Direct imaging of gas atomization process accompanying surface oxidation of tin droplets

Fine metal particles produced by gas atomization are recognized as an essential material for additive manufacturing. However, spatio-temporally resolved images for the atomization process are still lacking. In the present study, we apply a high-speed Schlieren optical system synchronized with a pulse light source with flashing period of 30 ns to a simple atomization setup, consisting of a single supersonic N2 jet at Mach number of 1.5 and free-falling tin droplets. Covering the tin droplets by Ar gas, we generate spherical droplets by minimizing the initial oxidation from the ambient. Impinging on the large-momentum gas jet, the tin droplet largely deforms to be shaved the bottom end and bounced above the jet with partially penetrating inside the gas core. The spreading ligaments above the jet thins at first along with the capillary timescale. As the surface oxidation proceeds, the neck transitionally shrinks according to the viscous-capillary timescale, which evidences the direct observation of gas atomization process superposed by the molecular diffusion across the interface. Statistics of collected metal particles demonstrate a bimodal distribution for the diameter, originated from the distinct atomization mechanisms of aerodynamic dominant breakup inside the gas jet and of capillary dominant breakup spreading above the jet.