 Preparation Methods
### Ultrasonication Method  
Ultrasonication is the act of applying ultrasonic sound waves of frequency \(>\)20 kHz to agitate the nanoparticles into the base fluid that reduces nano-sized cluster formation by breaking intermolecular interactions. [PERSON] and [PERSON] [57] followed the ultrasonication path while synthesizing TiO\({}_{2}\) nanofluid with different concentrations of TiO\({}_{2}\) nanoparticles in distilled water, prepared by using the sol-gel method. Figure 9 shows the XRD pattern and HR-TEM image of the prepared sample of TiO\({}_{2}\) nanoparticles. The nanoparticles were dispersed in water and ultrasonicated while at room temperature, to obtain the desired solution of Titania. The ultrasonication process prevented particle agglomeration by dispersing the particles into the base fluid. Figure 10 shows the stability of nanofluid with different concentrations of nanoparticles. [PERSON] et al. [58] performed experimentation to study the effect of ultrasonic quality during the fabrication of TiO\({}_{2}\) nanofluid on its stability and found that nanofluid prepared via continuous pulses of the ultrasonicator show high stability as compared to nanofluid prepared with discontinuous pulses. The TiO\({}_{2}\) nanofluids sample presented in this study were prepared by dispersing TiO\({}_{2}\) nanoparticles with different volume concentrations and ultrasonicated for 30 min to disperse the particles into the base fluid. Figure 9: (**a**) XRD pattern of TiO\({}_{2}\); (**b**) HR-SEM morphology of TiO\({}_{2}\)[57]. [PERSON] et al. [59] formulated and investigated the effects on crystallization characteristics of TiO\({}_{2}\) nanofluid by using rutile and brookite type nanoparticles. The results showed that, by varying the cooling rate (1.5-9.0 \({}^{\circ}\)C/min) for weight fractions of 0.30% and 0.70%, the rod shape (rutile TiO\({}_{2}\)) nanofluids performed best compared to the spherical shape (anatase TiO\({}_{2}\)) nanofluids and deionized water for ice storage systems. [PERSON] et al. [60] prepared TiO\({}_{2}\) nanofluid by dispersing spherical and cylindrical shaped nanoparticles in deionized water. For proper mixing of nanoparticles with different volume fraction into the base fluid, they used an ultrasonic dismembertor for 8-10 h. Experimental results showed that the thermal conductivity of the nanofluid increased with the variation of volume fraction. [PERSON] et al. [61] observed several important characteristics of the TiO\({}_{2}\) nanofluid that was prepared during their experiment. They found that the nanoparticle size decreased by increasing the sonication time and this decrease in size becomes stationary after reaching a specific sonification time. Figure 11 shows these results for the decrease in particle size as a function of sonication time. [PERSON] et al. [62] observed that the thermal conductivity of TiO\({}_{2}\) nanofluids was increased by increasing the concentration of the nanoparticles in the base fluid. They prepared three samples of TiO\({}_{2}\) nanofluids by dispersing the nanoparticles with concentrations of 0.2, 1.0, 2.0 vol. % in deionized water and ultrasonicated without adding any type of surfactant. Figure 11: TiO\({}_{2}\) particle size as a function of ultrasonication time [61]. Figure 10: [PERSON] and [PERSON]’s experimental results. [PERSON] et al. [63] prepared ethylene-glycol based nanofluid by using ultrasonication. The TiO\({}_{2}\) particles were submerged in the ethylene-glycol via ultrasonication and then magnetically stirred for 1 h. An enhancement in viscosity of 328 times was recorded at a volume fraction of 8.46% compared to the base fluid. [PERSON] et al. [64] developed a nanofluid of TiO\({}_{2}\) by using sol-gel derived nanoparticles for cutting tool applications, for the purpose of using it as a lubricant during face milling 7075 aluminum alloy. Nanofluid was dispersed in the ultrasonic bath in the presence of base fluid, to confirm the stability, for more than 24 h without any noticeable agglomeration and sedimentation.