Fragmentation of a liquid metal droplet falling in a water pool

This paper focuses on the experimental investigation of breakup regimes of a molten fusible metallic droplet in water, at intermediate Weber numbers with emphasis on mass and energy balance. The experiment consists in impacting perpendicularly a molten drop onto the interface of a deep water pool, at a controlled temperature. Using a drop-on-demand device and high-speed shadowgraph, a single drop can be visualized during its evolution. There is a noticeable velocity jump when the droplet crosses the interface that can be modeled using an unsteady Bernoulli equation. As observed for liquid-gas systems, the drop experiences different regimes of fragmentation, depending on its Weber number: oscillations, bag oscillations, prolate drop stretching breakup, and then bowl-shaped bag breakup. However, opposite to the gas-liquid case, a Rayleigh-Taylor instability mechanism seems to be absent and this seems related to the bowl-shaped bag breakup mechanism when compared to the dome-shaped gas-liquid case. Statistics of the daughter droplets are then given, using either image analysis for large droplets size distribution or sieving and weighting of the solidified fragments for measurement of the Sauter mean diameter and surface energy creation. Finally, a simple relation between the Sauter mean diameter and the Weber number is presented based on the energy and mass balances. When comparing with previous higher Weber number results, a viscous transition corresponding to a strong increase in the energy loss is also shown to occur for the higher Weber number.