Nanofluid Drop Impact on Heated Surfaces

We experimentally elucidate the impact dynamics of ethylene glycol (EG) droplets laden with both hydrophilic and hydrophobic SiO₂ nanoparticles (NPs) onto a flat heated surface in non-boiling, boiling, and Leidenfrost regimes. We use seven nanofluid concentrations (<i>C</i>_<i>p</i>), ranging from 0.89 to 64.3 wt %, and control the surface temperature (<i>T</i>_s) between 100 and 400 °C, while the nanofluid droplet's impact velocity is constant at 0.22 ± 0.02 m/s. Phase diagrams of impact outcomes are established to illustrate the effect of the additive nanoparticles on the droplets' impact dynamics, revealing that nanoparticles modify droplet impact behaviors differently in each regime. In the non-boiling regime, the droplet spreading profile remains unaffected by nanoparticles up to <i>C</i>_<i>p</i> < 11.9 wt % before reaching the maximum spreading diameter (β_max). For nanofluid drops with higher nanofluid concentration, the increasing viscosity with concentration is likely to be the primary factor that affects the droplets' spreading profile in the non-boiling regime <i>T</i>_s ≲ <i>T</i>_sat ≈ 200 °C, as the saturation temperature. In the boiling regime 200 °C < <i>T</i>_s ≲ 350 °C, a small amount of nanoparticle addition (<i>C</i>_<i>p</i> = 0.89 wt %) promotes atomization regardless of nanoparticle wettability. Finally, manifested in a complete rebound due to an intervening vapor layer, the Leidenfrost temperature (<i>T</i>_L) of the nanofluid droplets is affected by both nanofluid concentration and nanoparticles' wettability. The nanofluid droplets' <i>T</i>_L increases with higher nanofluid concentration; moreover, this Leidenfrost temperature increment is more significant for EG droplets laden with hydrophobic nanoparticles. Our results quantitatively reveal the significant influence of nanoparticle concentrations and wettability on drop spreading, impact outcome, and Leidenfrost temperature on heat surfaces, potentially benefiting applications in coating, spraying, and cooling.