Supersonic in-flight induced the air migration and crystallization at droplet and substrate interface

Droplet impact onto a substrate usually embedded the air film at the droplet/substrate interface. However, an in-depth understanding of the migration of air film and its influence on crystallization of supersonic droplet is still lacking. In this contribution, the high pressure of supersonic impingement effectively promoted the air solubility and broke up, resulting in the diameter of air bubble significantly reduced by one-sixteenth compared to the subsonic impingement. The thin air film was not insufficiently affected the heat diffusion and caused the formation of some amorphous films around nano-sized pores. The final height and width of columnar grains were approximately decreased by one-quarter and a half than that of flattened subsonic droplet. The intersection interface of splats atomically reconstructed and formed the partially epitaxial growth of [ 2 2 ‾ 0 ] direction of columnar grains, which essentially improved the cohesion property.