# Preparation Techniques of TiO\({}_{2}\) Nanofluids and Challenges: A Review
###### Abstract  
Titanium dioxide (TiO\({}_{2}\)) has been used extensively because of its unique thermal and electric properties. Different techniques have been used for the preparation of TiO\({}_{2}\) nanofluids which include single-step and two-step methods. In the natural world, TiO\({}_{2}\) exists in three different crystalline forms as anatase, brookite, and rutile. Nanoparticles are not used directly in many heat transfer applications, and this provides a major challenge to researchers to advance towards stable nanofluid preparation methods. The primary step involved in the preparation of nanofluid is the production of nano-sized solid particles by using a suitable technique, and then these particles are dispersed into base fluids like oil, water, paraffin oil or ethylene glycol. However, nanofluid can also be prepared directly by using a liquid chemical method or vapor deposition technique (VDT). Nanofluids are mostly used in heat transfer applications and the size and cost of the heat transfer device depend upon the working fluid properties, thus, in the past decade scientists have made great efforts to formulate stable and cost-effective nanofluids with enhanced thermophysical properties. This review focuses on the different synthesis techniques and important physical properties (thermal conductivity and viscosity) that need to be considered very carefully during the preparation of TiO\({}_{2}\) nanofluids for desired applications. titanium dioxide; nanofluids; anatase; brookite; rutile; synthesis techniques; thermophysical properties; heat exchangers +
Footnote †: journal: Applied Soft  
showed better performance with enhanced thermophysical properties without creating considerable stability problems. Fine particles in the range of millimeters and micrometers failed to exhibit better heat transfer performance because of obstacles such as particle sedimentation, excessive pressure drop, low thermal conductivity, corrosion of components and particle clogging, etc. This problem can be mitigated by suspending the nano-sized particle in the range of 100 nm in the suitable volume fraction of conventional base fluid. Ultrafast heat transfer, adequate durability, negligible agglomeration, stable dispersion, reduction in pumping power and better lubrication are the important features of nanofluids. All the nanoparticles that have a high thermal conductivity include metallic (Ag, Cu, Al, Fe, Au, etc.) and non-metallic (TiO\({}_{2}\), CuO, Al\({}_{2}\)O\({}_{3}\), SiC, Carbon nanotubes, Fe\({}_{3}\)O\({}_{4}\) etc.) particles can be used as an additive for the preparation of nanofluid in the presence of base fluid. Among all these nanoparticles, TiO\({}_{2}\) nanoparticles are used most commonly due to the stability of its chemical structure, electrical, optical, biocompatibility, and physical properties. We can easily understand the prodigious heat transfer feature from the [PERSON]'s law of cooling. \[Q=h\times A\times\Delta t \tag{1}\]  
Nanofluids can be helpful for enhancing the rate of heat transfer (\(Q\)) by increasing the convective heat transfer coefficient (\(h\)) for a constant value of heat transfer area (\(A\)) and temperature difference (\(\Delta t\)) due to the enhanced thermal conductivity. When the nanosized solid particles are dispersed into the base fluid each particle absorbs and desorbs an electron at the interface with the base fluid and gets electrically charged [4]. In the case of stable nanofluids, the Vander Waals repulsive forces become larger compared to the attractive forces between the particles and the Brownian motion is enhanced. Brownian motion plays a significant role in increasing the thermal conductivity by enhancing micro-convection due to the transfer of heat between solid particles during their collision [5; 6; 7].