OBSERVATIONS OF BUBBLE SELF-ORGANIZATION IN LASER-MATTER INTERACTIONS

Bubbles generated in laser–metal interactions (LMI) at the transition from planar-to-volume boiling behave like hard (and soft) spheres. Micrographic analysis indicates that the surface layer of vaporizing metal, which comprises small microscale bubbles, behaves like a two-dimensional granular system. Bubble collision leads to bubble self-organization which changes with laser power density Q_p, and falls into (i) the kinetic regime, (ii) the clustering regime, or (iii) the regime of inelastic collapse (when bubbles form chain-like clusters), in analogy with the results of numerical simulations of hard-sphere dynamics. At a certain power density, shock waves that travel over the surface become strong enough, so that the system of bubbles behaves like horizontally and vertically shaken granular material. Since bubbles in the vibrational process are decelerated, hydrodynamic forces deform them, causing them to stay coalesced after the collision, forming a cluster with plateau borders, which is actually metal foam. The results presented show the existence of various complexity levels in the self-organization of bubbles, their sensitivity to the variation of interaction parameters, and finally, they indicate some universal features of bubble system dynamics.