High-speed dynamics and temperature variation during drop impact on a heated surface

Drop impact on heated surfaces plays a vital role in numerous applications, such as spraying, cooling, and combustion. In addition to impact dynamics, we investigate both droplet and surface temperature changes using a high-speed infrared (IR) camera and thermocouples when a Milli-Q water droplet impacts on a heated surface under various Weber numbers (1.6≤We≤129, which compares the drop kinetic to surface energy) and initial surface temperature (100∘C≤Tsi≤445∘C). Theoretical models of droplet and surface temperature changes are deduced using the energy conservation principle. Both theoretical and experimental results show that temperature variations are dramatically influenced by impact dynamics, which shows spreading, spreading/splashing atomization, or complete rebound depending on We and Tsi. For non-Leidenfrost droplets, the experimental results show that the mean droplet temperature change (ΔTd) is increased with increasing We and Tsi and scales with ΔTd∼t3/2 and ΔTd∼t in the initial spreading and later-time sticking stages, respectively. The experimental results of solid surface temperature change, ΔTsc≡ΔTs(z=0), show that ΔTsc alters sharply with We for We<30, whereas insignificantly for We>30 with an identical Tsi. Under the same We, ΔTsc increases first for 100∘C≲Tsi≲300∘C and then reduces when 300∘C≲Tsi≲445∘C because of enhanced atomization at elevated temperatures. Finally, a negligible ΔTd and a nearly constant ΔTsc (≲4∘C) are observed for Leidenfrost droplets due to the formation of an insulating vapor layer formed underneath the bouncing droplet.