# Preparation Techniques of TiO\({}_{2}\) Nanofluids and Challenges: A Review
## 2 Preparation Methods
### Ultrasonication Method
[PERSON] and [PERSON] [65] added TiO\({}_{2}\) (0.2 vol. %) nanoparticle into the base fluid and investigated the heat transfer coefficient and friction factor of the prepared TiO\({}_{2}\) nanofluid by passing it into a horizontal double-tube counter flow heat exchanger. According to the results, the convective heat transfer coefficient was improved by about 6-10% as compared to the base fluid. Cetyltrimethylammonium bromide (CTAB) was added as a surfactant into the base fluid to ensure proper dispersion and better stability with very less concentration of about 0.01%. The desired nanofluid of Titania was then prepared by putting the nanoparticles into the base fluid and it was sonicated for 3-4 h to ensure dispersion with the help of the ultrasonicator. [PERSON] et al. [66] measured the viscosity of TiO\({}_{2}\) nanofluids prepared using an ultrasonicator at higher temperature and nanoparticle concentration and compared the viscosity variation trend that they observed by varying the particle concentration to the experimental results of several different researchers. [PERSON] et al. [67] prepared TiO\({}_{2}\) nanofluids by enhancing the nanoparticles up to a volume of 3% and observed the relative increase in thermal conductivity and viscosity. They concluded that by varying the volume of nanoparticles, the increase in viscosity was more as compared to the increase in thermal conductivity. The nanofluids that were used during their experimental study were prepared by dispersing the nanoparticles in the deionized water and ultrasonicated with a ultrasonic vibrator to homogenize the mixture for better stability. [PERSON] et al. [68] studied the convective heat transfer coefficient and pressure drop of water-based TiO\({}_{2}\) by passing it through uniformly heated horizontal circular tubes and observed an increase in the heat transfer coefficient by increasing the nanofluid volume concentration. The TiO\({}_{2}\) nanoparticles were sonicated for 1 h at 30 \({}^{\circ}\)C permitting it to place groups of hydrophilic ammonium on the surface of the TiO\({}_{2}\) nanoparticles after mixing with 1,1,1,3,3,3 hexamethyldisilazane. Then, the prepared nanoparticles were dried and dispersed into the distilled water with a particular volume fraction. To break down the agglomerations and obtain better stability, the solution was suspended for ultrasonic vibration for 3-5 h at 400 W and 24 kHz. Figure 12 shows the Field Emission Scanning Electron Microscopic (FESESSM) image of the prepared TiO\({}_{2}\) nanofluid. Figure 12: FESEM image of the dispersed TiO\({}_{2}\) nanofluid [68]. [PERSON] et al. [69] found that TiO\({}_{2}\) nanofluid showed higher thermal conductivity as compared to the nanofluid of Al\({}_{2}\)O\({}_{3}\). TiO\({}_{2}\) nanofluid containing 1 vol.% nanoparticles. It achieved a 14% increase in thermal conductivity which was much higher relative to the increase in the thermal conductivity of Al\({}_{2}\)O\({}_{3}\). They followed the two-step ultrasonication method for the preparation of TiO\({}_{2}\) nanofluid, by dispersing nanoparticles into the base fluid and ultrasonicating it with pulses of 700 W at 20 kHz to prevent cluster formation and proper dispersion of nanoparticles in the base fluid. Ultrasonication is an effective technique to overcome cluster formation by reducing the bonding forces between the particles after dispersing the powder into the base fluid. The sonication effect reduces the agglomeration effect and permits the utilization of the full potential of nanoparticles in the solution.