Towards hybrid nanofluids: Preparation, thermophysical properties, applications, and challenges

The objective of compiling this study is to seek the attention of young scholars and experts working in the field of heat transfer by discussing the applications and challenges of hybrid nanofluids with a concise discussion on its history, synthesis techniques, thermophysical properties, research gaps, future directions, current status, and the leading groups, organizations, and countries around the world. Applications in different fields like solar energy, heat pipes, automotive industry, manufacturing industry, heat exchangers, cooling of electronic equipment's etc. have been reviewed and discussed in this study. Nanofluids are part of these inventions that possess the ability of uprising the field of heat transfer, but instead of its superior thermal, electrical, and optical properties, advancement in the form of hybrid nanofluids and a numerous study conducted around the world, investigators and manufacturers are still facing the problem of practicing nanofluids in heat transfer devices used commercially and even in an individual's life. This problem works like a magnet, capturing the attention of research societies working around the world by inspiring them to take control and lead this field to the destination of domestic and commercial implementation. With the help of this article, authors endeavored to identify some important work orientations and existing problems that interrupted its performance and implementation for the convenience of captivated scholars. The most recent research work has improved the thermophysical properties of nanofluids by introducing hybrid nanofluids, working on preparation techniques, and by adding the surfactants to improve its stability but still need a lot of attention for the development of correlations/models that can predict the thermophysical properties and heat transfer characteristics, to make it cost-effective, improve its stability, and finally its implementation.