Effect of the Adsorbed on the Nanoparticles Surface Air Components on the Nanofluid Colloidal Stability: An Experimental Study

Nanofluids (NFs) can be considered promising working fluids and coolants for power systems. Nanoparticles (NPs) admixtures in liquids significantly affect their thermophysical properties. Thermophysical properties variation and colloidal stability of the NFs are determined by the concentration, size and shape of the NPs and by the influencing the NPs on the base liquid internal structure. Moreover, the NPs in the base liquid inhere in the presence of the surface layer of various air components that was sorbed during NPs storage. For today, the effect of the sorbed layer on the NPs' surface on the colloidal stability of NFs has not been examined. Here we demonstrate that preliminary NPs treatment to remove sorbed air components before NFs preparation contributes to obtaining the NFs with enhanced colloidal stability. Experimental studies of the desorption process regarding five different types of NPs that were stored under loose-fitting cover at the ambient conditions resulted that the adsorption layer consists mainly of water. For y-Al2O3 NPs (specific surface area 120 m ² .g- ¹ ) mass fraction of the adsorbed water was 2.2 %. Correspondingly, the following results have been obtained: a-Al2O3 (8.7 m ² .g ⁻¹ ) - 0.66 %; TiO 2 anatase (49.5 m ² .g- ¹ ) - 0.77 %; TiO 2 rutile (41 m ² .g- ¹ ) - 0.20 %; CuO (65.4 m ² .g- ¹ ) - 0.19 %. A technique for the NFs preparation has been developed. This technique involved preliminary NPs treatment by vacuuming together with heating up to 200 °C, followed NPs milling with liquid and subsequent ultrasonication. The measured hydrodynamic size (DLS) of the NPs aggregates in the NF obtained by the technique mentioned above was smaller than for similar NF prepared without NPs pre-treatment. The obtained results will contribute to improving the technique of preparation of the colloidal stable working fluids and coolants for power systems with high efficiency.