A guide to the preparation techniques of six classes of metal-, metal oxide-, and carbon-based nanofluids and the implications for their stability

Due to their encouraging thermal and, more recently, optical properties, nanofluids have attracted considerable interest and have become a viable option as working fluids in numerous heat transfer applications. The stability of nanofluids, on the other hand, is viewed as an impediment to their long-term use and a barrier to their commercialization. As a consequence, a number of researchers examined the various factors that affect stability. Preparation techniques are one of the most essential factors to consider when investigating the stability of nanofluid. This article examines studies conducted on metal-, metal oxide-, and carbon-based nanofluids, focusing on their preparation and stability evaluation. Part-I provides a concise overview of the stability challenges of nanofluids and the nanofluid preparation methods of nanoparticles as additives dispersed in water, oil, and ethylene glycol, as well as their impact on nanofluid stability. In part II, more emphasis is placed on the apparatus utilized in the preparation of nanofluids. Additionally, the preparation techniques of nanofluids based on hybrid nanomaterials and the stability status of such solutions are discussed. Lastly, the role of surfactants in sustaining the stability of nanofluids is highlighted. The size and shape of the nanomaterials used as the dispersed (solid) phase, nanomaterial concentration, formulation surfactant, preparation method, and other parameters all impact the stability of nanofluids, according to the present review. The stirring, sonication, surfactant, and surfactant-to-particle ratios are the four primary factors that influence the preparation's stability. Through manipulating these parameters, researchers enhanced stability from a few hours up to 6 months and even up to one year in some cases. Exceeding the prescribed range of these parameters may have negative effects on the nanofluids' stability. Several nanomaterials were tested as dispersants in water with stability of up to a year. In addition, various surfactants were used to maintain these stabilities. Researchers and academics interested in preparing stable nanofluids for a variety of potential applications are expected to find the results of this review beneficial.