Self-excited chaotic shedding of acoustically levitated drops through multi-mode spatiotemporal synchronization

We experimentally study the spatiotemporal dynamics of self-excited shedding of millimeter-sized water drops acoustically levitated in a single-node standing wave cavity. By decreasing the sound intensity below the threshold, the interplay of drop motion and its perturbed acoustic wave field lead to the transition from stable self-excited drop oscillation to chaotic drop oscillation with growing fluctuations and intermittent droplet shedding. Using azimuthal Fourier transform, the top-view drop shape can be decomposed into zonal and sectoral modes with varying amplitudes. The shedding is led by the increasing amplitudes of the low order sectoral modes (azimuthal mode number m = 2 and 3), which cause the strongest amplitude in the zonal mode (m = 0) in the re-expansion stage after the shrinking of the side lobes in the low order modes. It in turn causes synchronized excitations of high order sectoral modes with m > 3. Their constructive superposition at certain points along the flattened thin edge of the re-expanding drop leads to sharp protrusions, where the surface tension cannot hold the thin rapid expanding jets, and shedding occurs.