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Nanofluid droplet evaporation has attracted great interest due to its applications such as in painting, coating and patterning. In most studies, either the particle size or the concentration of nanofluid is considered as a factor in the formation of the residue pattern. This work aims to investigate the effect of both particle size and concentration on the residue pattern. A comprehensive study was made of the residue patterns of Al 2 O 3 and TiO 2 aqueous nanofluid droplets on different substrates (i.e. glass, stainless steel and Teflon). It was found that a ring-shaped pattern was formed at low concentrations and small particle sizes, while a uniform pattern was formed at high concentrations and large particle sizes for Al 2 O 3 nanofluids. In addition, only ring-shaped residue patterns were observed for all concentrations of TiO 2 nanofluids. In the case of different substrates, on a material with a high contact angle with water, it was difficult to form a ring-shaped pattern. The widths of the ring-shaped pattern were analyzed as well. The results showed that the width of the ring-shaped pattern was larger for small particles. The materials of substrate and nanoparticle also influenced the width. [ABSTRACT FROM AUTHOR]

• The MR-2 SGHF exhibits the highest synergistic thermal conductivity. • The low mixing ratio droplets give highest evaporation rates for 25 °C≤ T s ≤ 100 °C. • The high mixing ratio droplets give highest evaporation rates for 103 °C≤ T s ≤ 125 °C. • The SGHF droplet evaporation rate on residue is up to 10 times that on copper. • The latent heat flux removal up to 890 W/cm2 is obtained using SGHF droplets. Thermal management of many high heat flux devices depends on droplet based cooling, such as the spray cooling or electro-wetting for hotspot cooling. Recently, heat dissipation in these devices increased to unprecedented levels, pressing a need for advanced thermal fluids in droplet based cooling systems. In this paper, we address this challenge by investigating the evaporation and boiling performance of the silver-graphene hybrid nanofluid (SGHF) droplet for its various mixing ratios and droplet sizes on a heated copper and a residue surface, obtained from the evaporation of the first SGHF droplet. The results show that low mixing ratio (MR ≤ 0.1) SGHF droplets exhibit highest evaporation rates for substrate temperature (T s) in a range of 25 °C ≤ T s ≤ 100 °C. However, this trend is reversed in the nucleate boiling regime, where high mixing ratio (MR ≥ 0.9) droplets give highest evaporation rates. Moreover, all SGHF droplets, irrespective of their mixing ratio, exhibit similar evaporation rates in the film-boiling regime. Furthermore, the SGHF droplet evaporation rate on its porous residue surface increases up to 173% for 25 °C ≤ T s ≤ 100 °C and by an order of magnitude in the nucleate boiling regime as compared to a plain copper surface. We also show that besides the synergistic thermal effect, the thermal Marangoni convection also affects the SGHF droplet evaporation rate. Moreover, we develop a diffusion-convection evaporation model that can predict the evaporation rate for different mixing ratios of the SGHF droplet on heated copper and residue surfaces. Moreover, we demonstrate that the latent heat flux up to 890 W/cm2 and 850 W/cm2 can be achieved using a SGHF droplet on heated copper and residue surfaces, respectively, suggesting its potential application in high heat flux device cooling. Finally, we discuss the effects of spray hydrodynamic parameters on critical heat flux of the SGHF spray cooling. [ABSTRACT FROM AUTHOR]