On the colloidal and chemical stability of solar nanofluids: From nanoscale interactions to recent advances

The outdoor deployment of solar nanofluids in practical solar photothermal conversion devices for prolonged periods of time has been an ongoing challenge. This is due to their exposure to intense solar radiation, elevated temperatures, thermal and solar cycling, and other rough operation conditions that compromise their dispersion and chemical stability. This work is an attempt to address this challenge in light of state-of-the-art advances in understanding, characterizing, and mitigating issues pertaining to the stability of solar nanofluids. We first defined the concept of nanofluids, along with their types and applications, merits and limitations, and preparation techniques. This was followed by presenting the basic physical principles that govern the interparticle interactions, clustering and deposition kinetics, and theories of colloidal stability for nanoparticles in aqueous environments. Building on a sound understanding of these necessary fundamentals, the destabilization factors and instability effects on solar nanofluids were systematically identified. A critical evaluation of the most recent stability characterization techniques and stabilization strategies for solar nanofluids was then provided. A coverage of selected state-of-the-art works in the literature that target the enhanced stability of solar nanofluids in different photothermal conversion processes based on the direct volumetric absorption of solar radiation was presented and comparatively evaluated. Finally, recommendations were drawn regarding current knowledge gaps and future research directions in order to overcome the stability limitations hindering the deployment of solar nanofluids in practical solar photothermal conversion applications.