No abstract

Transition phenomena are observed when a water droplet train (with up to 18.9 m/s in velocity and 39.20 kHz in frequency) impinges onto a heated copper surface (up to 250 °C). The hydrodynamic flow pattern strongly depends on the wall temperature. The surface temperature does not apparently influence the spreading speed when the wall temperature is less than the boiling temperature, but it enhances the spreading rate significantly at higher surface temperatures. A “steady-state” wetting surface area can be reached when the water supply rate equals the water consumption rate. The time-independent spreading diameter decreases with an increase in the wall temperature until an ultimate diameter of the “steady-state” wetting area is observed at around 0.4 mm, 3.4 times the droplet diameter, when the surface temperature is higher than 190 °C. Moreover, unlike the random direction splashing when wall temperature is less than 180 °C, a stable splashing angle is established at higher wall temperatures. However, the angle reduces apparently with the increase in the wall temperature.

The hydrodynamic patterns of the impingement of a water droplet train on a high temperature substrate are captured with a high-speed camera, and then analyzed.