A snowflake? An ice crystal? Guess again – this photograph is of a liquid, not a solid.
Fuck Yeah Fluid Dynamics explains this mesmerizing overhead shot:
When a drop falls from a moderate height into a shallow pool, its impact creates a complicated pattern. The photo above is a composite image showing a top-down view 100 ms after such an impact. On the left side, the flow is visualized using dye whereas the right shows a schlieren photograph, in which contrast indicates variations in density.
A paper recently published to arXiv by physicists Andreas Wilkens, David Auerbach and GertJan van Heijst (from which the top image has been borrowed) discusses how both conditions give rise to a common fluid structure comprising two pairs of vortex rings:
The inner pair forms at the edge of the crater created by the impacting drop while the outer pair is laid off from the spreading wave. One ring of each of these pairs is short-lived while the other persists. Each of the rings has a measureable non-zero circulation (whose sign, however, is opposite to that of the well-known deep-water drop vortex rings) which persists long after the wave has receded.