Dynamic spreading and splashing of tin droplets: impact velocity effects and splashing boundary analysis

The spreading and splashing behavior of high-velocity metal liquid droplets upon impact with a substrate is a fundamental scientific concern in thermal spray and additive manufacturing for metal deposition. A thorough understanding of the spreading and splashing tendencies of individual liquid droplets, along with the factors influencing them, is crucial for improving the quality of sprayed coatings and additively manufactured components. In this research, high-speed photography was employed to document the propagation process, splashing events, and alterations in droplet structure following the impact of tin droplets onto stainless steel surfaces at varying velocities. The study unveiled that as the impact velocity rises, the droplet's dynamics undergo a series of transformations, encompassing retraction, spreading, and splashing, accompanied by dynamic fluctuations in the contact angle. Elevated impact velocities result in an augmentation of the droplet's maximum spreading diameter, coupled with a simultaneous reduction in spreading duration. Splashing occurs at relatively higher speeds and near the maximum forward contact angle. The paper verified a splashing prediction model based on the pressure balance of the liquid film and boundary conditions, successfully predicting the onset of droplet splashing. The results of the experiments can be applied to advanced technologies in thermal spray and additive manufacturing for metal deposition.