(a) Illustration of the vortex ring formation during drop impact on a pool surface. (b) Schematic of X-ray imaging coupled with the experimental set-up for the drop impact. (c) Sequential X-ray images (of the red square in a) in ethanol drop impact (with diameter ∼1.9 mm) from 80 mm height showing a positive vortical flow (Supplementary Movie 1). The interfacial boundaries between the drop fluid (dark contrast) and pool fluid are clearly resolved in a high temporal resolution. Scale bar, 100 μm long.