Evolution of drop-impact ejecta studied with color interferometry

When a droplet impacts a liquid pool, a thin ejecta sheet rapidly emerges from the neck connecting the two liquids. Initially propagating nearly horizontally, the sheet bends and stretches into curved shapes. We investigate the spatiotemporal evolution of its shape and thickness using high-speed three-color transmissive interferometry. We obtain absolute thickness measurements with tens of nanometer resolution by analyzing fringe patterns formed from the superposition of the three distinct wavelengths. The optical setup combines light from three LED sources using dichroic mirror and beam splitters, with wavelength selection achieved via narrow bandpass filters. Experiments are conducted with gravity-driven aqueous-glycerin drops of diameter 2R = 6.3 mm, viscosities ranging from that of water up to 210 cP, and impact velocities ranging from 1 to 6 m/s, marking the regime where extended ejecta shapes emerge. For higher viscosities (μ≳8 cP), the ejecta sheet exhibits pronounced radial and axial stretching, often forming an air torus when its tip contacts with the bottom free surface. The extended ejecta can remain intact while reaching heights up to 1.8 R. In contrast, for lower-viscosity impacts it transitions directly to a slingshot regime. The measured ejecta thickness spans from 470 nm to 8 μm for 8 cP, while 7- 20 μm for 90 cP, indicating a strong thickness dependence on viscosity.