  When vortex rings collide head-on at high enough Reynolds numbers, they
ultimately annihilate through a violent interaction which breaks down their
cores into a turbulent cloud. We experimentally show that this very strong
interaction, which leads to the production of fluid motion at very fine scales,
uncovers direct evidence of a novel iterative cascade of instabilities in a
bulk fluid. When the coherent vortex cores approach each other, they deform
into tent-like structures, and the mutual strain causes them to locally flatten
into extremely thin vortex sheets. These sheets then break down into smaller
secondary vortex filaments, which themselves rapidly flatten and break down
into even smaller tertiary filaments. By performing numerical simulations of
the full Navier-Stokes equations, we also resolve one iteration of this
instability and highlight the subtle role that viscosity must play in the
rupturing of a vortex sheet. The concurrence of this observed iterative cascade
of instabilities over various scales with those of recent theoretical
predictions could provide a new mechanistic framework in which the evolution of
turbulent flows can be examined in real-time as a series of discrete dynamic
instabilities.
