Though it resembles a stylized Christmas tree, this dark silhouette is a momentary structure sculpted by a collapsing cavitation bubble. As the bubble implodes near a particle‑laden oil‑coated free surface, it drives a powerful free surface jet toward air. The cavitation bubble propels and stretches the thin oil film, entraining particles and pulling the interface upward into a tall, tapering column that branches into fine ligaments and droplets—forming the “branches” and “ornaments” of the tree. This image freezes an event that unfolds over only microseconds, revealing how bubble dynamics interact with complex, contaminated interfaces. By varying particle concentration, oil properties, and bubble position, my research explores how such cavitation events control interfacial deformation, erosion, and material transport in natural and engineered flows. Here, an extreme multiphase phenomenon briefly takes on a familiar holiday shape, turning high‑speed fluid mechanics into a “Cavitation Christmas Tree.”

Hollow droplets, comprising a liquid shell encapsulating a gas bubble, exhibit core–shell morphologies with impact dynamics fundamentally distinct from those of single-phase droplets. These droplets play pivotal roles in both natural and advanced engineering processes, including aerosol generation, 3D printing, and thermal spray coating. Using high-speed imaging, we reveal the full sequence of hollow droplet impact on a thin liquid  lm, including splashing and counter-jet formation, governed by the intricate interplay between droplet impact dynamics and internal bubbles. By introducing combinations of modi ed non-dimensional numbers, we establish a uni ed scaling for the mechanics underlying these phenomena. This previously unexplored regime o ers new insights into  uid fragmentation pathways and provides a quantitative framework for interpreting and controlling splash phenomena across disciplines.

Droplet impact on solid surfaces plays a critical role in both natural and industrial contexts. Hollow droplets, composed of a gas bubble encapsulated by a liquid shell, exhibit a distinct core–shell structure that imparts unique physical and chemical properties compared to single-phase droplets. This morphology is prevalent in various practical processes, including raindrop impacts on the Earth's surface, the deposition of hollow spherical particles during thermal spray coating, and bubble-bursting aerosol transfer from the ocean. In many of these scenarios, the liquid shell can behave viscoelastically when the phase contains polymers, surfactants, or biological macromolecules commonly found in natural and industrial fluids. We are investigating this phenomenon to better understand and control the spreading and bouncing behavior of such droplets.