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An investigation on viscosity was conducted 2 weeks after the Al2O3-water nanofluids having dispersants were prepared at the volume concentration of 1-5%. The shear stress was observed with a non-Newtonian behavior. On further ultrasonic agitation treatment, the nanofluids resumed as a Newtonian fluids. The relative viscosity increases as the volume concentrations increases. At 5% volume concentration, an increment was about 60% in the re-ultrasonication nanofluids in comparison with the base fluid. The microstructure analysis indicates that a higher nanoparticle aggregation had been observed in the nanofluids before re-ultrasonication.

An experimental investigation was conducted on the effective thermal conductivity and viscosity of Al2O3 water-based nanofluids. The thermal conductivity was determined using the transient hot wire (THW) method. The results show that the thermal conductivity of Al2O3 nanofluids increases with increasing nanoparticle concentrations in a distinct linear trend. Adding (1 to 5) % of Al2O3 nanoparticles to water increases the effective thermal conductivity of Al2O3 nanofluids by (6 to 20) % at room temperature. There is a substantial increase in the enhancement of thermal conductivity as temperature increases. At 55 °C, this value increases to around 16 %. The thermal conductivity enhancement decreases from (30 to 10) % while the particle size increases from (10 to 35) nm, but enhancement increases when the particle size is above 35 nm. The increase in thermal conductivity is 27.5 % for a particle size of 150 nm. In the viscosity measurements, the effective viscosity increases up to 60 % as the volume concentration increases from 1 % to 5 % at 25 °C. As the temperature increases, the viscosity of Al2O3 nanofluids decreases exponentially. Experimental results also indicate that the viscosity of nanofluids is much higher when the nanoparticle size is smaller.

We investigate the effect of surfactant on the formation of nanoparticle aggregates that resulted from evaporation of sessile nanofluid droplets theoretically and experimentally. A Monte Carlo model is developed to explain the transition from the coffee-ring pattern to the uniform deposition in drying the pinned sessile nanofluid droplets. The model applies the diffusion limited cluster-cluster aggregation approach coupled with the biased random walk of nanoparticles. The experiments show that the addition of surfactant in nanofluids helps the formation of a coffee ring instead of the uniform domain coverage. The simulations suggest an explanation of this transition by controlling the sticking probability parameter between the particles. The simulated results statistically agree with the experimental observation of the finally dried graphite nanoparticle structures from the pinned nanofluid droplets.

Experiments and simulations to promote or attenuate the "coffee-ring effect" for pinned sessile nanofluid droplets are presented. The addition of surfactant inside a water suspension of aluminum oxide nanoparticles results in coffee-ring formation after the pinned sessile droplets are fully dried on a substrate, while droplets of the same suspension without the surfactant produce a fine uniform coverage. A mathematical model based on diffusion-limited cluster-cluster aggregation has been developed to explain the observed difference in the experiments. The simulations show that the particle sticking probability is a crucial factor on the morphology of finally dried structures.