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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.

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The remarkable enhancement in heat transfer capabilities of conventional fluids with the addition of nanosized metallic and non-metallic particles appealed the attention of investigators towards the suspension of hybrid nanocomposites as a substitute of mono particles. Although these fluids manifest captivating thermal characteristics, the drawbacks associated with their application include high frictional effects and pumping power requirements. The major cause of aforementioned problems is the elevated viscosity. The current study summarizes the work of different investigators and discusses the critical factors affecting the viscosity of hybrid nanofluids such as temperature, particle concentration, pH value, particle size and morphology with a concise discussion on the reasons reported in the literature for the viscosity augmentation. Furthermore, the models developed by different investigators have also been discoursed with specified limitations. Comparison between the viscosity of mono and hybrid nanofluid is also presented comprehensively. It is observed that most of the studies considered the effect of particle concentration and temperature that the effect of these factors is more significant. Water-based nanofluids delivered better results in comparison of ethylene glycol-based nanofluids while the oil-based nanofluids preferred in the applications where the pumping power is not more significant. It has been noticed that the fluids containing tube shaped nanoparticles comparatively showed enhanced viscosity than that of spherically shaped nanoparticles. It has also been observed that the studies preferred to develop their own models for the prediction of viscosity rather than to use the existing models and failed to provide a universal correlation.

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