Viscous-capillary thinning of molten metal ligament stimulated by surface oxidation

For producing fine metal powders, the critical step is ligament dynamics of molten metal, while the thinning and breakup processes subjected to the interfacial heat and mass transfer are elusive. Here, we conduct high-speed visualization and infrared pyrometry on a single tin ligament that enable us to distinguish the effects of surface oxidation and cooling. On the tin ligament, a thin oxide layer grows according to the molecular diffusion of ambient oxygen, leading to the significant increase of viscosity coefficient at the interface. The capillary destabilization is then suppressed by the surface oxidation. We discover that the tin ligament thins initially following the capillary timescale, while the necking is accelerated to obey the viscous-capillary timescale as the oxide layer becomes one-tenth of the neck radius, corresponding to the Ohnesorge number of larger than unity. Simultaneously, the neck is cooled by the ambient. The accelerated thinning due to the viscous effect leads to the prompt decrease of heat capacity. We successfully formulate the rapid temperature drop at the neck based on the interfacial heat transfer coupled with the viscous-capillary necking dynamics in a consistent manner. • Surface oxidation of molten tin increases viscosity and accelerates thinning. • Thinning of oxide tin ligament follows viscous-capillary timescale. • Heat transfer of tin ligament couples with interfacial mass transfer.