(a) Tracking the coiling vortex core with time in water drop impact. The x–y coordinates are set to the real size of the impacting drop. x=0 is the centre of the drop and y=0 is the top of the pool surface. (b) Circulation angle of the vortex core as a function of time for the water–ethanol (3:7 in mass fraction) drop impact with different impact speeds. (c) Circulation angle of the vortex core as a function of time normalized with the characteristic time U/D with the same data in b.