Experimental investigation of the relation between yield stress and ultrasonication period of nanofluid

Nanofluids are the mixtures of solid nanoparticles in liquid. Like most other fluids, nanofluids could behave as yield stress fluids, which require a certain amount of stress to flow. This flow characteristic may depend on the preparation of nanofluids. This paper investigates the effect of the ultrasonication periods (used during the nanofluid preparation) on the yield stress. The Al₂O₃ nanoparticle with 0.5 vol.% of solid concentration was added to the distilled water using an ultrasonic horn and applied for different durations from 0 to 5 h. The microstructures of the nanoparticles were observed by transmission electron microscopy. The flow characteristics as the shear stresses at different shear rates (12.23-305.75 s^-1) were observed for various temperatures from 10 to 50 °C. The yield stress, the flow behavior index, and the consistency index were studied using the Herschel-Bulkley rheological model. From this study, non-Newtonian flow characteristic was observed. The yield stress decreased with increasing the fluid temperature. At the start of the ultrasonication, the yield stress decreased rapidly; however, for further ultrasonication, it decreased slowly.