Experimental investigation of the surface wettability effect on dynamic characteristics during droplet impacting a tube surface at different velocities

The impact of liquid droplets on solid surfaces or liquid films occurs frequently in nature and industrial fields. In this paper, the droplet dynamic process is studied during impacting a tube surface with high speed photography. Experimental system is mainly included a high speed camera, a computer data acquisition system, a droplet generator, a tube surface, and some light sources. After the droplet impact the hydrophilic, hydrophobic and super hydrophobic tube surfaces respectively, with the curvature ratio of 0.43, the droplet morphological changing processes are observed at the impact velocities of 0.7m/s, 1.38m/s and 1.95m/s. The experimental results show that the worse the surface wettability is, the smaller the droplet maximum spreading factor is. The poor surface wettability hinders the droplet spreading, but helps the liquid film to retract and rebound. When the contact angle is a constant, the greater the impact velocity is, the greater the maximum spreading factor is. On the hydrophilic tube surface, the droplet spread only on the upper half of the horizontal tube at the impact velocities of 0.7m/s and 1.38m/s. The droplet spread not only on the upper half but also on the lower half of the horizontal tube at the impact velocity of 1.95m/s. On the super-hydrophobic tube surface, when the impact velocity is greater than 1.38m/s, the droplet presents splashing. The greater the impact velocity is, the earlier time the splashing occurred at. When the impact velocity is 1.95m/s, the worse the surface wettability is, the earlier time the liquid film begins to splashing at.