Contact angle dynamics during the impact of single water droplet onto a hot flat practical stainless steel surface under medium Weber numbers

An experimental study on the contact angle dynamic during the impact of single water droplet onto a hot flat practical stainless steel surface under medium weber number was performed. The tested solid material was practical stainless steel of SUS 304. The surface roughness of the hot surface, the static contact angle at ambient pressure, and the diameter of the solid materials were respectively Ra = 0.2 µm, 85.7 o , and 30 mm. The surface temperatures were varied from (60–200) o C. The liquid droplet diameter was 2.4 mm. The Weber numbers were 52.1, 57.6, and 63.1. The dynamic contact angles of single droplet during the impact were determined by implementing the developed image processing technique of the obtained video images from experiments. As a result, it was found that (1) the time needed to reach a maximum diameter of single droplet on a flat hot solid surface decreases with the increase of surface temperature, (2) the increase of the Weber number will postpone the spreading of the droplet from its receding, (3) there are three regions of the dynamic contact angle during the liquid evaporation, those are transient spreading region (region 1), a quasi-mechanical equilibrium region (region 2), and transient evaporation region (region 3), and (4) The contact angle in quasi-mechanical equilibrium region the under the nucleate boiling is insensitive to the surface temperature.