Effects of surface oxide layer on the impact dynamic behavior of molten aluminum droplets

Oxide layer is easily formed on the surface of some metal droplets, which affects the dynamic behavior of droplet impact. In this paper, experiments were conducted for the impacting process of a molten aluminum droplet on a Teflon substrate. The thickness of surface oxide layer of aluminum droplets was controlled by varying oxygen concentrations in the gas atmosphere. The effects of oxygen concentration and Weber number on droplet spread, rebound, and splash behavior were investigated. The results show that the oxygen concentration in the environment directly affects the droplet impact behavior. The differences in the droplet retraction process at different oxygen concentrations are more significant than the spread process. Strong retraction causes droplet rebound (We = 17.11) and splash (We = 102.3) in hypoxic environments, whereas only weak retraction in air. The viscous dissipation of the droplet impact in different oxygen concentration environment is estimated by counting the droplet bounce height, and the dissipation increases with the increase in the oxygen concentration. The thickness of the oxide layer measured by scanning electron microscope became thinner as the oxygen concentration decreases and, accordingly, the aluminum droplet collisions show different dynamic behavior. The experimental results are analyzed and explained regarding the reduction in surface tension, the increase in viscous forces, and the shear-thinning properties which increase the viscous dissipation during retraction. Ryan model was used to predict the droplet maximum spreading factor ξmax with the Weber number to the power of 0.5, better than the energy conservation-based Pasandideh-Fard model.