Laser-induced bursts of subsurface liquid Mo at transition from planar to volume vaporization: ballistic and percolation surface aggregation of ejected particles

Abstract

The quality of laser-driven processes associated with vaporization strongly depends on the vaporization regime. Transition from planar to ‘volume’ vaporization is connected with superheating which causes significant topological changes in the surface itself. A study of molybdenum surface has shown that the inrush and ejection of liquid molybdenum, caused by laser-induced subsurface heating in the periphery zone of the spot, occurs in the form of small droplets. Corresponding Reynolds numbers are estimated to be Re ≈ 10 3, similar to that for the turbulence in the boundary layer. Similarity with the morphology of the subsonic jets, which under strong disturbance break into droplets at the same value of the Reynolds number, is also established. Droplets of liquid molybdenum aggregate into two-dimensional structures of the columnar form which grow by ballistic aggregation, and into irregular, lace-clusters which grow by percolation. In the near-periphery zone a high radial momentum of ejected particles causes ballistic aggregation, while in a distant-periphery zone (near the edge of the spot), a random particle momentum causes growth of random aggregates. Numerical simulation of the random lace-clusters indicates that percolation conditions (number of particles, percolation probability, and the lattice) are different in different local basins, as a consequence of inhomogenous momentum distribution in the subsurface liquid molybdenum. As a consequence, these processes lead to loss of precise control of either laser film deposition, or laser-material removal.