Drop impact on unheated and heated flat surfaces

We experimentally investigate water droplet impact dynamics on a heated flat surface using a high-speed camera, an Infrared (IR) camera, and embedded thermocouples. As the increase of surface temperature, Ts, and Weber number, We (from 1.6 to 129), four typical outcomes are observed: spreading, complete rebound, spreading with atomization, and splashing breakup with atomization. The surface temperature variation, |ΔT|, analyzed from the thermocouple measurements, is found to increase in the low-Ts regime but decrease in the high-Ts regime. At low-Ts (100 – 200 ^oC), spreading is the primary impact outcome at various We. In contrast, a total rebound occurs at high Ts ( > 350 ^oC) but low We; The droplet temperature and surface temperature are nearly unchanged because of the Leidenfrost effect with the formation of an insulating vapor layer, limiting the heat transfer. Our experimental results reveal the combining effect of fluid motion and heat transfer on impact dynamics and temperature distribution.