Effect of SiO₂ nanoparticle addition on the wetting and rheological properties of solar salt

Molten salt based nanofluids are involved in Thermal Energy Storage (TES) Systems, both as heat transfer fluids and energy storage materials. Rheological and wetting properties play an acute role in the handling and storage of materials and are closely related with pumping and corrosion issues. In this study the effect of nanoparticle addition on the aforementioned properties of molten salt nanofluids is investigated. The solar salt (60% NaNO₃ – 40% KNO₃), as well as its individual components NaNO₃ and KNO₃, are mixed with various concentrations of SiO₂. The contact angle and viscosity are measured throughout the liquid phase. Addition of a small percentage of nanoparticles, significantly alters the contact angle and viscosity of the nanofluid. In the absence of silica all the molten salts display a linear behavior with respect to temperature. However, in the presence of nanoparticles the solar salt retains an elevated value until 300 °C in the case of the contact angle and 260 °C in the case of the viscosity, after which a steep reduction occurs. With larger concentrations of nanoparticles, this effect is shifted to higher temperatures. Similar behavior, however, is not present in the case of the NaNO₃ and KNO₃ individually, both of which, with the addition of nanoparticles, retain curve trends similar to their baseline cases. Further investigation, involving differential scanning calorimetry, suggests that nanoparticles delay the liquid/solid phase transition process of the molten salt mixture, which in turn affects the rheological and wetting behavior of the molten mixture.