Transient transition of high speed and high- frequency ethanol droplet train impingement on the heated indium tin oxide (ITO) surface

Hydrodynamic behaviors of ethanol are investigated when an ethanol droplet train impinges on the hot Indium Tin Oxide (ITO) surface at the Weber numbers of 1145, 1400, and 2067 that are obtained by different piezoelectric pinhole diameters and droplet train frequencies. At the Weber number of ~1400, the droplet train is impinged on both vertical and horizontal ITO surfaces in different experiments to better understand the gravity effect whilst the impingement studies are performed only for the horizontal ITO surface at Weber numbers of 1145 and 2067. The experiments are performed between 80 and 230°C with 30°C intervals. It is observed that the spreading diameter increases by rising of the Weber number. The largest spreading diameters are obtained in the nucleate boiling regime due to the huge nucleation bubbles with the values of 5.87, 5.92, and 7.05 mm while the smallest spreading diameters are calculated as 1.24, 1.74, and 2.89 mm in the film boiling region after the observed Leidenfrost effect. The liquid pools are accumulated due to the low evaporation rates between 80 and 140°C. For all Weber numbers, the distinguishable splashing lines were observed between 170 and 200°C for the vertical droplet train impingement on the horizontal ITO surface while the splashing lines are seen between 140 and 170°C for the horizontal droplet train impingement on the vertical ITO surface. Also, relevant splashing angles were investigated in the defined temperature range. On the other hand, the huge secondary droplets are observed at high temperatures.