Liquid rope coiling
N. M. Ribe (Instiut de Physique du Globe, Paris, France)
The coiling of a thin "rope" of viscous fluid falling onto a rigid surface is an example of a buckling instability, in which a thin object under compressional stress becomes unstable to bending disturbances.
Using a combination of analytical, numerical, and experimental approaches, we have determined a complete phase diagram for liquid rope coiling, comprising four distinct dynamical regimes (viscous, gravitational, inertio-gravitational, and inertial) depending on how the viscous forces that resist the bending of the rope are balanced. The most interesting behavior occurs during the gravitational-to-inertial transition, where multiple resonant modes coexist at a fixed fall height within a certain range. I will also describe a remarkable secondary instability in which spiral waves of air bubbles emanate from the pile of fluid formed by the coiling.