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The recent studies on nanotechnology have reported rapid development of nanofluids in various aspects due to the enhanced thermophysical and heat transfer properties of nanofluids. This paper reviews the preparation methods and some challenging issues of hybrid nanofluids during the Preparation of hybrid nanofluids. One-step and two-step are mainly the preparation methods of hybrid nanofluids. Compared to the one-step method, the two-step method is a widely used technique for preparing nanofluids due to its simplicity, whereas this technique has a complexity of achieving stability of hybrid nanofluids. On the contrary, the one-step is very flexible for achieving uniformity of nanofluids with comparatively high production cost. Some researchers followed various techniques such as surfactant addition, surface treatment, and pH modification for preparing a durable nanofluid. However, these methods also have their limitation, such as degrading the thermal attributes of hybrid nanofluids. So, future studies need to address these challenges along with the cost analysis during preparing the hybrid nanofluids.

The innovation of nanofluids, a novel working fluid, has presented the development of heat transfer properties in machining, automotive engine cooling systems, pumping power and others to optimize the overall system. Nanofluids have pulled in scientists' cogitation from various fields in designing new thermal systems for different engineering applications due to their distinctive thermophysical properties and prospective applications. Long term stability, improved thermal conductivity, and viscosity are the principal fundamental expectations in nanofluids research to achieve better heat transfer performance. In the previous couple of decades, various investigations have been completed to explore the nanofluids properties augmentation. For instance, kerosene-based oleic acid-coated Fe3O4 nanofluids showed 300% improvement of thermal conductivity, and water-based single-walled carbon nanotube revealed 320% improvement of viscosity. This paper presents a survey of recent exploration outcomes focusing on the thermal conductivity, viscosity, flow characteristics of hybrid nanofluids, including the preparation method of nanofluids utilized in different applications. Additionally, the elements that impact nanofluids' thermophysical properties, challenges of nanofluids, and fundamental outline and analysis of most recent research studies have been discussed and referenced. Finally, although the applications of nanofluid are increasing in several engineering sectors due to advanced discoveries of nanofluid, yet, requires more research focusing on the study of various sorts of nanofluids, different combinations of several types of nanoparticles, blending proportion and identifying the component which adds to the up-gradation of heat transfer to commercialize the nanofluids in practical fields.

This paper is presented to investigate experimentally on the stability of 40% ethylene glycol-based TiO2-Al2O3 hybrid nanofluids. Recently, the research is more highlighted on the thermophysical-properties of nanofluids. Hence, the stability of the hybrid nanofluids thoroughly assessed in this research work. The study uses the two-step method for preparing 40% ethylene glycol-based TiO2-Al2O3 hybrid nanofluids. The experiment is carried out for the various combination of mixture ratios including 20:80, 40:60, 50:50, 60:40 and 80:20 of TiO2-Al2O3 nanoparticles with a volume concentration of 0.1%. The stability assessment of hybrid nanofluids is accomplished through visualisation effect, transmission electronic microscopic observation, UV-Vis spectrophotometry and zeta potential value from particle size analyser. The findings show the optimum mixing ratios of TiO2-Al2O3 nanoparticles in terms of stability which is further confirmed by Zeta potential and absorbency from UV-Vis spectrophotometry. The results from the study reveal that 80:20 ratio of TiO2-Al2O3 nanoparticles possesses the best uniformity for over 21 days without any noticeable settlement of particles in the colloidal suspension. Moreover, 50:50 and 60:40 ratios show modest stability for almost two weeks while 20:80 and 40:60 ratios of TiO2-Al2O3 nanoparticles show minimum stability along with rapid sedimentation in the dispersion. However, it is also evident that the optimum mixing ratio of TiO2-Al2O3 hybrid nanofluids has excellent stability which will lead to further study on the thermal properties of nanofluids and finally motivate engineers to the real-life application of nanofluids.