Abstract
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.
Highlights
Small droplet shedding from a liquid drop is important for a wide range of applications such as ink-jet printing, spray combustion in the engine, and spray cooling
Only a few studies focus on droplet shedding in a contact-free environment
We experimentally investigate the spatiotemporal dynamics of self-excited intermittent shedding of millimeter sized water drops levitated by a weak standing acoustic wave in a single-node cavity
Summary
Small droplet shedding from a liquid drop is important for a wide range of applications such as ink-jet printing, spray combustion in the engine, and spray cooling. The most common mechanism involved in the formation of highly stretch lobes is often explained by the growth of perturbation on the surface through the Rayleigh and Kelvin-Helmholtz type of instabilities. Shedding can be induced by strong external perturbations such as mechanically vibrating the drop, exposing to surface acoustic waves, or shearing the drop using high-speed gas jets.. A more detailed experimental investigation on the spatiotemporal dynamics of self-excited droplet shedding in a weak acoustic levitation field is conducted. It is found that as the oscillation instability grows, the self-excited low-order sectoral modes change from alternate emerging to collective excitation, which is associated with the growth of the shedding occurring rate. Shedding events occur when the amplitude of the zonal mode reaches the temporal maximum, accompanied with synchronized excitation of high-order sectoral modes with m > 3. The induced thinning and protrusion spike formation of the drop edge are key for droplet shedding
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