Migration dynamics of molten droplets impacting on vertical solid surfaces

Droplet impact onto solid surfaces is a widespread phenomenon in various engineering applications, including metal droplet ejection three-dimensional (3D) printing, an innovative metal additive manufacturing technique. Despite extensive research on droplet behavior upon impact, the physics of molten droplets impacting on vertical or inclined substrates, particularly in the scenarios of omnidirectional deposition and conformal fabrication, remains understudied. This paper introduces a novel phenomenon termed “migration,” distinct from slipping and rolling, observed when a droplet impacts vertical surfaces. The study investigates the migration behavior of molten droplets on vertical substrates with varying roughness and wettability, elucidating the underlying mechanisms and influences of these surface properties on droplet migration. Meanwhile, the coupled effects of fluid dynamics and thermodynamics on the migration of the droplet are numerically analyzed. It was found that the migration results from the asymmetric spreading and receding along the longitudinal direction caused by gravity or gravity-induced lateral inertial forces. The migration distance of the droplet increases with the substrate's roughness and non-wettability, which resulting from a reduction in interfacial binding force. The migration dynamics depend on the interaction between the surface flow and rapid solidification, allowing control over the final form and migration displacement of droplets. These findings enhance our understanding of the minor migration phenomena in droplets impacting on the vertical substrates along the gravity direction, contributing valuable insights for practitioners aiming to minimize forming defects and improve the quality of metal droplet ejection 3D printing.